The 1972 AP Chemistry exam reflects a period when the free-response section was significantly longer and more flexible than today’s format, consisting of 18 free-response questions
to be completed in 110 minutes. Key topics covered in that year included complex ion coordination, stoichiometry in mixtures, and chemical kinetics. chemmybear.com
Below is an analysis of prominent questions from the 1972 exam and their solutions. Coordination Chemistry and Complex Ions
One notable question required students to account for the properties of three compounds with the same empirical formula, , based on experimental measurements. chemmybear.com The Problem
: Students had to relate structural formulas to experimental data, such as the amount of cap A g cap C l precipitated upon adding excess cap A g cap N cap O sub 3 The Answer
: The solution involves understanding how many chloride ions are "labile" (ionic) versus coordinated within the complex. For example, in
, only the one chloride ion outside the coordination sphere precipitates immediately as cap A g cap C l chemmybear.com Acid-Base and Stoichiometry: The Carbonate Mixture
A recurring problem from 1972 (often used in gas law topics) involves a 5.00-gram dry mixture of cap K cap O cap H cap K sub 2 cap C cap O sub 3 cap K cap C l reacted with Part (a): Percentage of cap K sub 2 cap C cap O sub 3 cap C cap O sub 2 gas is produced at 22 raised to the composed with power C Calculate Moles of cap C cap O sub 2
n equals the fraction with numerator open paren 740 / 760 atm close paren cross 0.249 L and denominator 0.08206 L center dot atm/mol center dot K cross 295 K end-fraction is approximately equal to 0.0100 mol cap C cap O sub 2 open bracket 1.2 .6 close bracket cap K sub 2 cap C cap O sub 3 , there is Find Mass and Percent Mass equals 0.0100 mol cross 138.2 g/mol equals 1.38 g Percentage
Percentage equals open paren 1.38 g / 5.00 g close paren cross 100 % equals 27.6 % open bracket 1.3 .7 close bracket Part (b): Percentage of cap K cap O cap H cap K cap C l The excess cap H cap C l is titrated with cap H cap C l cap H cap C l cap H cap C l reacted with cap K sub 2 cap C cap O sub 3 cap H cap C l reacted with cap K cap O cap H Final Calculation of the sample), leaving cap K cap C l AP Chemistry Olympics Kinetics and Reaction Rates The 1972 kinetics question presented data for the reaction 2 cap A plus 2 cap B right arrow cap C plus cap D
, asking students to determine the rate law based on initial concentration experiments. Archival Resources While the College Board typically highlights the three most recent years of FRQs , historical archives such as the Adrian Dingle Worked Answer Archive provide solutions for exams dating back to the early 1970s. Adrian Dingle's Chemistry Pages from that year or perhaps see how modern scoring guidelines would grade these 1972 responses?
The 1972 AP Chemistry Free Response section included a mix of quantitative analysis, acid-base chemistry, and organic isomerism. Below are the key problems and solutions from that exam. 1. Quantitative Analysis & Gas Laws A 5.00-gram dry mixture of KOHcap K cap O cap H , K2CO3cap K sub 2 cap C cap O sub 3 , and KClcap K cap C l is reacted with 0.100 liters of 2.00 M HClcap H cap C l . Part (a): 249 mL of dry CO2cap C cap O sub 2
gas (at 22°C and 740 torr) is obtained. Calculate the percentage of K2CO3cap K sub 2 cap C cap O sub 3 in the mixture. Solution: First, find the moles of CO2cap C cap O sub 2 using .
