2021 RVHS Prelim Paper 3 (Solutions)

River Valley High School 9729/03/PRELIMS/21 2021 Preliminary Examination [Turn over RIVER VALLEY HIGH SCHOOL JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME CLASS 2 0 J CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9729/03 Paper 3 Free Response 21 September 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and index number in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all the questions. Section B Answer one question. Circle the question number of the question you attempted. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Question Number 1 2 3 4 5 s.f. units Total Marks 20 22 18 20 20 80 This document consists of 32 printed pages and 0 blank pages.

2 River Valley High School 9729/03/PRELIMS/21 2021 Preliminary Examination Section A Answer all the questions in this section. 1 In a 2014 paper published in the Journal of Agricultural and Food Chemistry, Hendon and Colonna-Dashwood discovered the effect of water hardness on coffee flavour. Compounds in hard water tend to attach to the flavo urful elements in roasted coffee beans during brewing. Water with higher levels of magnesium will likely extract more flavour from a coffee bean. Water described as "hard" is high in concentration of Total Dissolved Solids (TDS) , specifically calcium and magnesium. The hardness of water may be reported in parts per million (ppm). The solute concentration of a dilute aqueous solution in units of mg dm3 is called parts per million, or ppm. Classification ppm Soft 0  17.1 Slightly hard 17.1  60.0 Moderately hard 60.0  120 Hard 120  180 Very hard > 180 (a) In a sample of Singapore’s tap water, the concentration of magnesium and calcium ions present are found to be 5.97  105 mol dm 3 and 5.49  104 mol dm3 respectively. These two ions can be separated by selective precipitation with potassium hydroxide. The numerical values of solubility product of magnesium hydroxi de and calcium hydroxide at 25 ° C are 1.50  1011 and 5.50  106 respectively. (i) Calculate the total concentration of magnesium and calcium ions in ppm, and hence classify the hardness of water in this sample of tap water. [2] [Mg2+] in ppm = 5.97  105  24.3  1000 = 1.45 ppm [Ca2+] in ppm = 5.49  104 × 40.1 × 1000 = 22.0 ppm Total [Mg2+] and [Ca2+]