A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Empirical and Molecular Formulae — mark scheme explained
The short answer
In chemistry, understanding the composition of compounds is fundamental. Two key concepts in this area are empirical formula and molecular formula. These formulas provide different levels of detail about the elements present in a compound and their ratios.
The question
A compound is found to contain 27.3% carbon and 72.7% oxygen by mass. Calculate its empirical formula. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Convert percentages to grams (assume a 100 g sample).
- S2
Carbon = 27.3 g, Oxygen = 72.7 g
- S3
Step 2: Convert grams to moles using the molar masses of carbon and oxygen.
- S4
Moles of Carbon = 27.3 / 12 ≈ 2.28 moles
- S5
Moles of Oxygen = 72.7 / 16 ≈ 4.54 moles
- S6
Step 3: Find the simplest ratio by dividing each mole value by the smallest number of moles.
- S7
Ratio of Carbon to Oxygen = 2.28 / 2.28 : 4.54 / 2.28 ≈ 1 : 2
- S8
Step 4: The values are already whole numbers, so no further scaling is needed.
- S9
Ratio = 1 : 2
- S10
Step 5: Write the empirical formula.
- S11
Empirical Formula = CO 2
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Convert percentages to grams (assume a 100 g sample).
- S2
Carbon = 27.3 g, Oxygen = 72.7 g
- S3
Step 2: Convert grams to moles using the molar masses of carbon and oxygen.
- S4
Moles of Carbon = 27.3 / 12 ≈ 2.28 moles
- S5
Moles of Oxygen = 72.7 / 16 ≈ 4.54 moles
- S6
Step 3: Find the simplest ratio by dividing each mole value by the smallest number of moles.
- S7
Ratio of Carbon to Oxygen = 2.28 / 2.28 : 4.54 / 2.28 ≈ 1 : 2
- S8
Step 4: The values are already whole numbers, so no further scaling is needed.
- S9
Ratio = 1 : 2
- S10
Step 5: Write the empirical formula.
- S11
Empirical Formula = CO 2
Final answer: CO 2
Common mistakes
- Forgetting to convert percentages to grams when calculating empirical formula. — Always assume a 100 g sample and use the given percentages as masses in grams.
- Making arithmetic errors when converting grams to moles. — Double-check your calculations and use a calculator if necessary.
- Not simplifying ratios correctly when finding the empirical formula. — Always ensure that the ratio is in its simplest form and use multiplication if needed to achieve whole numbers.
- Using the wrong molar mass for the empirical formula when calculating the molecular formula. — Double-check your calculation of the empirical formula's molar mass and ensure it is accurate.
- Forgetting to multiply each subscript in the empirical formula by the ratio when calculating the molecular formula. — Always multiply each subscript in the empirical formula by the calculated ratio and double-check your final answer.
- Misinterpreting experimental data when calculating empirical or molecular formulas. — Practice analyzing different types of experimental data and focus on extracting the necessary values for your calculations.
Where the marks go
- Full worked solution (all marking points)4 marks