n=(740/760 atm)(0.249 L)(0.08206 L⋅atm/mol⋅K)(295 K)≈0.0100 mol CO2n equals the fraction with numerator open paren 740 / 760 atm close paren open paren 0.249 L close paren and denominator open paren 0.08206 L center dot atm/mol center dot K close paren open paren 295 K close paren end-fraction is approximately equal to 0.0100 mol cap C cap O sub 2 Since 1 mole of K2CO3cap K sub 2 cap C cap O sub 3 produces 1 mole of CO2cap C cap O sub 2 , there is 0.0100 mol of K2CO3cap K sub 2 cap C cap O sub 3 . Mass of . Percentage: . Part (b): The excess HClcap H cap C l is titrated with 86.6 mL of 1.50 M NaOHcap N a cap O cap H . Calculate the percentages of KOHcap K cap O cap H and KClcap K cap C l . Solution: Total moles . Moles HClcap H cap C l used by . Excess HClcap H cap C l (from titration) . Moles HClcap H cap C l reacted with . Mass ( 56.2%56.2 % ). Mass ( 16.2%16.2 % ). 2. Acid-Base Buffers Given a solution of ammonium chloride ( NH4Clcap N cap H sub 4 cap C l ), what is needed to prepare a buffer? Answer: You need a weak base, such as ammonia ( NH3cap N cap H sub 3 ). Mechanism: Adding Strong Acid ( H+cap H raised to the positive power ): The base ( NH3cap N cap H sub 3 ) reacts with it:
NH3+H+→NH4+cap N cap H sub 3 plus cap H raised to the positive power right arrow cap N cap H sub 4 raised to the positive power . Adding Strong Base ( OH−cap O cap H raised to the negative power ): The acid ( NH4+cap N cap H sub 4 raised to the positive power ) reacts with it:
NH4++OH−→NH3+H2Ocap N cap H sub 4 raised to the positive power plus cap O cap H raised to the negative power right arrow cap N cap H sub 3 plus cap H sub 2 cap O . Dilution: The remains essentially unchanged because the ratio of stays the same. 3. Organic Chemistry: Isomerism What types of isomerism are possible when substituting one and one atom for two atoms in: Ethane ( C2H6cap C sub 2 cap H sub 6 ):
Structural Isomers: 1-bromo-1-chloroethane vs. 1-bromo-2-chloroethane.
Optical Isomers: 1-bromo-1-chloroethane has a chiral center and exists as a pair of enantiomers. Ethene ( C2H4cap C sub 2 cap H sub 4 ):
Structural Isomers: 1-bromo-1-chloroethene vs. 1-bromo-2-chloroethene.
Geometric Isomers: 1-bromo-2-chloroethene exists as cis and trans (or E and Z) isomers.
Detailed solutions and scoring guides for similar legacy problems can be found on resources like Scribd or Coach Coker's Chemistry.
For the 1972 AP Chemistry Free Response section, students were required to answer several comprehensive problems covering core chemical principles. Detailed worked solutions for the entire set can be found in the Adrian Dingle's AP FRQ Archive.
Below are key solutions and concepts for specific questions from that year: Acid-Base & Stoichiometry (Question 1) This problem involved a 5.00-gram mixture of KOHcap K cap O cap H K2CO3cap K sub 2 cap C cap O sub 3 KClcap K cap C l reacting with HClcap H cap C l Part (a): You must determine the percentage of K2CO3cap K sub 2 cap C cap O sub 3 by calculating the moles of CO2cap C cap O sub 2 gas produced ( ). Using the stoichiometry of
K2CO3+2HCl→2KCl+CO2+H2Ocap K sub 2 cap C cap O sub 3 plus 2 cap H cap C l right arrow 2 cap K cap C l plus cap C cap O sub 2 plus cap H sub 2 cap O , 0.0100 mol of CO2cap C cap O sub 2 corresponds to 1.38 g of K2CO3cap K sub 2 cap C cap O sub 3 , resulting in 27.7% K2CO3cap K sub 2 cap C cap O sub 3 .
Part (b): The remaining percentages are found by titrating excess HClcap H cap C l NaOHcap N a cap O cap H HClcap H cap C l HClcap H cap C l reacted with K2CO3cap K sub 2 cap C cap O sub 3 and excess HClcap H cap C l leaves the amount reacted with KOHcap K cap O cap H Organic Chemistry & Isomerism
The exam also tested the types of isomerism possible when substituting one atom into ethane ( C2H6cap C sub 2 cap H sub 6 ) and ethene ( C2H4cap C sub 2 cap H sub 4
Ethane: Potential for constitutional (structural) isomers like 1-bromo-1-chloroethane and 1-bromo-2-chloroethane.
Ethene: Includes geometric (cis/trans) isomers and structural isomers. Energy & Electrochemistry One question focused on calculating free energy ( ΔGcap delta cap G ) and enthalpy ( ΔHcap delta cap H ) using electrochemistry data. Key Formula: Calculation: For a specific redox reaction yielding ΔGcap delta cap G was determined to be
. By rearranging the free energy formula with entropy data, the ΔHcap delta cap H was calculated as . portion of the first question? AP FRQ WORKED ANSWER ARCHIVE
The 1972 AP Chemistry Free Response section is characterized by a high volume of questions and a significant emphasis on classical chemical calculations and descriptive chemistry, which distinguishes it from modern exam formats. Exam Structure & Format
In 1972, the Free Response section (Section II) was significantly more extensive than current versions: Duration: 110 minutes total.
