12.2: CHEMICAL CALCULATIONS

STANDARD 3.e. Students know how to

OBJECTIVES Construct mole ratios from balanced chemical equations and apply these ratios in stoichiometric calculations Calculate stoichiometric quantities from balanced chemical equations using units of moles, particles, and volumes of gases at STP

WRITING AND USING MOLE RATIOS Mole Ratio: a conversion factor derived from the coefficients of a balanced chemical equation interpreted in terms of moles. In chemical reactions, mole ratios are used to convert between moles of reactant and moles of product, between moles or reactants, or between moles of products.

SAMPLE PROBLEM 1

PRACTICE PROBLEM 2

IN EXCESS “A substance in excess” means that you have more than enough of that substance. For example, if the recipe calls for 2 eggs, but I have a dozen, then eggs are in excess.

H2H2 3 mol NH 3 2 mol H 2 1 mol H g H g NH 3 1 mol NH 3

Known: 5.00 g CaC 2 Unknown: g C 2 H 2 We have to go from g CaC 2  mole CaC 2  mole C 2 H 2  g C 2 H 2 g C 2 H 2 = 5.00 g CaC 2 x ___________ g CaC 2 1 mol C 2 H 2 1 mol CaC 2 26 g C 2 H 2 1 mol C 2 H 2 X ___________ X _________

Known = 49.0 g H2O Unknown = mole CaC2 We have to go from g H2O  mol H2O  mole CaC2 Mole CaC2 = 49.0 g H2O x __________ 18.0 g H 2 O 1 mol H 2 O1 mol CaC 2 2 mol H 2 O X ___________

FIGURE

OTHER STOICHIOMETRIC CALCULATIONS In a typical stoichiometric problem, the given quantity is first converted to moles. Then the mole ratio from the balanced equation is used to calculate the number of moles of the wanted substance. Finally, the moles are converted to any other unit of measurement related to the unit mole (depending on the problem).

CALCULATING MOLECULES OF A PRODUCT

#7

#8

VOLUME-VOLUME CALCULATIONS

PRACTICE PROBLEMS 10

PRACTICE PROBLEM 11