Skip to content

Course home

Sign up

Atoms and reactions

EasyMediumHard
123456
Question 3

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

In this reaction, 1 mol1\text{ mol}1 mol of ibuprofen 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 three ibuprofen tablets with 75.0 cm375.0\text{ cm}^375.0 cm3 of 0.400 mol dm−3 NaOH(aq)0.400\text{ mol dm}^{-3}\ \text{NaOH(aq)}0.400 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 500.0 cm3500.0\text{ cm}^3500.0 cm3 with distilled water in a volumetric flask.
  • Step 3: A 50.00 cm350.00\text{ cm}^350.00 cm3 sample of the diluted solution from Step 2\text{Step 2}Step 2 is titrated with 0.120 mol dm−3 HCl(aq)0.120\text{ mol dm}^{-3}\ \text{HCl(aq)}0.120 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}^3cm324.1046.8523.60
Initial reading / cm3\text{cm}^3cm30.5024.301.15

Determine the mass, in mg\text{mg}mg, of ibuprofen in one tablet. (Molar mass of ibuprofen, Mr=206 g mol−1M_r = 206\text{ g mol}^{-1}Mr​=206 g mol−1).

[6]
Markscheme

Atoms and reactions Questions

  1. A Level
  2. /Chemistry
  3. /Atoms and reactions

199 exam-style questions on OCR A A Level Chemistry Atoms and reactions, covering Atomic structure and isotopes, Compounds, formulae and equations, Amount of substance, Acids, and Redox. Each one has a worked solution and a mark scheme showing where the marks go.

Question bank