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Chemistry 2007.Pdf NATIONAL SCIENCE OLYMPIAD 2007: CHEMISTRY SECTION The answers have been compiled from a variety of sources, mainly Wikipedia (the free online encyclopaedia), Encyclopaedia Britannica, the Kirk-Othmer Encylopaedia of Chemical Technology and a variety of chemistry textbooks. (1) In the following list, __________ is Answer: not an example of matter. 3 3 To form 5.00 dm NO 2 requires 2.50 dm O2 (a) planets V2 = ( P1V1/T1) × ( T2/P2) (b) elemental phosphorus = (75 kPa × 2.50 dm 3/293 K) x (288 K/80 kPa) (c) table salt = 2.30 dm 3 [Option (b) ] (d) light (3) If SO 2 as an oxidizing agent is to be Answer: placed on the list of standard reduction potentials, it would be written as: + - From the definition of matter (physical SO 2 + 4 H + 4 e ↔ S + 2 H 2O substance in general, anything that occupies If the following half-reaction is borne in mind: space and possesses rest mass), the only S + 2 H + + 2 e - ↔ H S (E° = 0.14 V), correct answer is (d) . 2 choose a possible E° value for the SO 2 half-reaction from the following list: (2) Given the general gas equation pV = nRT , calculate the volume (in dm 3) (a) 0.16 V (b) 0.08 V of O at a pressure of 75 kPa and 2 (c) -0.30 V 20 °C needed to form 5.00 dm 3 of (d) -0.70 V NO 2 at 80 kPa and 15 °C according Answer: to the equation: For the reaction SO 2 + H 2S → 2 S + 2 H 2O, 2 NO 2 (g) + O 2 (g) → 2 NO 2 (g) E° = E° - E° cell O.A. R.A. = E° – 0.14 (a) 1.76 S/SO2 The reaction is spontaneous, so E° – 0.14 > 0, (b) 2.30 S/SO2 therefore E° > 0.14 V. Only option (a) is (c) 4.61 S/SO2 greater thus correct . (d) 5.40 2 (4) All atoms of a given element have For a neutral atom, atomic number equals the the same __________. number of electrons thus the correct option is (d) . (a) number of protons (b) number of neutrons (6) The element X has three naturally (c) number of electrons and neutrons occurring isotopes. The masses (amu) and (d) mass % abundances of the isotopes are given in the table below. Answer: Isotope Abundance Mass All atoms of a given element have the same 221 X 74.22 220.9 number of protons (atomic number). 220 X 12.78 220.0 Variation in the number of neutrons gives 218 X 13.00 218.1 rise to isotopes whilst variation in the number of electrons gives rise to ions The average atomic mass of the element is (oxidation states). Thus, option (a) is ___________ amu. correct . (a) 220.4 (5) There are __________ electrons, (b) 219.7 __________ protons, and (c) 22042 __________ neutrons in an atom of (d) 221.0 132 54 Xe Answer: (a) 78, 78, 132 (b) 78, 78, 54 To calculate the average mass of an atom of the (c) 54, 54, 132 element we simply multiply the fractional (d) 54, 54, 78 abundance of each isotope by its mass and then add the products: Answer: (0.7422×220.9) + (0.1278×220.0) + Subtracting the atomic number (no of (0.1300×218.1) = 220.4. Thus, option (a) is the protons) from the given mass number gives only one correct . the number of neutrons: 132-54 = 78. 3 (7) The formula and name of the ester Answer: formed when ethanoic acid reacts with propanol are, respectively, As the name suggests, a combination reaction is simply one in which two or more reactants (a) CH 3CH 2COOCH 2CH 3; combine to form one product or a number of ethyl propanoate products fewer than the number of starting (b) CH 3COOCH 2CH 2CH 3; materials. Reaction equations 2 and 3 fit this propyl ethanoate criterion and thus option (d) is correct . (c) CH 3COOCH 2CH 2CH 3; ethyl propanoate (9) Which one of the following molecular (d) none of the above formulas is also an empirical formula? Answer: (a) C6H6O2 (b) C6H6 The name of an ester is always two words (c) H2P4O6 and is derived from the parent name of the (d) C2H6SO alcohol with suffix yl followed by the parent name of the acid and ending with the suffix Answer: ate . Thus from prop anol and ethano ic acid we get the ester prop yl ethano ate [ option An empirical formula merely indicates the relative (b)] . ratios of each kind of atom in a substance whilst a molecular formula gives the actual numbers of (8) Which