A chemist analysed aspirin tablets for quality control. The initial step of the analysis was the standardisation of a NaOH solution. Three 25.00 mL samples of a 0.1034 mol L⁻¹ solution of standardised HCl were titrated with the NaOH solution. The average volume required for neutralisation was 25.75 mL. (a) Calculate the molarity of the NaOH solution. Three flasks were prepared each containing a mixture of 25 mL of water and 10 mL of ethanol. An aspirin tablet was dissolved in each flask. The aspirin in each solution was titrated with the standardised NaOH solution according to the following equation: C₉H₈O₄(aq) + NaOH(aq) → C₉H₇O₂Na(aq) + H₂O(l) The following titration results were obtained. Tablet Volume (mL) 1 16.60 2 16.50 3 16.55 (b) (i) Calculate the average mass (mg) of aspirin per tablet. (ii) Why was it necessary to include the ethanol in the mixture?

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A chemist analysed aspirin tablets for quality control - HSC - SSCE Chemistry - Question 30 - 2012 - Paper 1

Question 30

A chemist analysed aspirin tablets for quality control. The initial step of the analysis was the standardisation of a NaOH solution. Three 25.00 mL samples of a 0.10... show full transcript

Worked Solution & Example Answer:A chemist analysed aspirin tablets for quality control - HSC - SSCE Chemistry - Question 30 - 2012 - Paper 1

Step 1

Calculate the molarity of the NaOH solution.

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Answer

To calculate the molarity of the NaOH solution, we first find the number of moles of HCl used in the titration. The reaction between HCl and NaOH is given by:

$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$

The moles of HCl can be calculated using the formula:

$\text{Moles} = \text{Concentration (mol/L)} \times \text{Volume (L)}$

For HCl:

$\text{Moles HCl} = 0.1034 \text{ mol L}^{-1} \times 0.02500 \text{ L} = 0.002585 \text{ moles}$

Since the reaction is a 1:1 ratio, the moles of NaOH is also 0.002585. Now, we can use the volume of NaOH used for neutralisation:

$\text{Volume of NaOH} = 25.75 ext{ mL} = 0.02575 ext{ L}$

The molarity of NaOH can be calculated as follows:

$\text{Molarity (NaOH)} = \frac{\text{Moles NaOH}}{\text{Volume NaOH (L)}} = \frac{0.002585}{0.02575} = 0.1005 ext{ mol L}^{-1}$

Step 2

Calculate the average mass (mg) of aspirin per tablet.

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Answer

To find the average mass of aspirin, we first need to determine the moles of NaOH used in each titration. Using the average volume from the three titrations:

$\text{Average Volume} = \frac{16.60 + 16.50 + 16.55}{3} = 16.55 ext{ mL} = 0.01655 ext{ L}$

The moles of NaOH used for the average titration:

$\text{Moles NaOH} = M \times V = 0.1005 ext{ mol L}^{-1} \times 0.01655 ext{ L} = 0.00166 ext{ moles}$

From the balanced equation, we know it takes 1 mole of NaOH to neutralize 1 mole of aspirin. Thus, the moles of aspirin in each flask is also 0.00166 moles. The molar mass of aspirin (C₉H₈O₄) is 180.16 g/mol. Therefore, the mass of aspirin is:

$\text{Mass}_{\text{aspirin}} = \text{moles} \times \text{molar mass} = 0.00166 \text{ moles} \times 180.16 \frac{g}{mol} = 0.299 g = 299 mg$

Thus, the average mass of aspirin per tablet is approximately 299 mg.

Step 3

Why was it necessary to include the ethanol in the mixture?

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Answer

Ethanol serves several purposes in the dissolution of aspirin tablets. Primarily, aspirin is more soluble in ethanol than in water, which helps ensure that the aspirin completely dissolves and is available for the titration reaction.

Moreover, the presence of ethanol can enhance the accuracy of the titration, as it maintains a consistent environment for the chemical reactions to take place. Ethanol may also help in stabilizing the solution and preventing potential hydrolysis of aspirin, thus ensuring more reliable outcomes in the experiment.

A chemist analysed aspirin tablets for quality control - HSC - SSCE Chemistry - Question 30 - 2012 - Paper 1

Worked Solution & Example Answer:A chemist analysed aspirin tablets for quality control - HSC - SSCE Chemistry - Question 30 - 2012 - Paper 1

Calculate the molarity of the NaOH solution.

Calculate the average mass (mg) of aspirin per tablet.

Why was it necessary to include the ethanol in the mixture?

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