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Amount of substance

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Question 4

A student uses the reaction of aspirin with cold NaOH(aq)\text{NaOH(aq)}NaOH(aq) to determine the mass of aspirin in one tablet.

In this reaction, 1 mol1\text{ mol}1 mol of aspirin reacts with 1 mol1\text{ mol}1 mol of cold NaOH(aq)\text{NaOH(aq)}NaOH(aq).

The student’s method is outlined below.

  • Step 1: The student reacts two aspirin tablets with 50.0 cm350.0\text{ cm}^350.0 cm3 of 0.500 mol dm−3 NaOH(aq)0.500\text{ mol dm}^{-3}\ \text{NaOH(aq)}0.500 mol dm−3 NaOH(aq). The NaOH\text{NaOH}NaOH is in excess. A colourless solution forms.
  • Step 2: The colourless solution from Step 1 is made up to 250.0 cm3250.0\text{ cm}^3250.0 cm3 with distilled water.
  • Step 3: A 25.00 cm325.00\text{ cm}^325.00 cm3 sample of the diluted solution from Step 2 is titrated with 0.100 mol dm−3 HCl(aq)0.100\text{ mol dm}^{-3}\ \text{HCl(aq)}0.100 mol dm−3 HCl(aq) in the burette.

The HCl(aq)\text{HCl(aq)}HCl(aq) reacts with excess NaOH(aq)\text{NaOH(aq)}NaOH(aq) that remains in Step 1:

NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l) \text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} NaOH(aq)+HCl(aq)→NaCl(aq)+H2​O(l)

The student repeats the titration to obtain concordant (consistent) titres.

Titration results The trial titre (Titre 1) is not used because it is not concordant. The burette readings have been read to the nearest 0.05 cm30.05\text{ cm}^30.05 cm3:

123
Final reading / cm3\text{cm}^3cm321.5041.8022.30
Initial reading / cm3\text{cm}^3cm30.0021.502.10

Determine the mass, in mg\text{mg}mg, of aspirin in one aspirin tablet. (Molar mass of aspirin, Mr=180 g mol−1M_r = 180\text{ g mol}^{-1}Mr​=180 g mol−1).

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Amount of substance Questions

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