JC1 H2 Chemistry Tutorial Atoms, Molecules and Stoichiometry

1. When aqueous ammonium dichromate(VI), (NH4)2Cr2O7 is added gradually to melted ammonium thiocyanate, NH4SCN, an ammonium salt known as Reinecke s salt, A, is formed. A has the formula NH4[Cr(SCN)x)(NH3)y], and analysis produced the following composition by mass: Cr: 15.5% S: 38.1% N: 29.2%

Calculate the values of x and y in the above formula. (x =4,y=2) 2. A particular vitamin C tablet contains 250 mg of vitamin C (C6H8O6). How many molecules of vitamin C does a tablet contain? (8.52 x 1020 molecules) 3. When a mixture of solid sodium carbonate and sodium hydrogen carbonate is heated, only sodium hydrogen carbonate decomposes. The products of this decomposition are sodium carbonate, carbon dioxide and water. If there is a loss of mass of 8.42 g when 60.42 g of this mixture is heated, calculate the percentage by mass of sodium hydrogen carbonate in the mixture. (37.8%) 4. Calculate the mass of carbon tetrachloride (CCl4) which can be produced by the reaction of 10.0 g of carbon with 100 g of chlorine gas. (108 g) 5. 150 cm3 of oxygen were added to 20 cm3 of a gaseous hydrocarbon D. After explosion and cooling, the gaseous mixture occupied 130 cm3 and , after absorption by potassium hydroxide, 90 cm3 of oxygen remained. All volumes were being measured at r.t.p. Find the molecular formula of D. (C2H4) 6. 10 cm3 of a hydrocarbon is exploded with an excess of oxygen. A contraction of 25 cm3 occurs. On treating the products with aqueous sodium hydroxide, a further contraction of 40 cm3 occurs. Deduce the formula of the hydrocarbon. (Ans : C4H6) 7. of What volume of 0.50 mol dm-3 Ba(OH)2 is needed to neutralize a mixture of 50.0 cm3

0.100 mol dm-3 HCl and 100 cm3 of 0.200 mol dm-3 H2SO4? (0.0450 dm3) 8. 2.50 g of a mixture of anhydrous sodium carbonate and sodium chloride were made up to 250 cm 3 with distilled water. 25.0 cm3 of this solution required 20.00 cm3 of

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0.100 mol dm-3 hydrochloric acid for titration. Calculate the percentage by mass of sodium chloride in the anhydrous mixture. (57.6%) 9. 1.600 g of a metallic oxide, MO, was dissolved in 100 cm3 of 1.0 mol dm-3 HCl. The resulting liquid was made up to 500 cm3 with distilled water. 25.0 cm3 of this solution then required 21.05 cm3 of a 0.1020 mol dm-3 sodium hydroxide for neutralisation. Calculate (a) (b) the mass of the oxide combining with 1 mole of the HCl and hence the relative formula mass of the oxide and the relative atomic mass of the metal.

((a)28.04 g ; (b) Mr of MO=56.08;Ar = 40.1) 10. A 1.500 g sample of an organic compound containing only C, H and O was burned completely. The only combustion products were 1.738 g carbon dioxide and 0.711 g water. What could be its empirical formula? (C2H4O3) Extra questions 11. 25.0 cm3 of a solution containing sodium hydroxide and sodium carbonate was titrated against 0.100 mol dm-3 hydrochloric acid, using phenolphthalein as indicator. After 30.0 cm3 of acid had been used, the indicator was decolourized. Methyl orange was added, and a further 12.5 cm3 of hydrochloric acid was needed to turn the indicator orange. Calculate the concentration of (i) sodium carbonate and (ii) sodium hydroxide in the solution. (0.0500 mol dm-3; 0.07 mol dm-3) 12. The relative atomic mass of boron, which consists of isotopes 10B and 11B is 10.8. What is the percentage of 11B atoms in the isotopic mixture? (80%) 13. In the absence of a catalyst, ammonia burns in an excess of oxygen to produce steam and nitrogen gas. What is the volume of oxygen remaining when 60 cm3 of ammonia is burnt in 100 cm3 of oxygen, all volumes being measured at the same temperatures and pressure? (55 cm3) 14. When metal M is heated in halogen X2, a compound MXn is formed. In a given experiment, 1.00 g of titanium reacts with chlorine to give 3.22 g of a compound. What is the corresponding value of n? [Ar (Ti) = 47.9, Ar(Cl) = 35.5] (n = 3) 15. A sample containing acidic ammonium sulfate was warmed with 250 cm3 of 0.800 mol dm-3 sodium hydroxide. After the evolution of ammonia has ceased, the remaining sodium hydroxide solution was neutralised by 85.00 cm3 of 0.500 mol dm-3 hydrochloric acid. What mass of ammonium sulfate did the sample contain? (10.4g)

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