Question Count: The exam featured 18 free-response questions in total. Modular Scoring: 1972 ap chemistry free response answers
Part A & B: Focused on core conceptual questions (15% and 20% of the section grade). Part C: Required choosing one out of two questions (15%).
Part D (Net Ionic Equations): Students had to complete five out of eight equations (15%). Historically, these were presented using chemical names rather than formulas, adding a layer of nomenclature difficulty.
Part E (Problems): Students chose four out of six quantitative problems (35%). Content Highlights
A review of released questions from 1972 reveals several recurring themes that remain central to chemistry but were tested with different nuances:
Acid-Base Chemistry: One prominent question involved a complex mixture of potassium hydroxide, potassium carbonate, and potassium chloride. Students had to use titration data (NaOH and HCl) and gas volume (CO₂) to calculate the percentage composition of the original sample.
Organic Chemistry: The exam tested isomerism by asking students to identify and draw structures for chlorinated and brominated substitutes of ethane and ethene. Equilibrium: Questions often utilized ammonia ( NH3cap N cap H sub 3 ) and ammonium ( NH4+cap N cap H sub 4 raised to the positive power
) buffer systems to test understanding of hydrogen ion concentration stability. Historical Comparison
Tools: Unlike modern exams where graphing calculators are standard, 1972 students were provided with log tables for their free-response calculations.
Question Presentation: Net ionic equations were numbered as individual questions (e.g., questions 4–12) rather than sub-parts of a single larger question, which is why the total question count appears so much higher than today's seven-question format.
Nomenclature: There was a heavier reliance on knowing chemical names by heart, as formulas were often omitted in the prompts.
For students looking to practice with these archival materials, resources like the Adrian Dingle AP FRQ Archive and ChemmyBear provide historical compilations of these questions and their solutions. AP Chemistry Acid-Base FRQ Solutions | PDF - Scribd
A comprehensive guide to the 1972 AP Chemistry Free Response section requires a bit of historical context. Please note: The College Board does not officially distribute scanned copies of the 1972 exam for public download, and specific numerical values (like the exact mass of a sample or a specific heat capacity) can sometimes vary slightly between different third-party archives.
However, the concepts tested in 1972 remain fundamental to modern AP Chemistry. Below is a reconstructed guide based on the archived curriculum and common problems referenced in AP history.
This guide breaks down the typical question types found on the 1972 exam, provides the conceptual solutions, and explains the reasoning.
Searching for the 1972 AP Chemistry free response answers isn’t just about cheating on a half-century-old test. It serves three modern purposes:
Typical Prompt: Calculate the pH of a 0.100 M solution of acetic acid (Ka = 1.8 × 10^-5). Show all work using the approximation method.
Typical Prompt (1972): A solution contains Ag⁺, Pb²⁺, and Zn²⁺. Describe a procedure to separate and confirm each ion using 1M HCl, 1M H2SO4, and 1M NH3(aq). Write net ionic equations.
If you are comparing your work to a 1972 answer key, understand the scoring philosophy:
Let’s reconstruct a typical question from that year (paraphrased from actual historical prompts):
Question 3: "A 0.500 gram sample of a pure metal, X, reacts completely with excess hydrochloric acid to produce 280. mL of hydrogen gas collected over water at 25.0°C and a total pressure of 740. mm Hg. The vapor pressure of water at 25.0°C is 23.8 mm Hg. Determine the equivalent weight of the metal."
Why this question was cruel: Modern students would use a P-table and an ICE chart. 1972 students had to:
The Answer (1972 Style):
Typical Prompts: Write balanced net ionic equations for the following:
The 1972 AP Chemistry Exam is often cited by educators as a "classic" era of the program. It was a time when the curriculum leaned heavily into rigorous physical chemistry, equilibrium, and thermodynamics.
