Stoichiometry – Part #2

1. Stoichiometry – Part #2 Chemistry: Review and Essentials

2. Types of Stoichiometric Calculations • mole-mole • mass-mass calculations • mass-volume • volume-volume • particle-mass Any quantity that is given must be converted to moles before any other quantities can be determined. In all stoichiometric calculations, the mole ratio can be used to calculate the number of moles of the desired substance. Once you know the number of moles for the desired substance, you can convert them to any other unit of measurement. This all starts with a balanced chemical equation.

3. Some units of measurement • Molarity  (mol/L) • Particles  (6.02×1023particles/mol) • Molar mass (g/mol) • Molar volume of a gas at STP, standard temperature(0°C) and pressure (1atm) (22.4L/mol) • Molar volume of a gas at SATP, standard ambient temperature (25°C) and pressure (1atm)  (24.8L/mol)

4. Try these! • Mole-mole calculations 4Al(s) + 3O2(g)  2Al2O3(s) How many moles of aluminum are required to produce 3.7 mol of Al2O3? (7.4 mol Al) • Mass-mass calculations N2(g) + 3H2(g)  2NH3(g) How many grams of NH3 are produced if you react and excess amount of nitrogen gas with 5.4g of hydrogen gas? (31g NH3)

5. Excess Reagents • When a reactant is present in excess it will not run out; this means that it will not determine how much product is formed. • The remaining excess reagent will remain unreacted once the reaction is complete.

6. Limiting Reagent • When a reactant is presented in limited quantities it determines the quantity of product(s) that is (are) formed. • The limiting reagent gets completely used up; there is none of it left at the end of the reaction. • If you are not given the quantities of your reactants in moles, the first step is to convert them into moles. Once this has been done, you can determine the quantity (in grams) of desired product that each of the reactants can produce. • The limiting reagent is the one that produces the least product; this is the amount of product that can be formed.

7. Try these! • 2Na(s) + Cl2(g)  2NaCl (s) • What is the limiting reagent when 6.70 mol Na react with 3.20 mol Cl2? (Cl2) • How many moles of NaCl are produced? (6.40 mol) • C2H4(g) + 3O2(g)  2CO2(g) + 2H2O(g) • What is the limiting reagent when 2.70 mol C2H4 reacted with 6.30 mol O2? (O2) • How many moles of water are produced? (4.20 mol)  • 2Cu(s) + S(s)  Cu2S(s) • What is the limiting reagent when 80.0g Cu reacts with 25.0g S? (Cu) • What is the maximum number of grams of Cu2S can be formed? (1.00x102g) • Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) • What is the limiting reagent when 6.00g HCl reacts with 5.00g Mg? (HCl) • How many grams of H2 are produced? (0.160g)

8. Yield • Theoretical yield: The maximum amount of product(s) that can be formed with the given amounts of reactants. These can be calculated using stoichiometric calculations. • Actual yield: This is the amount of product(s) that are actually formed when an experiment is carried out in the lab; this is an experimental value. The actual yield is usually less than the theoretical yield. • Percent yield: You can calculate the ratio of actual yield to percent yield according to the following equation.

9. Calculating Percent Yield Some factors that could affect yield: • The reaction does go to completion • Impure reactants or competing side reactions cause unwanted products to form • Loss of product during the filtration process or when transferring containers