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Mass-Mass Problems Knowing the molar relationships between the substances involved allow us to calculate the mass relationships. This quantitative study of chemical reactions is called stoichiometry. Mass-Mass problems generally involve the following steps: 1. Changing the mass given to the number of moles with the formula; mass given # of moles = -----------------------molar mass 2. Determine the number of moles of the unknown by comparing the molar ratio; number of moles of given number of moles of unknown -------------------------------- = -------------------------------------coefficient of given coefficient of unknown 3. Change the number of moles of the unknown to mass with the formula; mass = # of moles x molar mass Ex: 1. How many grams of water are produced when 7.00 grams of oxygen react with an excess of hydrogen according to the reaction shown below? Given Unknown 2H2(g) + O2(g) ----> 2H2O(g) 7.00g xg 1. Changing the mass given to the number of moles with the formula; mass given = 7.00g 7.00g # of moles = -----------------------32.0g/mole # of moles of oxygen = 0.219 mole 2. Determine the number of moles of the unknown by comparing the molar ratio; 0.219 X ------- = ------1 2 Number of moles of water (unknown) = 0.438 mole 3. Change the number of moles of the unknown to mass with the formula; # of moles of water = 0.438 mole mass of water = 7.89 g Ex: 2. How many grams of sulfuric acid are required to react completely with 15.0 grams of zinc in a single displacement reaction? given unknown Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g) 15.0g Xg 1. Changing the mass given to the number of moles with the formula; moles of zinc = 0.229 mole 2. Determine the number of moles of the unknown by comparing the molar ratio; 0.229 X ------- = ------1 1 Number of moles of sulfuric acid (unknown) = 0.229 mole 3. Change the number of moles of the unknown to mass with the formula; # of moles of sulfuric acid = 0.229 mole mass of sulfuric acid = 22.5 g 22.5 grams of sulfuric acid to completely react with 15.0 grams of zinc. 2. How many grams of sulfuric acid are required to react completely with 15.0 grams of zinc in a single displacement reaction? given unknown Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g) 15.0g Xg 1. Changing the mass given to the number of moles with the formula; mass given = 15.00g 15.00g # of moles = ------------------------ = 0.229 mole of Zn 65.4g/mole 2. Determine the number of moles of the unknown by comparing the molar ratio; 0.229 X ------- = ------1 1 Number of moles of sulfuric acid (unknown) = 0.229 mole 3. Change the number of moles of the unknown to mass with the formula; # of moles of sulfuric acid = 0.229 mole mass of sulfuric acid = 0.229 mole x 98.1 g/mole mass of sulfuric acid = 22.5 g