STOICHIOMETRY

What is stoichiometry? Stoichiometry is the quantitative study of reactants and products in a chemical reaction.

What You Should Expect Given : Amount of reactants Question: how much of products can be formed. Example 2 A + 2B 3C Given 20.0 grams of A and sufficient B, how many grams of C can be produced?

What do you need? You will need to use i. molar ratios, ii. molar masses, iii. balancing and interpreting equations, and iv. conversions between grams and moles. Note: This type of problem is often called "mass-mass."

Steps Involved in Solving Mass-Mass Stoichiometry Problems Balance the chemical equation correctly Using the molar mass of the given substance, convert the mass given to moles. Construct a molar proportion (two molar ratios set equal to each other) Using the molar mass of the unknown substance, convert the moles just calculated to mass.

Mole Ratios A mole ratio converts moles of one compound in a balanced chemical equation into moles of another compound.

Example Reaction between magnesium and oxygen to form magnesium oxide. ( fireworks) 2 Mg(s) + O2(g) 2 MgO(s) Mole Ratios: 2 : 1: 2

1) N H 2 ---> 2 NH 3 Write the mole ratios for N 2 to H 2 and NH 3 to H 2. 2) A can of butane lighter fluid contains 1.20 moles of butane (C 4 H 10 ). Calculate the number of moles of carbon dioxide given off when this butane is burned. Practice Problems

Mole-Mass Problems Problem 1: 1.50 mol of KClO 3 decomposes. How many grams of O 2 will be produced? [k = 39, Cl = 35.5, O = 16] 2 KClO 3 2 KCl + 3 O 2

Three steps…Get Your Correct Answer Use mole ratio Get the answer in moles and then Convert to Mass. [Simple Arithmetic] Hello! If you are given a mass in the problem, you will need to convert this to moles first. Ok?

Stoichiometric Calculations The coefficients in the balanced equation give the ratio of moles of reactants and products

Mass-Mass Problems There are four steps involved in solving these problems: Make sure you are working with a properly balanced equation. Convert grams of the substance given in the problem to moles. Construct two ratios - one from the problem and one from the equation and set them equal. Solve for "x," which is usually found in the ratio from the problem. Convert moles of the substance just solved for into grams.

Stoichiometric Calculations From the mass of Substance A you can use the ratio of the coefficients of A and B to calculate the mass of Substance B formed (if it’s a product) or used (if it’s a reactant)

Stoichiometric Calculations Starting with 1.00 g of C 6 H 12 O 6 … we calculate the moles of C 6 H 12 O 6 … use the coefficients to find the moles of H 2 O… and then turn the moles of water to grams C 6 H 12 O O 2  6 CO H 2 O

Limiting Reactants

How Many Cookies Can I Make? In this example the sugar would be the limiting reactant, because it will limit the amount of cookies you can make

Limiting Reactants The limiting reactant is the reactant present in the smallest stoichiometric amount

Limiting Reactants The limiting reactant is the reactant present in the smallest stoichiometric amount – In other words, it’s the reactant you’ll run out of first (in this case, the H 2 )

Limiting Reactants In the example below, the O 2 would be the excess reagent

Theoretical Yield The theoretical yield is the amount of product that can be made – In other words it’s the amount of product possible as calculated through the stoichiometry problem This is different from the actual yield, the amount one actually produces and measures

Percent Yield A comparison of the amount actually obtained to the amount it was possible to make Actual Yield Theoretical Yield Percent Yield =x 100