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

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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 standardized NaOH solution may differ from the expected value due to several reasons. Firstly, the purity of the NaOH pellets could be less than 100%, meaning some of the mass used in calculations does not contribute to the NaOH in solution. Secondly, the NaOH may have absorbed moisture from the air, leading to an increase in weight without a corresponding increase in the amount of NaOH. Lastly, inaccuracies in measuring the volume of the solution during preparation could result in a different concentration than intended.

Step 2

Determine the concentration of citric acid in the lemon cordial.

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Answer

To find the concentration of citric acid in the lemon cordial, we first calculate the molarity of the NaOH solution. The molarity is calculated as follows:

  1. Calculate moles of HCl used:

    nHCl=CHCl×VHCln_{HCl} = C_{HCl} \times V_{HCl}

    where CHCl=0.1011 mol L−1C_{HCl} = 0.1011 \, \text{mol L}^{-1} and VHCl=0.0250 L.V_{HCl} = 0.0250 \, \text{L}. So,

    nHCl=0.1011 mol L−1×0.0250 L=0.0025275 moln_{HCl} = 0.1011 \, \text{mol L}^{-1} \times 0.0250 \, \text{L} = 0.0025275 \, \text{mol}

  2. Moles of NaOH used:

Since it reacts in a 1:1 ratio:

nNaOH=nHCl=0.0025275 moln_{NaOH} = n_{HCl} = 0.0025275 \, \text{mol}

  1. Calculate concentration of diluted citric acid solution:

The average volume of NaOH from the titrations:

VNaOH=26.55+27.25+27.30+27.204=27.00 mL=0.0270 LV_{NaOH} = \frac{26.55 + 27.25 + 27.30 + 27.20}{4} = 27.00 \, \text{mL} = 0.0270 \, \text{L}

Concentration of NaOH is:

CNaOH=0.0025275 mol0.0270 L=0.0935 mol L−1C_{NaOH} = \frac{0.0025275 \, \text{mol}}{0.0270 \, \text{L}} = 0.0935 \, \text{mol L}^{-1}

  1. Finally, to find the concentration of citric acid:

    The original citric acid concentration can be calculated using:

    Ccitric=CNaOH×VNaOH×dilution factorVdilutedC_{citric} = \frac{C_{NaOH} \times V_{NaOH} \times dilution \ factor}{V_{diluted}}

    Plugging in values gives:

    Ccitric=0.0935 mol L−1×0.0250 L×200.0500 L=0.0935 mol L−1×500=4.675 mol L−1C_{citric} = \frac{0.0935 \, \text{mol L}^{-1} \times 0.0250 \, \text{L} \times 20}{0.0500 \, \text{L}} = 0.0935 \, \text{mol L}^{-1} \times 500 = 4.675 \, \text{mol L}^{-1}

    Therefore, the concentration of citric acid in the lemon cordial is approximately 4.675 mol L⁻¹.

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