If you are a student or a teacher looking to deconstruct the 1972 AP Chemistry Free Response section, you’ll find that while the formatting has changed, the core principles of chemical logic remain identical to today's standards. Overview of the 1972 Free Response Section
In 1972, the Free Response (now known as Section II) required students to demonstrate deep conceptual knowledge without the aid of modern graphing calculators. The focus was on "first principles"—the ability to derive relationships and explain why a reaction occurs, rather than just plugging numbers into a formula. Key Questions and Conceptual Answers 1. Equilibrium and Solubility Product ( Kspcap K sub s p end-sub
A major focus in '72 was the behavior of sparingly soluble salts.
The Concept: Students were often asked to calculate the molar solubility of a compound (like silver chloride or lead iodide) in both pure water and in the presence of a common ion. The Answer Key Logic: Set up the dissociation equation:
AB(s)⇌A+(aq)+B−(aq)cap A cap B open paren s close paren is in equilibrium with cap A raised to the positive power open paren a q close paren plus cap B raised to the negative power open paren a q close paren Apply the Law of Mass Action:
Common Ion Effect: Remember that the solubility decreases when a common ion is added because the equilibrium shifts to the left (Le Chatelier’s Principle). 2. Gas Laws and Kinetic Molecular Theory
Gas behavior was a cornerstone of the '72 exam, specifically focusing on the Ideal Gas Law ( ) and Graham’s Law of Effusion. The 1972 AP Chemistry exam reflects a period
The Concept: Comparing the rates of two gases escaping through a pinhole.
The Answer Key Logic: The rate of effusion is inversely proportional to the square root of the molar mass.
The 1972 exam expected students to explicitly state that at a constant temperature, all gases have the same average kinetic energy, but different velocities. 3. Thermodynamics: Entropy and Enthalpy
1972 was a year that tested the "spontaneity" of reactions before the term "Gibbs Free Energy" was as ubiquitous in high school classrooms as it is today. The Concept: Predicting the sign of ΔScap delta cap S (entropy) and ΔHcap delta cap H (enthalpy). The Answer Key Logic: If a gas is produced from solids, ΔScap delta cap S is positive (disorder increases). If the reaction vessel gets hot, ΔHcap delta cap H is negative (exothermic).
A reaction is spontaneous if it leads to a lower energy state and higher disorder. 4. Atomic Structure and Periodic Trends
Questions often asked for the electron configuration of transition metals or the explanation of ionization energy trends.
The Concept: Why does Ionization Energy increase across a period?
The Answer Key Logic: It’s all about Effective Nuclear Charge ( Zeffcap Z sub e f f end-sub
). As you move across a period, you add protons without adding new shielding shells, pulling the electrons closer and making them harder to remove. Why Study the 1972 Exam Today?
You might wonder why a 50-year-old exam matters. The reason is complexity. Modern AP Chemistry exams sometimes use "guided" questions (parts a through f) that lead you to the answer. The 1972 questions were often "broad," requiring you to organize a multi-step scientific argument from scratch. Benefits of practicing with '72 prompts:
Mastering Stoichiometry: The math is "cleaner" but requires a better grasp of mole ratios.
Conceptual Clarity: Without the distraction of complex data sets, you focus on the chemistry.
Historical Context: Seeing how the AP program has evolved helps you identify the "Big Ideas" that College Board has valued for half a century. Tips for Solving Legacy AP Problems
Show Your Work: Even in 1972, partial credit was king. Always write out your units.
Sig Figs: While the 1972 graders were slightly more lenient than today's, maintaining proper significant figures shows mathematical maturity.
Lewis Structures: If asked for a shape, always draw the Lewis diagram first to determine the VSEPR geometry. Conclusion
The 1972 AP Chemistry Free Response answers reveal a rigorous standard for scientific literacy. Whether you are prepping for the upcoming May exam or just a fan of chemical history, these legacy problems are excellent tools for sharpening your analytical skills.
The 1972 AP Chemistry exam remains a fascinating benchmark in the history of science education, reflecting a period when the curriculum emphasized classical analytical techniques, descriptive chemistry, and complex structural logic. Analyzing the free-response questions (FRQs) and their answers provides a masterclass in how student expectations have evolved from the "calculator-light" era to the data-heavy modern exam. The Rigor of 1970s Analytical Chemistry
The 1972 exam was notably lengthy, featuring 18 total free-response questions compared to the 7 questions found on today’s exams. While the modern exam focuses heavily on particle-level representations and experimental design, the 1972 answers reveal a deep focus on stoichiometric precision and complex inorganic coordination.
