Polychlorinated biphenyls (pcbs), which were formerly used in the manufacture of electrical transformers, are environmental and health hazards. they break down very slowly in the environment. the decomposition of pcbs can be represented by the equation:

2c12h4cl6 + 23o2 + 2h2o yields 24co2 + 12hcl

a) how many moles of water are needed to react 10.0 mol o2?

b) how many grams of hcl are produced when 15.2 mol h2o react?

c) how many moles of co2 are produced when 76.5 g hcl are produced?

d) how many grams of c12h4cl6 are reacted when 100.25 g co2 are produced?

e) how many grams of hcl are produced when 2.5 kg c12h4cl6 react?

With the balanced reaction we're able to use the stoichiometric ratios in order to solve each of the questions:

2C₁₂H₄Cl₆ + 23O₂ + 2H₂O → 24CO₂ + 12HCl

a) 10.0 mol O₂ * = 0.870 mol H₂O

b) When we work with mass, we use the molecular weight:

15.2 mol H₂O * 3324.24 g HCl

c) 76.5 g HCl *   50.37 mol CO₂

d) 100.25 g CO₂ *  =68.48 g C₁₂H₄Cl₆

e) 2.5 kg C₁₂H₄Cl₆ = 2500 g C₁₂H₄Cl₆

2500 g C₁₂H₄Cl₆ * 1515.80 g HCl

1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%