C7 Calculation Practice. Using Bond Energies. Reacting hydrogen and oxygen to form water: 2H2 + O2 2H2O Covalent bond energies are given below: H-H is 436kJ O=O is 496kJ O-H is 436kJ Calculate the energy change for the formation of water. Hydrogen and halogens react to form hydrogen halides. H2 + Cl2 2HCl H-H is 436kJ Use the bond energy data to calculate the energy change. Cl-Cl is 243kJ H-Cl is 432kJ Chromatography – calculate Rf values. Calculate the Rf values for the four colours. Concentration. Calculate the concentration of solution when 12.6g of sucrose is dissolved in 350cm3 water. What is the mass of sodium chloride in 25 cm3 of a solution with a concentration of 36g/dm3? Titration. 50cm3 of Sodium hydroxide (concentration 25g/dm3) was placed in a conical flask. It was titrated with hydrochloric acid of unknown concentration. The following titres were recorded: 10.0 cm3, 9.9 cm3, 10.1 cm3. Estimate the concentration of the acid. Calculating the degree of uncertainty of titration results… Results of 5 titrations: 54.7 g/dm3 49.3 g/dm3 53.1 g/dm3 52.2 g/dm3 54.3 g/dm3 a) Calculate the mean. b) What is the range? c) The degree of uncertainty is the difference between the highest and the lowest. Percentage error = (degree of uncertainty/mean) x 100 Certainty is therefore 100 – percentage error Atom economy. There are two ways to extract copper. a) Copper oxide can be heated with carbon: 2CuO + C 2Cu + CO2 Calculate the atom economy. b) Copper sulphide can be heated with oxygen: CuS + O2 Cu + SO2 Calculate the atom economy. Ethanol can be made by reacting ethane with steam or fermentation of sugar. Write equations for the reactions and calculate the atom economies. Percentage Yield. Limestone (CaCO3) can be heated to make quicklime (CaO) for cement making. CaCO3 CaO + CO2 50 tonnes of limestone was heated and 24 tonnes of quicklime made. Calculate the percentage yield.