of the following are each kind of atom in a molecule. Options (a), (b) combination reactions? and (c) have the ratios that are all divisible by 2 and can thus not be empirical formulae. Thus, the (1) CH 4 (g) + O2 (g) → CO 2 (g) + H2O (l) only correct option is (d) . (2) CaO (s) + CO 2 (g) → CaCO 3 (s) (3) Mg (s) + O2 (g) → MgO (s) (10) Calculate the percentage by mass of (4) PbCO 3 (s) → PbO (s) + CO 2 (g) nitrogen in PtCl 2(NH 3)2. (a) 1, 2, 3, and 4 (a) 4.67 (b) 4 only (b) 9.34 (c) 2, 3, and 4 (c) 4.95 (d) 2 and 3 (d) 12.67 4 Answer: (12) Zinc is more active than cobalt and iron but less active than aluminium. Cobalt is Look up the molar masses of the elements in more active than nickel but less active the Periodic Table. The percentage by mass than iron. Which of the following of nitrogen is calculated as follows: correctly lists the elements in order of increasing activity? 2 ×14 ×100 195 08. + 2( × 25 45. ) + 2( ×14 ) + 2( × 3× .1 008 ) (a) Co < Ni < Fe < Zn < Al (b) Zn < Al < Co < Ni < Fe = 9.34% [ Option (b) ] (c) Fe < Ni < Co < Al < Zn (d) Ni < Co < Fe < Zn < Al (11) The balanced reaction between aqueous nitric acid and aqueous Answer: strontium hydroxide is __________. This is a question of simple logic and only option (a) HNO 3(aq) + Sr(OH) 2(aq) → (d) is correct . H2O(l) + Sr(NO 3)2(aq) (b) HNO 3(aq) + SrOH(aq) → (13) Element X has 2 valence electrons, and H2O(l) + SrNO 3(aq) element Y has 5 valence electrons. The (c) 2HNO 3(aq) + Sr(OH) 2(aq) → most likely type and formula of the 2H 2O(l) + Sr(NO 3)2(aq) compound formed when X and Y (d) 2HNO 3(aq) + Sr(OH) 2(aq) → combine chemically are Sr(NO 3)2(aq) + 2H 2(g) (a) covalent, X 3Y2 Answer: (b) ionic, X 3Y2 (c) ionic, X 2Y3 Strontium is in group two and its hydroxide, (d) ionic, X 5Y2. which is a base, has the formula Sr(OH) 2. For complete neutralisation, it requires two Answer: moles of nitric acid (a monoprotic acid) to form a salt (strontium nitrate) and water. With two valence electrons (a group 2 element), Thus, only option (c) is correct . element X will act as a reducing agent whilst Y, with 5 valence electrons (a group 5 element), will 5 act as an oxidizing agent and acquire three (15) Which of the following statements electrons to achieve an octet: correctly describes what happens when potassium dichromate reacts with ethanol X – 2e - → X 2+ in an acid medium? Y + 3e - → Y 3- ____________________ 3 X + 2 Y → X 2+ Y3- 3 2 1. Ethanoic acid is formed 2. An ester is formed Thus, option (b) is the only one correct . 3. The solution turns green 4. The ethanol is oxidized (14) The weak acid HA ionises in water as follows: + - (a) 1, 3 and 4 only HA (aq) + H 2O (l) → H 3O + A (aq) (b) 1 and 3 only Given that the ionisation constant of (c) 2 only HA is 4.50 × 10 -4, the pH of a 0.10 (d) 1 only. mol.dm -3 aqueous solution of HA is Answer: (a) 1.00 (b) 2.17 The correct option is (a) . (c) 2.19 3CH CH OH + 16H + + 2Cr O 2- → (d) none of the above 3 2 2 7 ethanol orange 3CH COOH + 11H O + 4Cr 3+ Answer: 3 2 ethanoic acid green -4 + - Ka = 4.50 × 10 = [H 3O ][A ]/[HA] + 2 (16) Of the following statements, __________ = [H 3O ] /[HA] is not true for oxygen. Upon dissociation the concentrations of the ions will be: + - (a) Oxygen forms peroxide and superoxide [H 3O ] = [A ] = x and [HA] = 0.10 - x anions 4.50 × 10 -4 = x2/(0.10 – x) (b) The most stable allotrope of oxygen is O x2 + 4.50 × 10 -4 x – 4.50 × 10 -5 = 0 2 (c) Oxygen is a colourless gas at room x = 6.49 × 10 -3 temperature pH = - log (6.49 × 10 -3) = 2.19 (d) Dry air is about 79% oxygen 6 Answer: As more of the copper salt is consumed, the intensity of the colour of the electrolyte in the Oxygen has 2 allotropes (O 2 and ozone, O 3) copper half-cell gradually decreases [ Option (d) ]. the most stable of which is O2. It is a 2- colourless gas that we breathe in all the (18) The Lewis structure of the CO 3 ion is time. It reacts to form oxides (O 2-), __________. 2- - peroxides (O 2 ) and superoxides (O 2 ) with other elements. Oxygen constitutes 21% of 2- O O (a) (b) air.
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