# Science C5 Essay

10105 Words41 Pages
Moles and empirical formulae 1 Calculate the molar mass of the following: (Relative atomic masses: H = 1; N = 14; O = 16; Na = 23; S = 32; Cl = 35.5; K =39.) (a) nitric acid, HNO3 (b) sodium sulfate, Na2SO4 (c) ammonia, NH3 (d) potassium chloride, KCl (e) chlorine, Cl2 2 John says that there is no change in mass during a chemical reaction. Anna disagrees. She says that when calcium carbonate reacts with hydrochloric acid to form carbon dioxide, calcium chloride and water, the mass of the reaction vessel and contents decreases. Who is right, John or Anna? Explain your answer. 3 When 200 g of calcium nitrate, Ca(NO3)2.2H2O is heated at 120ºC the mass decreases by 36 g. (a) Why does the mass decrease? (b) How much calcium nitrate is left after heating? 4 Calculate the molar mass of these compounds: (relative atomic masses: H = 1; N = 14; O = 16; S = 32; Cu = 64; Br = 80; Pb = 207) (a) copper nitrate, Ca(NO3)2 (b) lead bromate, Pb(BrO3)2 (c) ammonium sulfate, (NH4)2SO4 5 The equation for the complete combustion of methane is shown below. CH4 + 2O2 → CO2 + 2H2O When 80 g of methane is completely combusted, 220 g of carbon dioxide and 180 g of water are formed. (a) Why is the mass of carbon dioxide formed greater than the mass of methane burnt? (b) Calculate the mass of oxygen that reacted. (c) Calculate the mass of water formed when 20 g of methane undergoes complete combustion. 6 Deduce the empirical formulae of the following: Moles and empirical formulae 1 Calculate the molar mass of these compounds. (Relative atomic masses H = 1; N = 14; O = 16; Mg = 24; S = 32; Ca = 40; K =39; Cr = 52; Fe = 56.) (a) calcium ethanoate, Ca(CH3CO2)2 (b) chromium sulfate Cr2(SO4)3 (c) magnesium hydroxide, Mg(OH)2 (d) potassium cyanoferrate(III), K3Fe(CN)6 2 When 2.4 g of magnesium