ACJC H1 CHEM MCQ solutions

© ACJC 2017 8872/01/Prelims/17 [Turn over 2 Section A For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct. 1 Ether has the formula, CH3OCH3. In a sample of ether, 8.7 % contains the 18O isotope, with the rest contains the 16O isotope. What is the relative molecular mass of ether in this sample? A 46.2 B 46.8 C 47.2 D 47.8 Ar of O = 8.7 x 18 + 91.3 x 16 /100 = 16.174 Relative Molecular Mass = 16.174 + 12 + 3+ 12 + 3 = 46.174 = 46.2 2 Copper reacts with dilute nitric acid to produce nitrogen dioxide gas. The balanced ionic equation is given. Cu + 2H + + 2HNO3  Cu2+ + 2NO2 + 2H2O Which of the following is correct? Oxidation state of N in Role of copper HNO 3 NO 2 A +5 -3 Reducing agent B +5 +4 Reducing agent C -5 -3 Oxidising agent D -5 +4 Oxidising agent Cu is being oxidised. It is a reducing agent. HNO3 = (+1)+ ? + 3(-2) =0 ? = +5 NO 2 = ？ + 2（-2）= 0 ？= +4

© ACJC 2017 8872/01/Prelims/17 [Turn over 3 3 10 cm 3 of propane was completely burnt in x cm3 of excess oxygen. After cooling to room temperature, the volume of the residual gas was 60 cm 3. The residual gas was passed through aqueous sodium hydroxide was passed through and the volume reduced to y cm3. Which of the following is correct? x y A 50 30 B 60 20 C 70 20 D 80 30 C3H8 + 5O2  3CO2 + 4H2O 10cm 3 requires 50cm3 of oxygen for complete combustion and 30cm3 of CO2 will be produced. Hence x = 50 + (60-30) = 80 cm3. y = volume of excess oxygen = 60-30 cm3 4 An organic compound with the formula C xHyO2 has undergone incomplete combustion, producing carbon dioxide and carbon monoxide in the ratio of 99 : 1. The equation may be represented as follows: CxHyO2 + a O2  b CO + c CO2 + 0.5y H2O a, b and c can be expressed in terms of x and y. Which of the following is correct? a b c A 99x + 0.5x + 0.25y - 1 x 99x B 99x + 0.5x + 0.25y x 99x C 0.99x + 0.005x + 0.25y- 1 0.01x 0.99x D 0.99x + 0.005x + 0.25y 0.01x 0.99x C = 0.01x + 0.99x = x Check by balancing O atoms on both sides, 2 + 2(0.99x + 0.005x + 0.25y- 1) = 0.01x + 0.99x(2) + 0.5y 1.98x+ 0.01x+0.50y = 1.99x + 0.5y

© ACJC 2017 8872/01/Prelims/17 [Turn over 4 5 Use of the Data Booklet is relevant to this question. The components of a 100 g sample of fertilizer is as shown in the table below: Element Mass / g N 15 P 30 K 15 Other Elements 40 The recommended usage of fertilizer is 14 g of fertilizer per 5 dm3 of water. What is the concentration of nitrogen atoms in this recommended solution? A 0.03 mol dm-3 B 0.15 mol dm-3 C 0.42 mol dm-3 D 0.75 mol dm-3 15/100 x 14grams = 2.1 grams No of moles of N = 2.1/14 = 0.15mol Concentration = 0.15/5 = 0.03 mol dm-3 6 The elements X and Y are in Group 16 and 17 respectively in the same period. Which of the following statements regarding X and Y is most likely to be true? A Y has more unpaired electrons than X. B Y atom is bigger than X atom. C X is more electronegative than Y. D The first ionisation energy of X will likely be less endothermic than that of Y. X: ns2np4 Y: ns2np5 => less unpaired electrons than X

© ACJC 2017 8872/01/Prelims/17 [Turn over 5 Y has more protons than X. Thus Y would be smaller than X. The electrons are more strongly attracted to the nucleus in Y than X. Thus Y would be more electronegative than X. More energy is needed to remove 1st electron from its nucleus. 7 The table gives the successive ionisation energies for an element X. 1st 2nd 3 rd 4th 5th 6th ionisation energy/ kJ mol –1 950 1800 2700 4800 6000 12300 What could be the formula of the fluoride of X? A XF B XF2 C XF3 D XF4 The greatest jump in energy is between 3rd and 4th IE. Thus 3 valence electrons are in X which means that the highest oxidation number of X is +3. 8 ‘Dot-and-cross’ diagrams for carbon monoxide are shown below. Which circle pair of electrons represent a co-ordinate bond? A x x B x x C x x D x x Coordinate bond is a covalent bond in which both electrons come from the same atom.

