STOICHIOMETRY 436 5.6 – To determine, using stoichiometric calculations, the quantity of a substance involved in a chemical reaction. 5.8 – To solve numerical problems related to stoichiometry.

Recall: A mole is the amount of substance that contains 6.02 x 1023 particles of that substance n (moles) = mass of substance (g) molar mass of substance (g/mol)

Stoichiometry: Stoichiometry: uses ratios of a chemical reaction to determine the amounts of elements/ compounds (reactants & products) involved in a chemical change

Ex. 2Mg + O2  2MgO 2 : 1 : 2 Mole Ratio – Magnesium metal reacts with oxygen to produce magnesium oxide. 2Mg + O2  2MgO 2 : 1 : 2 Mole Ratio – describes the ratio in which the substances combine Ex. for every 2 Mg, 1 O2 needed used to predict what will happen Ex. for every 2 Mg & 1 O2, 2 MgO will be produced

Ex #1 First Second 2Mg + O2  2MgO 2 : 1 : 2 4 moles of magnesium metal reacts with oxygen to produce magnesium oxide. How many moles of MgO are produced? 2Mg + O2  2MgO First Mole Ratio 2 : 1 : 2 Second from mole ratio Set up ratio 2 moles Mg 2 moles MgO = 4 moles Mg ? moles MgO from question

Ex #1 cont... Third 2 moles Mg 2 moles MgO = 4 moles Mg ? moles MgO Solve by cross multiplying 2 moles Mg 2 moles MgO = 4 moles Mg ? moles MgO 4 x 2 ÷ 2 4 moles MgO are produced

Ex #2 First Second 4Fe + 3O2  2Fe2O3 4 : 3 : 2 Iron metal reacts with oxygen to produce 3 moles of iron (III) oxide. How many moles of oxygen were needed in the reaction? 4Fe + 3O2  2Fe2O3 First Mole Ratio 4 : 3 : 2 Second from mole ratio Set up ratio 3 moles O2 2 moles Fe2O3 = ? moles O2 3 moles Fe2O3 from question

Ex #2 cont... Third 3 moles O2 2 moles Fe2O3 = ? moles O2 Solve by cross multiplying 3 moles O2 2 moles Fe2O3 = ? moles O2 3 moles Fe2O3 3 x 3 ÷ 2 4.5 moles O2 are needed

Ex #3 How many moles of nitrogen gas & hydrogen gas are required to make 7.5 mol of ammonia? N2 + 3H2  2NH3 1 : 3 : 2 Solve for moles of N2: 11.25 moles H2 are required 1 mole N2 2 moles NH3 = ? moles N2 7.5 moles NH3 Solve for moles of H2: 3.75 moles N2 are required 3 mole H2 2 moles NH3 = ? moles H2 7.5 moles NH3

Ex. Ex. We can now add mass into problems involving stoichiometry. Recall, n (moles) = mass of substance (g) molar mass of substance (g/mol) Ex. How many moles are in 12.5g of Ca? n (moles) = 12.5g 40g/mol 0.3125 mol of Ca Ex. How many moles are in 2.4g of NO2? n (moles) = 2.4g 46g/mol 0.052 mol of NO2

Ex #4 First Second: 2Li + F2  2LiF 2 : 1 : 2 How many moles of Li are needed to react with fluorine gas to make 435g of lithium fluoride? 2Li + F2  2LiF 2 : 1 : 2 First Solve for moles of LiF: Second: n (moles) = 435g 26g/mol Solve for moles of Li: 16.73 mol of LiF 2 moles Li 2 moles LiF = ? moles Li 16.73 mol LiF 16.73 moles Li are needed

Ex #5 First Second 2NaClO3  2NaCl + 3O2 2 : 2 : 3 How many grams of NaClO3 must be decomposed to produce sodium chloride & 1.65 moles of oxygen? Ex #5 2NaClO3  2NaCl + 3O2 2 : 2 : 3 First Solve for moles of NaClO3: 2 mol NaClO3 3 mol O2 = ? mol NaClO3 1.65 mol O2 1.1 mol of NaClO3 Second Solve for mass of NaClO3: 116.6g of NaClO3 1.1 mol = ? g 106 g/mol

Ex #6 First Third Second Mg + 2HCl  MgCl2 + H2 1 : 2 : 1 : 1 3g of magnesium react with how many grams of hydrochloric acid to produce hydrogen gas & magnesium chloride? Ex #6 Mg + 2HCl  MgCl2 + H2 1 : 2 : 1 : 1 First Solve for moles of Mg: Third n = 3 g 24 g/mol Solve for mass of HCl: 0.25mol = ? g 36 g/mol 0.125 mol of Mg Second 9g of HCl Solve for moles of HCl: 1 mol Mg 2 mol HCl = 0.25 mol of HCl 0.125 mol Mg ? mol HCl

Key Points: Always find moles first!!! Use mole ratio to find the moles of each reactant & product Remember, the equation must be balanced to give you the mole ratio n (moles) = mass of substance (g) molar mass of substance (g/mol)

Practice: Worksheet #