Stoichiometry

Latin:

• Stoichon ➝ element

• Metron ➝ measure

Study of quantitative relationships between the amounts of reactants used and the amounts of products formed by a chemical reaction

Why do we calculate stuff?

• Cost - you don't want to buy any more than you need

• Saving on chemicals - less waste

• Saving time - reducing guesswork by knowing exactly how much you need

• Purity - you have a better idea of purity

Example 1 - Haber Process

N2 + 3H2 ➝ 2NH3

Atomic level: (conserved? yes)

2 nitrogen atoms are combining with 6 hydrogen atoms to form a compound with 8 atoms.

Molecules: (conserved? no.)

1 molecule of nitrogen is combining with 3 molecules of hydrogen to make two molecules of ammonia

Moles: (conserved? no.)

1 mole of nitrogen and 3 moles of hydrogen combine to make 2 moles of ammonia.

Grams: (conserved? yes)

28g of nitrogen and 6g of hydrogen combine to make 34g of ammonia.

Molar Volume: (conserved? no)

22.4L + 67.2L ➝ 44.8L

Example 2 - Decomposition of Sodium Chlorate

2NaClO3 ➝ 2NaCl + 3O2

100g / 106.3 (MM) x 2/2 x 58.35 (MM) ➝ 54.9g NaCl

Example 3 - Hexane

2C6H14 + 19O2 ➝ 12CO2 + 14H2O

26/(12.011 x 6)x(1.00794\cdot14\right)

Percent Yield

%Yield = Actual Amount / Theoretical Amount

Molarity

50mL of .1M Lead Nitrate solution is reacted with excess Zinc. How many grams of Pb is made?

Zn (excess) + Pb(NO3)2 (50mL .1M) ➝ Pb (?) + Zn(NO3)2 (not relevant)

M = mol/Litres

.1 * .05L ➝ .005mol Lead Nitrate in solution

.005mol * 207.2g ➝ 1.036g Lead

"Limiting Reactant" Problem

How do you determine the limiting reactant?

Example 1

3AgNO3 (aq)+ Na3PO4 (aq)➝ Ag3PO4 (s) + 3Na2NO3 (aq)

3g AgNO3 + 1g Na3PO4 ➝ (?) Ag3PO4 + (?) 3Na2NO3

Block out one, solve for 1 gram.

1g/163.87 (AgNO3) x 3/1 (mole ratio) x 169.8 (Ag3NO3) ➝ 3.11g AgNO3

AgNO3 runs out first. AgNO3 is the limiting reactant - because you need 3.11 grams for a complete reaction and there are only 3 grams.

Example 2

3g AgNO3 + 1g Na3PO4 ➝ (?) Ag3PO4 + (?) 3Na2NO3

3g/169.8 (AgNO3) x 1/3 x 418.5 (Ag3PO4) ➝ 2.4g Ag3PO4f