RI Mole Concept and Stoichiometry Tutorial 2025

-1- Raffles Institution Year 5 H2 Chemistry 2025 Tutorial 1 – The Mole Concept and Stoichiometry Practice Questions 1 It is recommended that drinking water contain fluoride (F −) for prevention of tooth decay. During the purification process, fluoride ions are added to 1000 cm 3 of reservoir water. A 10.00 cm 3 sample of the fluorinated water is taken and transferred to a volumetric flask. Deionised water is added to make a total volume of 100.0 cm 3 of the diluted solution. This diluted solution contains 0.16 ppm of fluoride ions. (1 ppm of fluoride ions = 1 mg of fluoride ions in 1000 cm3 of solvent.) (a) Calculate the mass of F− ions present in 1000 cm3 fluorinated reservoir water. (b) Fluoride is provided by hydrogen hexafluorosilicate, H 2SiF6. Calculate the mass of H2SiF6 added to the reservoir water given that six F− ions are formed per unit of H2SiF6. [Mr of H2SiF6 = 144.1] 2 A carbonate M2CO3 of mass 5.30 g was dissolved in deionised water to produce a 500 cm3 solution. During titration, 25.0 cm3 of this solution required 26.0 cm3 of 0.192 mol dm−3 HCl for complete neutralisation. Calculate the concentration of M2CO3 in g dm −3, and determine the relative atomic mass of M. 3 2014 CT Qn B2(b) Propane, C3H8, is a gaseous hydrocarbon fuel that undergoes combustion to produce carbon dioxide and water. 10 cm3 of C3H8 and z cm3 of N2 were sparked with excess O2 in a flask and the flask was then cooled to room temperature. NO2(g) was the only N containing product formed. The gaseous mixture was then passed through 50 cm 3 of 0.50 mol dmˉ3 NaOH(aq). The excess NaOH required 24.40 cm3 of 0.35 mol dmˉ3 H2SO4(aq) for complete reaction. (a) Write balanced equations with state symbols for the two combustion reactions, where C3H8 and N2 each reacted with O2. (b) Calculate the volume of gaseous product(s) formed from the combustion of C3H8. (c) Calculate the amount of NaOH that has reacted with the gaseous mixture. (d) NO2 reacts with NaOH according to the following reaction: 2NO2(g) + 2NaOH(aq) → NaNO2(aq) + NaNO3(aq) + H2O(l) Determine the value of z.

-2- 4 The percentage of nitrogen in foodstuffs gives an indication of how much protein is present. It can be estimated by the following method. I. The foodstuff is weighed and boiled under reflux with concentrated sulfuric acid for some time. This converts all the nitrogen into ammonium sulfate. II. An excess of aqueous sodium hydroxide is added, and the mixture again boiled. The liberated ammonia gas is passed into a known excess of dilute hydrochloric acid. III. The unreacted hydrochloric acid is then titrated with aqueous sodium hydroxide of known concentration. (a) Write equations for the two reactions occurring in step II. (b) When 1.00 g of a foodstuff was subjected to the above procedure , and the gas in step II was passed into 50.0 cm 3 of 0.100 mol dm −3