Practice Questions for Chemistry WA3 (Answers)

PRACTICE QUESTIONS FOR CHEMISTRY WA3 Name: ……..……………………………… ( ) Year 4 ..……. Date: …….………. Titration Question P is a solution that has been made by mixing dilute hydrochloric acid with dilute sulfuric acid. You are to determine the concentration of hydrogen ions in P by titrating this solution with aqueous sodium hydroxide, Q . Read all the instructions below carefully before starting the experiment. You are going to carry out a titration experiment. P - mixture of hydrochloric acid and sulfuric acid Q - 0.450 mol/dm 3 sodium hydroxide solution (a) (i) Put P into the burette. Pipette a 25.0 cm 3 portion of Q into a flask and titrate with P , using the indicator provided. Record your results in a suitable format, repeating the titrations as many times as you consider necessary to achieve consistent results. Results: Titration number 1 2 Final burette reading / cm 3 19.00 38.00 Initial burette reading / cm 3 0.00 19.00 Volume of P used / cm 3 19.00 19.00 Best titration results (√) √ √ [5] (ii) From your titration results, obtain an average volume of P to be used in your calculations. Show clearly how you obtained this volume. Average volume of P = (19.00 + 19.00) ÷ 2 = 19.00 cm 3 Working for the average must be shown

Average volume of P ……………………19.00 cm 3 ……. [1] PRACTICE QUESTIONS FOR CHEMISTRY WA3 (b) (i) Q is 0.450 mol/dm 3 sodium hydroxide. Calculate the concentration, in mol/dm 3 , of hydrogen ions in P . NaOH + H + → Na + + H 2 O No. of moles of NaOH used = concentration x volume in dm 3 = 0.450 x 1000 . 025 = 0.01125 mol no. of moles of H + = 1 1 no. of moles of NaOH No. of moles of H + = 0.01125 mol [1] ∴ Concentration of H + in P = no. of moles ÷ volume in dm 3 . 0019 = 0.01125 ÷ 1000 = 0.59211 mol/dm 3 (5 s.f.) = 0.592 mol/dm 3 (3 s.f.) [1] ‘ concentration of hydrogen ions in P …….0.592 mol/dm 3 [2] (ii) The concentration of hydrochloric acid in P is 0.110 mol/dm 3 . Using your answer from (i) , calculate the number of moles of hydrogen ions in 1 dm 3 of solution P due to sulfuric acid. . No. of moles of hydrogen ions in 1 dm 3 of solution P due to H 2 SO 4 = 0.59211 - 0.110 = 0.48211 (5 s.f.) = 0.482 mol (3 s.f ) number of moles of hydrogen ions in 1 dm 3 of solution P due to sulfuric acid…. 0.482 mol [1] (iii) Using your answer from (ii) , deduce the concentration of sulfuric acid, in mol/dm 3 , present