WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

 1.3 Chemical calculations

Test yourself 1. Calculate the mass of gold that contains the same number of atoms as there are molecules in 5.75 g of nitrogen dioxide, NO 2 . [1] 2. Silicon has three stable isotopes 28 Si, 29 Si and 30 Si. (a) The mass spectrum of silicon gave the following results:

Isotope

% abundance

28 Si 29 Si 30 Si

92.21

4.70 3.09

(i) State two changes which must be made to a sample of solid silicon before it can be investigated in a mass spectrometer. (ii) Calculate the relative atomic mass of silicon, giving your answer to four significant figures.

[2]

[2] [1]

(iii) Explain what is meant by relative atomic mass.

(b) A sample of water containing 1 H, 2 H and 16 O was analysed in a mass spectrometer. The trace showed a number of peaks. Suggest which ions are responsible for the peaks at mass numbers 1, 18 and 20. [2] 3. An oxychloride of phosphorus which is used to dehydrate amides has a percentage [2] (b) What other information would you need to determine the molecular formula of this compound? [1] 4. Sodium nitrate is widely used in the production of fertilisers. (a) Sodium nitrate can be formed by the reaction between sodium carbonate and nitric acid as shown by the equation: Na 2 CO 3 (aq) + 2HNO 3 (aq) → 2NaNO 3 (aq) + H 2 O(l) + CO 2 (g) In an experiment, 25.0 cm 3 of a solution of sodium carbonate of concentration 0.0450 mol dm –3 required 23.6 cm 3 of the acid for complete neutralisation. Calculate the concentration of the acid. [3] (b) Sodium nitrate decomposes on heating as shown by the equation: 2NaNO 3 (s) → 2NaNO 2 (s) + O 2 (g) When a sample was decomposed completely, the oxygen occupied a volume of 700 cm 3 at a pressure of 101 kPa and a temperature of 28 °C. Calculate the amount, in moles, of oxygen produced. [3] 5. Elinor investigates the reaction between calcium carbonate and dilute hydrochloric acid. She is given 20.0 cm 3 of acid of concentration 1.20 mol dm –3 . CaCO 3 (s) + 2HCl(aq) → CaCl 2 (aq) + CO 2 (g) + H 2 O(l) (a) Calculate the minimum mass of calcium carbonate needed to react completely with this amount of acid. [3] (b) Calculate the volume of carbon dioxide that would be produced at a temperature of 25 ºC and a pressure of 1 atm. [2] (c) Use your answer to (b) to calculate the volume of carbon dioxide that would be produced at 50 °C and 1 atm. [2] 47 DRAFT composition, by mass, of P 20.0%; O 10.4%; Cl 69.6%. (a) Calculate the empirical formula of this compound.