QUESTION 148 (5 marks)
A synthesis must produce 2.50 g of ethyl butanoate at an expected isolated yield of 75.0%. Butanoic acid and ethanol react in a 1:1 ratio. Ethanol is supplied in excess. Use M(butanoic acid) = 88.1 g mol⁻¹ and M(ethyl butanoate) = 116.2 g mol⁻¹.
(a) Calculate the minimum mass of butanoic acid required for this yield model. [3 marks]
(b) Explain how excess ethanol can increase the equilibrium ester yield without changing Kc at fixed temperature. [2 marks]
Practice marking scheme
Answer
Minimum butanoic acid mass = 2.53 g. Excess ethanol shifts the esterification equilibrium towards products; Kc remains unchanged at fixed temperature.
Working
The theoretical ester mass must be g. The theoretical ester amount is mol, requiring the same amount of butanoic acid. Acid mass is g. Extra ethanol is a reactant, lowering Qc relative to Kc and promoting net ester formation until equilibrium is re-established. It changes composition, not the temperature-dependent constant.
Marking criteria
- Divides target ester mass by 0.750 to obtain the theoretical mass. [1 mark]
- Uses the 1:1 ratio to obtain required acid amount ≈ 0.02869 mol. [1 mark]
- Calculates butanoic acid mass ≈ 2.53 g. [1 mark]
- Explains that excess ethanol promotes the product side of the reversible reaction. [1 mark]
- States Kc remains unchanged at fixed temperature. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.