In this experiment you will determine the relative formula mass, Mr, of basic zinc carbonate, ZnCO3•2Zn(OH)2•xH2O, by an alternative method. A known mass of basic zinc carbonate is reacted with a known volume and concentration of hydrochloric acid, HCl. The acid added is in excess. You will titrate portions of the resulting solution with sodium hydroxide, NaOH, of known concentration. FA 2 has been prepared as follows: 3.52g of basic zinc carbonate, ZnCO3•2Zn(OH)2•xH2O, is reacted with 100 cm³ of 2.00 mol dm⁻³ hydrochloric acid, HCl. The resulting solution is diluted to 1.00 dm³ with distilled water. FA 3 is 0.150 mol dm⁻³ sodium hydroxide, NaOH. FA 4 is bromophenol blue indicator.

1. (a) Method• Fill the burette with FA 3. • Pipette 25.0 cm³ of FA 2 into a conical flask. • Add several drops of FA 4 to the conical flask. • Perform a rough titration and record your burette readings in the space below. The rough titre is .............................................. cm³. • Carry out as many titrations as you think necessary to obtain consistent results. • Make sure any recorded results show the precision of your practical work. • Record, in a suitable form below, all your burette readings and the volume of FA 3 added in each accurate titration. I II III IV V VI VII[7]
2. (b)From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm³ of FA 2 required .............................................. cm³ of FA 3.[1]
3. (c)(i)Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(vi) to an appropriate number of significant figures.[1]
4. (c)(ii)Calculate the amount, in mol, of sodium hydroxide present in the volume of FA 3 in (b). amount of NaOH = .............................................. mol[1]
5. (c)(iii)Use your answer to (c)(ii) to calculate the amount, in mol, of hydrochloric acid present in 1.00 dm³ of FA 2. amount of HCl in 1.00 dm³ of FA 2 = .............................................. mol[1]
6. (c)(iv)Use your answer to (c)(iii) and the information about the preparation of FA 2 to calculate the amount, in mol, of hydrochloric acid that reacted with 3.52 g of basic zinc carbonate. amount of HCl that reacted = .............................................. mol[1]
7. (c)(v)Using the equation given in question 1(b) as a guide, complete the equation for the reaction of hydrochloric acid with basic zinc carbonate. State symbols are not required. ZnCO3•2Zn(OH)2•xH2O + 6HCl → .............................................. + .............................................. + ..............................................[1]
8. (c)(vi)Use your answer to (c)(iv) and the equation in (c)(v) to calculate the amount, in mol, of basic zinc carbonate in 3.52 g. amount of ZnCO3•2Zn(OH)2•xH2O = .............................................. mol Hence, calculate the relative formula mass, Mr, of basic zinc carbonate. Mr of ZnCO3•2Zn(OH)2•xH2O = ..............................................[2]
9. (d)A student correctly followed all the instructions in (a) to (c) of this question. The formula mass the student calculated was smaller than the correct value. The student suggests that this may be because the concentration of sodium hydroxide, FA 3, is greater than 0.150 mol dm⁻³. Explain why the student is correct.[2]