Stoichiometry

Do Now A recipe calls for one cup of milk and three eggs per serving. You quadruple the recipe because you are expecting guests. How much milk and eggs do you need?

Stoichiometry Stoichiometry = the proportional relationship between two or more substances during a chemical reaction. Reaction stoichiometry = problems involving the amount of products in relation to the amount of reactants – In order to correctly solve stoichiometry problems, you MUST be able to correctly balance equations! Stoichiometry problems are solved by using ratios from the balanced equation.

Balanced Equations Balanced equations show proportions A balanced chemical equation is very similar to a recipe in that the coefficients show the proportions of the reactants and products involved in the reaction. – For example: 2 H 2 + O 2  2 H 2 O So, 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.

Mole Ratio You can use mole ratios to determine how much of a reactant is needed to produce a quantity of product, and vice versa. Since the coefficients in a balanced equation show the relative numbers of moles of the substances in a reaction, ALWAYS convert between amounts in moles! – For example: Al 2 O 3 (l)  Al (s) + O 2 (g)

Moles to Moles Steps to converting between amounts in moles: 1.Identify the amount in moles that you know from the problem. 2.Using coefficients from the balanced equation, set up the mole ratio with the known substance on the bottom and the unknown substance on top. 3.Multiply the original amount by the mole ratio.

Moles to Moles Setup Amount of known moles x unknown moles = unknown moles known moles Mole Ratio

Moles to Moles Example How many moles of hydrogen are needed to prepare 312 moles of ammonia? N 2 + 3H 2  2NH 3

Moles to Moles Example How many moles of hydrogen are needed to prepare 312 moles of ammonia? N 2 + 3H 2  2NH 3 Amount of NH 3 = Amount of H 2 = From the equation: ___ mol H 2 = ___ mol NH 3

Moles to Moles Example How many moles of hydrogen are needed to prepare 312 moles of ammonia? N 2 + 3H 2  2NH mol NH 3 x 3 mol H 2 = ? 2 mol NH 3 = 468 mol H 2

Mole to Mole Problems Al 2 O 3 (l)  Al (s) + O 2 (g) How many moles of Al (s) will be produced from the decomposition of 13.0 mol of Al 2 O 3(l) ?

Mole to Mole Problems Al 2 O 3 (l)  Al (s) + O 2 (g) How many moles of O 2(g) will be produced when 36 mol of Al (s) are produced from the decomposition of Al 2 O 3(l) ?

Mole to Mole Problems Al 2 O 3 (l)  Al (s) + O 2 (g) How many moles of O 2(g) are produced when 15 mol of Al 2 O 3(l) react?

Moles to Mass Stoichiometry (and vice versa)

Mass Calculations Substances are usually measured by mass, so before using the mole ratio, you will need to convert from mass to moles! The conversion factor for converting between mass and moles is the molar mass of the substance. Molar mass = the sum of the atomic masses of the elements using the chemical formula – Molar mass is also known as: Gram-Formula Mass, and Molecular Mass

Moles to Mass (and vice versa) To convert between Moles and Mass, you will need to find the gram-formula mass and use your reference tables The formula for mole to mass calculations is on Table T (page 12) Number of moles = given mass gram-formula mass

Mass to Mole Example N 2 + 3H 2  2NH 3 How many moles of H 2 can be made from 22 g of H 2 and excess N 2 ?

Mass to Mole Example N 2 + 3H 2  2NH 3 How many moles of H 2 can be made from 22 g of H 2 and excess N 2 ? Given: mass of H 2 = 22 g molar mass of H 2 = 2.02 g/mol moles of NH 3 = unkwown

Mass to Mole Example N 2 + 3H 2  2NH 3 How many moles of H 2 can be made from 22 g of H 2 and excess N 2 ? Moles of H 2 = 22 g H 2 2 g/mol H 2 11 Moles of H 2

Mole to Mass Practice Fe 2 O 3 + Al  Fe + Al 2 O 3 How many moles of Fe 2 O 3 are needed to completely react with 135 g Fe 2 O 3 ?

Mass to Moles Practice Fe 2 O 3 + Al  Fe + Al 2 O 3 How many grams of Al 2 O 3 can form when 23.6 mole Al 2 O 3 react with excess Fe 2 O 3 ?

Mass to Moles Practice Fe 2 O 3 + Al  Fe + Al 2 O 3 How many moles of Fe 2 O 3 react with excess Al to make 475 g Fe 2 O 3 ?

Moles to Mass Practice Fe 2 O 3 + Al  Fe + Al 2 O 3 How many grams of Fe will form when.976 moles Fe form?