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 in solution P . H 2 SO 4 → 2H + + SO 4 2- Concentration of H 2 SO 4 = ½ x concentration of H + = ½ x 0.48211 = 0.241 mol/dm 3 (3 s.f )

concentration of sulfuric acid present in P …0.241 mol/dm 3 [1] (c) A student repeated the experiment and accidentally used a 20.0 cm 3 pipette to measure solution Q into the flask for each titration. The student thought that he has used a 25.0 cm 3 pipette. Describe and explain the effect this would have on the calculated number of moles of hydrogen ions in 1 dm 3 of solution P due to sulfuric acid in b(ii) . • Volume of P needed in the titration will be lower than actual. [1] • Calculated concentration of hydrogen ions in P in b(i) will be higher than expected, [1] • Hence the calculated number of moles of hydrogen ions in 1dm 3 of P due to sulfuric acid in b(ii) will also be higher than actual. [2] PRACTICE QUESTIONS FOR CHEMISTRY WA3 Planning Question R is a nitric acid solution. Describe a method you could use to determine its concentration. You can assume that all the apparatus and reagents normally found in a school laboratory are available. You should include the measurements you would take and how you would use your results to calculate the concentration of the acid in R . You may not use a titration method. Method 1 Procedures 1. Measure a known volume of R using a pipette/burette and place it into a conical flask . 2. Add excess calcium carbonate or other carbonate to the flask. 3. Collect all the carbon dioxide produced using a graduated gas syringe . 4. Record the total volume of carbon dioxide collected at r.t.p Data Processing CaCO 3 + 2HNO 3 → Ca(NO 3 ) 2 + H 2 O + CO 2 1. Number of moles of CO 2 = volume of CO 2 in dm 3 ÷ molar volume in dm 3 = volume of CO 2 ÷ 24 2. From equation, number of moles of HNO 3 = 2 x no. of moles of CO 2 3. Concentration of HNO 3 in R in mol/dm 3 = no. of moles of HNO 3 ÷ volume of R in dm 3

Method 2 Procedures 1. Measure a known volume of R using a pipette/burette and place it into a conical flask . 2. Place the conical flask on an electronic mass balance . Record the total mass of the flask and R . Let this mass be m 1 g. 3. Add a known mass ( m 2 g) of calcium carbonate or other carbonate (in excess) to the flask. Place a loose plug of cotton wool over the mouth of the flask to prevent loss of acid spray. 4. Record the final total mass of the flask and its content when the total mass does not decrease further . Let this total mass be m 3 g. PRACTICE QUESTIONS FOR CHEMISTRY WA3 Data Processing CaCO 3 + 2HNO 3 → Ca(NO 3 ) 2 + H 2 O + CO 2 1. Mass of CO 2 produced = ( m 1 + m 2 – m 3 )g 2. Number of moles of CO 2 = mass of CO 2 in g ÷ molar mass of CO 2 in g/mol 3. From equation, number of moles of HNO 3 = 2 x no. of moles of CO 2 4. Concentration of HNO 3 in R in mol/dm 3 = no. of moles of HNO 3 ÷ volume of R in dm 3 Note : The reaction between a metal and nitric acid is not suitable as the products formed depend on the concentration of the acid and the nature of the metal. Oxides of nitrogen (NO, NO 2 ) may be formed instead of hydrogen gas.