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Complete combustion of 0.0100 mol of an organic acid produced 0.0200 mol of carbon dioxide - AQA - A-Level Chemistry - Question 7 - 2022 - Paper 3
Question 7
Complete combustion of 0.0100 mol of an organic acid produced 0.0200 mol of carbon dioxide. The same amount of the acid required 20 cm³ of 1.00 mol dm⁻³ NaOH (aq) f... show full transcript
Worked Solution & Example Answer:Complete combustion of 0.0100 mol of an organic acid produced 0.0200 mol of carbon dioxide - AQA - A-Level Chemistry - Question 7 - 2022 - Paper 3
Step 1
Calculate the moles of acid
Answer
The balanced equation for the combustion of an organic acid can be represented as:
ightarrow n ext{CO}_2 + p ext{H}_2 ext{O}$$ From the question, we know that 0.0100 mol of acid produces 0.0200 mol of carbon dioxide. This indicates that every 1 mole of the acid produces 2 moles of carbon dioxide. Therefore, the acid must contain 1 carbon atom at least. This suggests that the formula of the organic acid can be simplified to $C_xH_yCOOH$, where $x$ must equal 1.Step 2
Determine the equivalence of acid and NaOH
Answer
The volume of NaOH required for neutralisation is given as 20 cm³ of 1.00 mol/dm³. To convert this, we first calculate the number of moles of NaOH used:
ext{Moles of NaOH} = ext{Concentration} imes ext{Volume} = 1.00 ext{ mol/dm}^3 imes rac{20 ext{ cm}^3}{1000} = 0.020 ext{ mol}
Since the acid reacts with NaOH in a 1:1 ratio, the organic acid must also have been 0.020 mol, confirming the stoichiometry.
Step 3
Identify the possible formula of the acid
Answer
Given that the organic acid reacts with NaOH in a 1:1 ratio and produces 2 moles of CO₂ from the combustion of 0.010 mol, it can be inferred that the acid is likely to be a dicarboxylic acid. Thus, the potential candidates among the provided options are:
- A) HCOOH (formic acid, a monoacidic)
- B) CH₃COOH (acetic acid, also monoacidic)
- C) HOOCCH₂COOH (malonic acid, dicarboxylic)
- D) HOOC(CH₂)₂COOH (succinic acid, dicarboxylic)
The correct answer would be C) HOOCCH₂COOH, as it matches the criteria of both the molar ratios and the types of compounds formed during combustion.