Quantitative Stoichiometry: One of the hallmark questions involved a complex mixture of potassium hydroxide, potassium carbonate, and potassium chloride. The answer required a multi-step titration analysis, where students had to account for gas evolution ( CO2cap C cap O sub 2 ) and excess HClcap H cap C l
neutralization to determine the mass percentages of three different salts in a single dry sample.
Coordination Chemistry: Question 1 featured the transition metal complex
. The answers required students to draw three different structural isomers based on experimental data like silver nitrate precipitation and electrical conductivity. This type of "puzzle-solving" chemistry, which links physical observations directly to molecular architecture, was a cornerstone of the 1972 test. Thermodynamics and Organic Foundations
The 1972 FRQs also tackled foundational concepts in energy and structural isomerism that remain core to the AP curriculum today, though often framed with different levels of mathematical complexity.
Energy and Electrochemistry: Students were tasked with calculating changes in Gibbs Free Energy ( ΔGcap delta cap G ) and enthalpy ( ΔHcap delta cap H
) by flipping reduction potentials and reconciling units (switching between joules for entropy and kilojoules for enthalpy). These answers highlighted the perennial student challenge of "unit trap" management that still plagues modern test-takers.
Organic Isomerism: The exam pushed students on their knowledge of isomers for ethane and ethene derivatives. Unlike modern exams, which might ask for the effect of a functional group on boiling point, the 1972 answers required hand-drawing every possible geometric and structural isomer resulting from substituting chlorine and bromine atoms into hydrocarbons. Comparison: 1972 vs. The Modern Exam
Looking back at the 1972 solutions, there is a distinct lack of the "justify your answer" prompts that dominate today’s scoring guidelines. In 1972, the "answer" was often the numerical result or a correct structure; today, the answer is the reasoning behind that result. AP Chemistry Exam Questions - AP Central - College Board
The 1972 AP Chemistry Free-Response Questions cover a wide range of fundamental chemistry concepts, including thermodynamics, kinetics, and acid-base equilibria. You can find a complete set of worked answers for all nine questions on Adrian Dingle’s Chemistry Pages.
Below are solutions to two prominent problems from that exam: 1. Thermodynamics and Electrochemistry
Question: A 1972 problem involves calculating energy changes for a specific electrochemical reaction. Calculate Standard Cell Potential ( E∘cap E raised to the composed with power Why Study These Answers
):By analyzing the half-reactions and determining which is flipped (oxidation vs. reduction), you combine the potentials to find the overall E∘cap E raised to the composed with power
Ecell∘=+0.3 Vcap E sub c e l l end-sub raised to the composed with power equals positive 0.3 V Determine Gibbs Free Energy ( ΔG∘cap delta cap G raised to the composed with power
):Use the relationship between cell potential and free energy:
ΔG∘=−nFE∘cap delta cap G raised to the composed with power equals negative n cap F cap E raised to the composed with power For this specific reaction, the result is:
ΔG∘=57.9 kJ/molcap delta cap G raised to the composed with power equals 57.9 kJ/mol Solve for Enthalpy ( ΔH∘cap delta cap H raised to the composed with power ):Rearrange the Gibbs free energy formula ( ) to solve for enthalpy.
ΔH∘=-73.5 kJ/molcap delta cap H raised to the composed with power equals negative 73.5 kJ/mol 2. Acid-Base Equilibria Question: A dry mixture of containing KOHcap K cap O cap H K2CO3cap K sub 2 cap C cap O sub 3 KClcap K cap C l is reacted with Determine Limiting Reactants:Calculate the moles of HClcap H cap C l available (
) and compare it to the molar amounts of the basic components ( KOHcap K cap O cap H K2CO3cap K sub 2 cap C cap O sub 3 ) to find the excess or limiting reagent. Analyze Buffer Effects:When a strong base ( OH−cap O cap H raised to the negative power ) is added to a system containing NH4+cap N cap H sub 4 raised to the positive power , the ammonium ion reacts to keep the H+cap H raised to the positive power
concentration relatively stable, demonstrating the principles of a buffer system. Results Summary Gibbs Free Energy: Enthalpy Change: Standard Cell Potential: AP FRQ WORKED ANSWER ARCHIVE
The 1972 AP Chemistry Exam: A Time Capsule of Chemical Rigor
The 1972 AP Chemistry exam stands as a fascinating benchmark in the history of American science education. Taken by a much smaller, highly specialized group of students compared to today’s massive cohorts, the exam provides a window into what was considered "college-level" mastery five decades ago. The Structure of the 1972 FRQ
In 1972, the Free Response section was a marathon of chemistry. Unlike the modern format of 3 long and 4 short questions, the 1972 exam featured a high volume of tasks packed into 110 minutes: Part A & B: Core required questions (35% of total). A choice between two major conceptual problems (15%).
