A manufacturer makes lemon cordial by mixing flavouring, sugar syrup and citric acid. The concentration of the citric acid is determined by titration with NaOH. The sodium hydroxide solution is prepared by dissolving 4.000 g of NaOH pellets in water to give 1,000.0 mL of solution. This solution is standardised by titrating 25.00 mL with a 0.1011 mol L⁻¹ standardised solution of HCl. The average titration volume is found to be 24.10 mL. To analyse the lemon cordial 50.00 mL of the cordial is diluted to 500.0 mL. Then 25.00 mL of the diluted solution is titrated with the NaOH solution to the phenolphthalein endpoint. The following data were collected during one of the analysis runs of the lemon cordial. Titration #1 volume 26.55 mL Titration #2 volume 27.25 mL Titration #3 volume 27.30 mL Titration #4 volume 27.20 mL (a) Why is the calculated concentration of the standardised NaOH solution different from the concentration calculated using the mass given, assuming no human error occurred? (b) Determine the concentration of citric acid in the lemon cordial.

SSCE HSC Chemistry Brønsted–Lowry Theory: A manufacturer makes lemon cordi

A manufacturer makes lemon cordial by mixing flavouring, sugar syrup and citric acid - HSC - SSCE Chemistry - Question 26 - 2011 - Paper 1

Question 26

A manufacturer makes lemon cordial by mixing flavouring, sugar syrup and citric acid. The concentration of the citric acid is determined by titration with NaOH. The... show full transcript

Worked Solution & Example Answer:A manufacturer makes lemon cordial by mixing flavouring, sugar syrup and citric acid - HSC - SSCE Chemistry - Question 26 - 2011 - Paper 1

Step 1

Why is the calculated concentration of the standardised NaOH solution different from the concentration calculated using the mass given, assuming no human error occurred?

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Answer

The calculated concentration of the standardised NaOH solution may differ from the concentration calculated using the mass given due to various factors such as:

Purity of NaOH: The NaOH pellets used might contain impurities or moisture, which would affect the actual mass of NaOH available for the reaction, leading to a concentration that is less than theoretically expected.
Dissolution Issues: Incomplete dissolution of NaOH pellets may also result in a lower concentration than anticipated, as the total amount of solute that effectively contributes to the solution could be less than the mass calculated.
Measurement Errors: Any errors in measuring the volume of the solution or the mass of the NaOH could lead to discrepancies in the calculated concentration.
Evaporation: If any water evaporated during the preparation or while measuring concentrations, it could result in a higher-than-expected concentration calculation.

Step 2

Determine the concentration of citric acid in the lemon cordial.

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Answer

To determine the concentration of citric acid in the lemon cordial, perform the following calculations:

Calculate the average volume of NaOH used in titrations:

$ext{Average volume} = rac{26.55 + 27.25 + 27.30 + 27.20}{4} = 27.05 \text{ mL}$

Calculate the moles of NaOH used:

$ext{Moles of NaOH} = \text{Concentration} \times \text{Volume (in L)}$

$= rac{0.1011 \text{ mol L}^{-1}}{1000} \times 0.02705 \approx 2.73 \times 10^{-3} \text { moles}$

Relate moles of NaOH to moles of citric acid: Citric acid (C₆H₈O₇) reacts with NaOH in a 1:3 ratio (since it has 3 acidic protons). Hence, the moles of citric acid are:

$ext{Moles of citric acid} = \frac{\text{Moles of NaOH}}{3} = \frac{2.73 \times 10^{-3}}{3} \approx 9.10 \times 10^{-4} \text{ moles}$

Find the concentration of citric acid in the diluted solution:

$\text{Concentration of citric acid (diluted)} = \frac{\text{Moles of citric acid}}{\text{Volume (in L)}} = \frac{9.10 \times 10^{-4}}{0.025} \approx 0.0364 \text{ mol L}^{-1}$

Calculate the concentration in the original solution: The original lemon cordial was 50.00 mL of a solution diluted to 500.0 mL, hence:

$\text{Concentration of citric acid (original)} = \text{Concentration (diluted)} \times 10 = 0.0364 \times 10 \approx 0.364 \text{ mol L}^{-1}$

A manufacturer makes lemon cordial by mixing flavouring, sugar syrup and citric acid - HSC - SSCE Chemistry - Question 26 - 2011 - Paper 1

Worked Solution & Example Answer:A manufacturer makes lemon cordial by mixing flavouring, sugar syrup and citric acid - HSC - SSCE Chemistry - Question 26 - 2011 - Paper 1

Why is the calculated concentration of the standardised NaOH solution different from the concentration calculated using the mass given, assuming no human error occurred?

Determine the concentration of citric acid in the lemon cordial.

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