SST Class Test 2 Chem Marking Scheme

School of Science and Technology, Singapore 2023 Secondary 3 Chemistry Class Test 2 Marking Scheme Section A Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 D B B C A D B A B A Section B Question Accepted Answers Marks 1a magnesium 1 b silicon 1 c sulfur/chlorine/ phosphorus 1 d phosphorus 1 e chlorine 1

2a C F Cl % by mass 8.7 13.8 77.5 Ar 12 19 35.5 no. of moles (calculation shown) 8.7/12 = 0.725 13.8/19 = 0.72631 77.5/35.5 = 2.18309 ratio (calculation shown) 0.725/0.725 =1 0.72631/0.725 =1.001 ≈1 2.18309/0.725 = 3.01 ≈3 Empirical Formula: CFCl3 1 1 1 2b(i) sharing of electrons between C and F is correct sharing of electrons between C and Cl is correct 1 1 2b(ii) CCl4 + 2HF → CF2Cl2 + 2HCl 1 2b(iii) no. of mol of HF = 10/20 = 0.5 mol no. of mol of CCl4 = 36/24 =1.5 mol Based on molar ratio, HF is the limiting reactant. mass of of CF2Cl2 = 0.5/2 x [12+(19)(2) + (35.5)(2)] = 0.25 x 121 = 30.25 = 30.3 g (to 3 sf) 1 1 1 1(answer with units)

3a 1. Fill a✓ burette with (dilute) sulfuric acid. Note the initial burette reading, V1 cm3. 2. ✓Pipette (25.0 cm3 )of potassium hydroxide solution into a conical flask. 3. Add ✓a few drops of suitable named indicator (e.g. methyl orange) to the solution in conical flask. 4. Add the ✓acid from the burette (slowly), ✓swirling the conical flask, until the ✓indicator changes colour. (e.g. red to orange) Record the final burette reading, V2 cm3. 5. Determine the ✓volume of (dilute) sulfuric acid required for complete neutralisation of potassium hydroxide solution. 6. ✓Repeat the titration without adding the indicator. Add V2 – V1 cm3 of the acid into 25.0 cm3 of potassium hydroxide solution into a conical flask. 7-8 ✓ : 4m 5-6 ✓ : 3m 3-4✓ : 2m 2 ✓ : 1m 1 ✓ : 0m 3b carbon dioxide will be formed from this reaction, as an acidic oxide/, it will lower(affect) the pH of the resultant solution/, requiring greater volume of potassium carbonate to neutralise sulfuric acid. 1 4a C is a giant covalent molecular structure as it has very high melting and boiling points. F comprises of simple covalent/discrete molecule/structure as it has low melting and boiling points. C and F are unable to conduct electricity in its solid and liquid states, as there are no mobile charged carriers (ions/electrons)in its structure. 1 1 b Nitric acid is a strong acid, thus it (completely) ionises/dissociates to form 0.01 mol / dm3 of hydrogen ions. 1 1

Nitrous acid is a weak acid, it (partially) ionises/dissociates to form less hydrogen ions of concentration 4.50 x 10-6 0.0004 mol / dm3 c(i) reactivity of metal : A is unreactive while B is a reactive metal. No gas was produced when A reacted with both acids. basicity of acid: twice the volume of gas was produced when B reacted with sulfuric acid compared to nitric acid, though sa