The "Net Ionic" gauntlet—students had to choose 5 out of 8 equations to write from names alone (15%).
Quantitative problems where students chose 4 out of 6 (35%). Notable Questions and Solutions 1. The Stoichiometry Puzzle (Acid-Base/Gases)
One of the most cited problems from 1972 involved a complex dry mixture of potassium hydroxide ( cap K cap O cap H ), potassium carbonate ( cap K sub 2 cap C cap O sub 3 ), and potassium chloride ( cap K cap C l The Challenge: A 5.00g sample is reacted with 0.100L of . Students had to: Calculate the % of cap K sub 2 cap C cap O sub 3 based on 249mL of cap C cap O sub 2 gas produced. Use back-titration data with cap N a cap O cap H to find the percentages of the remaining components. The Solution Path: Use the Ideal Gas Law ( ) to find the moles of cap C cap O sub 2 . At 740 torr and 22°C, Relate moles of cap C cap O sub 2 cap K sub 2 cap C cap O sub 3 (1:1 ratio). Calculation: cap K sub 2 cap C cap O sub 3 2. Transition Metal Coordination
Another problem asked students to differentiate between isomers of and octahedral complexes like The Key Logic:
Students had to understand that the number of "immediately precipitatable" chlorides (using cap A g cap N cap O sub 3
) depended on which chloride ions were outside the coordination sphere vs. inside. 1972 vs. Today: What has changed?
Looking at the 1972 answers reveals several stark differences in how chemistry is tested: No Calculators:
In 1972, students relied on log tables and manual arithmetic. Modern exams allow graphing calculators, but the questions have shifted to focus more on conceptual "why" rather than just "how much". Memorization vs. Inquiry:
The 1972 exam required heavy memorization of solubility rules and complex ion colors. Today’s AP Chemistry Exam (hosted on AP Central ) emphasizes "Big Ideas" and laboratory design. The "Net Ionic" Era:
Writing net ionic equations from scratch (e.g., "solutions of silver nitrate and sodium phosphate are mixed") was a standalone, high-stakes skill. Today, this is typically integrated into larger multipart questions. Where to Find Old Exams
For those looking to practice with these "legacy" problems, resources like Adrian Dingle’s Chemistry Pages
maintain archives of worked answers dating back to the early 70s. of the 1972 gas law problem?
Problem: Given the following data for the reaction: $C(s) + H_2O(g) \rightarrow CO(g) + H_2(g)$
(a) Calculate $\Delta H^\circ$ for the reaction. (b) Calculate $\Delta S^\circ$ for the reaction. (c) Calculate $\Delta G^\circ$ at $298\text K$.
Goal: Find ΔH for: 2C(s) + 3H2(g) + ½O2(g) → C2H5OH(l)
Step 1: Reverse equation (3) (since we want ethanol as product, not reactant). [ 2CO2 + 3H2O → C2H5OH + 3O2 \quad ΔH = +1367 , \textkJ ]
Step 2: Multiply equation (1) by 2 (to get 2C on left). [ 2C + 2O2 → 2CO2 \quad ΔH = -787.0 , \textkJ ]
Step 3: Multiply equation (2) by 3 (to get 3H2 on left). [ 3H2 + 1.5O2 → 3H2O \quad ΔH = -857.4 , \textkJ ]
Step 4: Add all three equations.
Step 5: Sum ΔH values. [ ΔH = (+1367) + (-787.0) + (-857.4) ] [ ΔH = 1367 - 1644.4 = -277.4 , \textkJ/mol ]
Final answer: ΔH°f (C2H5OH, l) = -277.4 kJ/mol
(Modern value is ~-277.7 kJ/mol – remarkably accurate for 1972 data.)