© ACJC 2017 8872/01/Prelims/17 [Turn over 6 9 Which of the following molecules is linear and non-polar? A CS2 B SCN C SO2 D SiO2   10 Consider the following four compounds. 1 CH 3CH2CH2F 2 CH 3CH2CH2OH 3 CH 3CH2CH2CH3 4 (CH 3)3CH What is the order of increasing boiling points of the compounds (from lowest to highest)? A 2 → 1 → 3 → 4 B 4 → 3 → 2 → 1 C 3 → 1 → 2 → 4 D 4 → 3 → 1 → 2 Compound 2 has hydrogen bonds between its molecules. Compound 1 is polar and has pd-pd interactions between its molecules. Compound 3 and 4 are both structural isomers. Compound 3 is linear whereas compound 4 is branched. Thus compound 3 has more id-id interactions due to bigger surface area than compound 4.

© ACJC 2017 8872/01/Prelims/17 [Turn over 7 11 The enthalpy changes for two reactions are given by the equations: 2Cr(s) + 2 3 O2(g)  Cr2O3(s) ∆H = - 1130 kJ mol-1 C(s) + ½ O2(g) CO(g) ∆H = - 110 kJ mol-1 What is the enthalpy change, in kJ mol-1, for the following reaction? 3C(s) + Cr2O3(s)  2Cr(s) + 3CO(g) A -800 B +800 C -1460 D +1460 Cr2O3(s) 2Cr(s) + 2 3 O2(g) 3C(s) + 3/2 O2(g) 3CO(g) 3C(s) + Cr2O3(s)  2Cr(s) + 3CO(g) Enthalpy change of reaction = -110(3) + 1130 = + 800 kJ mol-1 12 Hydrogencarbonate may react with acid according to the equation below. HCO3–(aq) + H+(aq) H2O(l) + CO2(g) Ho = +12.7 kJ mol–1 Using the following enthalpy changes of formation provided, what is the standard enthalpy change of formation of H+(aq)? species H  f / kJ mol–1 H2O(l) –285.8 CO2(g) –393.5 HCO3–(aq) –692.0 A –25.4 kJ mol–1 B 0.0 kJ mol–1 C +25.4 kJ mol–1 D +1384 kJ mol–1 ∆H = ∆Hf(products) - ∆Hf(reactants) +12.7= –285.8–393.5 –( –692.0 + ∆HH+) ∆HH+ = 0

© ACJC 2017 8872/01/Prelims/17 [Turn over 8 13 In an experiment, 70 cm3 of water at 25 ºC was brought to boil by burning butane in excess oxygen. Calculate the volume of butane required if this process is only 85 % efficient. [∆H c (butane) = –2877 kJ mol –1; c = 4.2 J g –1 K–1; Molar volume of gas under the given conditions = 24 dm3] A 0.0721 dm3 B 0.156 dm3 C 0.184 dm3 D 0.216 dm3 Heat absorbed = Q = mc∆T = 70 x 4.18 x 75 = 21 945 J Heat released = 100/85 x 21 945 = 25 817 J No of moles of butane = 25 817x10 -3 / 2877 = 0.008973 mol Volume = 0.008973 x 24 = 0.216 dm3 14 Which statement concerning the equilibrium reaction given below is correct? 2CrO 42–(aq) + 2H+(aq) Cr2O72–(aq) + H2O(l) yellow orange A It is a redox reaction. B The equilibrium constant, Kc, has the units of mol-2 dm6. C The colour of the solution change from orange to yellow when pH increases. D The addition of a catalyst will result in an increase in the concentration of Cr2O72–(aq). There is no change in oxidation state of Cr (+6 in both species). Kc = [Cr2O72–]/[CrO42–]2[H+]2 hence, units is mol-3dm9 pH increases implies that there is a decrease in H+. By LCP, system shifts to the left as more H+ ions are produced.

© ACJC 2017 8872/01/Prelims/17 [Turn over 9 15 The kinetics of the reaction between iodine and compound J is investigated. 5 10 time / min [I2] 2y [ J ] = 2a [ J ] = a y What conclusions can be drawn from the graphs? A The reaction is second order with respect to compound J because rate of reaction increases by four times when its concentration is increased by two times. B Both iodine and compound J react in equal mole ratio.