2022 YIJC H2 CHEM PRELIM P2 MS

©YIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUGGESTED ANSWERS CG INDEX NO CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 29 August 2022 2 hours READ THESE INSTRUCTIONS FIRST This document consists of 28 printed pages. For Examiner’s Use 1 / 14 2 / 17 3 / 12 4 / 16 5 / 16 Penalty units significant figures Overall / 75 Write your name and class in the spaces at the top of this page. Write in dark blue or black pen on both sides of the paper. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, highlighters, glue or correction fluid/tape. Answer all questions in the spaces provided on the Question Paper. 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.

©YIJC [Turn over 2 Answer all the questions in the spaces provided. 1 Gallium, chromium and iron are metals that are found in Period 4 of the Periodic Table. (a) (i) Naturally occurring gallium, Ga, is a mixture of two isotopes. Table 1.2 shows the relative percentage abundance of the two isotopes. Table 1.2 relative mass relative % abundance 68.9256 60.11 70.9247 39.89 Calculate the relative atomic mass of Ga to four significant figures. Show your working. [2] Relative atomic mass of Ga = (68.9256 × 60.11) + (70.9247 × 39.89) 100 = 69.72 (ii) Gallium reacts with nitrogen to form gallium nitride, GaN. Calculate the volume of nitrogen gas needed to react with 10 kg of gallium at s.t.p.. [2] Ga(s) + ½N2(g) → GaN(s) Amount of Ga = 10000  69.7 = 143.47 mol Amount of N2 needed = 0.5 × 143.47 = 71.736 mol Volume of N2 needed = 71.736 × 22.7 = 1630 dm3 (b) Chromium(III) hydroxide, Cr(OH)3, is sparingly soluble in water. Cr(OH)3(s) ⇌ Cr3+(aq) + 3OH−(aq) (i) Write an expression for the solubility product, Ksp, of chromium(III) hydroxide, stating its units. [2] Ksp = [Cr3+][OH−]3, units: mol4 dm−12

©YIJC [Turn over 3 (ii) Calculate the concentration of OH −(aq) in a saturated solution of chromium( III) hydroxide, given the value of Ksp is 6.7 × 10−31.