SToichiometrySToichiometry

Law of Conservation of

Mass

Antoine LavoisierStudied combustion

Showed it involved oxygen&life is supported by oxygen

in a process similar to combustion

Carefully weighted reactants & products

In an (ordinary) chemical reaction, mass is neither created nor destroyed.

Wrote first modern chemistry book

Worked as a tax collector during French RevolutionConsidered enemy of the peopleSent to guillotine

Law of Conservation of MassAll atoms you start with you end up with

Need to show conservation of mass with balanced equation

Reactant + Reactant --> Product

Combustion:Burning in the presence of oxygen

Heat is evolvedCombustion of hydrocarbonProduces CO2 & Water

Combustion:It is usually easiest to balancecarbon first

hydrogen secondcount how many oxygen needed in products

C5H12 + O2 --> H2O + CO256Need 16 oxygen for products

8

Mass-Mass & Mole-Mole Relations in ReactionsSuggested method:

1. write balancedequation

CO + 2 H2 --> CH3OH2. Make Chart

Grams

Molar Mass

Moles have

Given 100.0 g of each reactant,a. which is limiting reactant

b. how much of reactant in excess is leftc. How much product, in grams

100.0 g 100.0 g

28.0 g/mol 2.00 g/mol

______ ______/

//

/

3.57 mol 50.0 mol

3. Pick onetake moles have

& divide by balanced coefficient

This gives you a ratioto multiply other

coefficients by

3.57/1 = 3.57

Moles use 3.57 mol4. Take answer from step 3 multiply other

coefficient by it

3.57 x 2 = 7.14

7.14 mol

Moles left

- -

0 42.9 mol

Grams left 0 85.8 g

3.57 x 1 = 3.57

3.57 mol

32.0 g/mol

114. g

CO85.8 g

114. g

Theoretical yieldPercent yield = (actual yield/ theoretical yield) 100Let’s say you got 100.0 g for this reaction

Percent yield = (100.0 g / 114. g) 100 = 87.7 %

You might have your own method for You might have your own method for stoichiometry--that’s fine too.stoichiometry--that’s fine too.