The Mathematics of Chemical Equations Stoichiometry

I. Stoichiometry- study of quantitative, or measurable, relationships that exist in chemical formulas and chemical reactions. II. Balanced Equations- The coefficients in a balanced chemical reaction indicate the number of moles of each substance participating in the chemical reaction. –Ex: 2H 2 + O 2  2H 2 O –2 moles of hydrogen + 1 mole of oxygen yields 2 moles of water.

Mole-Mole problems Using a balanced equation you can determine amounts of reactants or products. –Ex: Nitrous oxide is produced by the decomposition of ammonium nitrate. NH 4 (NO 3 )  N 2 O + 2H 2 O How many moles of N 2 O are produced from 2.25 moles of NH 4 NO 3? Moles of given Moles of unknown Use molar ratio to convert

Stoichiometry problems All stoichiometry problems are solved using a balanced chemical equation By doing so, you verify the Law of Conservation of Mass.

Mass-Mass Problems How many g of Cu(NO 3 ) 2 will be produced from 128 g of AgNO 3 ? Cu + 2 AgNO 3  Cu(NO 3 ) 2 + 2Ag Mass of given Moles of givenMoles of unknown Mass of unknown Use molar mass Use molar ratio Use molar mass

Mass-mole problems How many moles of CO 2 are produced from 64.8 g CO 2 ? 2C 2 H 2 + 5O 2  4CO H 2 O Mass of given Moles of given Moles of unknown convert Use molar ratio

Mole-mass problems What mass of H 2 O can be produced from 5.42 moles of glucose? C 6 H 12 O O 2  4CO H 2 O Moles of given Moles of unknown Mass of unknown Use molar ratio Use molar mass

Mass-Volume problems Sodium azide is the compound found in air bags. In a collision a sensor sets off a spark causing the NaN 3 to decompose explosively, producing solid sodium and nitrogen gas: 2 NaN 3  2Na + 3N 2 Assume an air bag contains 125 g of NaN 3, what volume of N 2 is produced? (Flow chart on next slide)

Mass-volume problems Mass of given Moles of given Moles of unknown Volume of Unknown gas Use molar mass Use molar ratio Use molar volume

Volume-volume problems What volume of hydrogen gas is needed to react with 17.3 L of nitrogen gas? N 2 + 3H 2  2NH 3 Volume of given Moles of given Moles of unknown Volume of Unknown Use molar volume Use molar ratio Use Molar volume

Limiting Reactants The limiting reactant (LR) is completely used up in the reaction. The other reactants will be left over, or “in excess” The quantity (amount) of products formed in a rxn are always determined by the quantity of the limiting reactant To determine the LR you do mass-mass problems

Limiting Reactant Determine the LR: Cu + 2AgNO 3  Cu(NO 3 ) 2 + 2Ag Mass of given Moles of given Moles of unknown Mass of unknown Determine LR

Percent Yield Ratio of the expected yield/actual yield Expected yield – amt of product that should be produced according to the balanced chemical equation. Actual yield- amt of product actually produced in the reaction

Percent yield % yield = (actual yield/expected yield) x 100 Reasons for less than 100% yield –1. Reactants don’t react –2. Side reactions –3. Loss of reactants/products –4. Operator error