A technician has 350 cm3\mathrm{cm^{3}}cm3 of potassium nitrate, KNO3\mathrm{KNO_3}KNO3, solution with a concentration of 0.240 mol/dm3\mathrm{mol/dm^{3}}mol/dm3.
Some water evaporates until the volume of the solution is 280 cm3\mathrm{cm^{3}}cm3. No potassium nitrate is lost.
Relative atomic mass (ArA_rAr): K = 39.1, N = 14.0, O = 16.0
Calculate the concentration of the solution after evaporation in g/dm3\mathrm{g/dm^{3}}g/dm3.
Concentration = ...... g/dm3\mathrm{g/dm^{3}}g/dm3
A sample of the solution after evaporation must contain 5.05 g of potassium nitrate.
Calculate the volume of this sample in cm3\mathrm{cm^{3}}cm3.
Volume = ...... cm3\mathrm{cm^{3}}cm3
The technician then leaves the 280 cm3\mathrm{cm^{3}}cm3 of solution to stand, and a further 70.0 cm3\mathrm{cm^{3}}cm3 of water evaporates. No potassium nitrate is lost.
A student says:
“The concentration will increase by the same amount as it did during the first evaporation.”
Explain why the student is wrong.
152 exam-style questions on OCR GCSE Chemistry 5.1 Monitoring chemical reactions, covering 5.1.1 The technique of titration, 5.1.2 Theoretical mass, percentage yield and atom economy, 5.1.3 Concentration of a solution in mol/dm3, 5.1.4 Concentration calculations and titration results, 5.1.5 Molar gas volume and volumes of gases in reactions, and 5.1.6 Choosing a reaction pathway. Each one has a worked solution and a mark scheme showing where the marks go.