Stoichiometry & Limiting Reagent

What is stoichiometry?

Determining product amount from a reactant using mass conservation.

Origin of "stoichiometry"?

Greek: stoicheion = elements, metron = measuring.

Cheeseburger analogy ratio?

2 bread + 1 cheese + 1 patty
\rightarrow 1 cheeseburger.

How many bread slices for 30 cheeseburgers?

60 slices.

Cheeseburgers from 25 bread slices?

12 (limited by whole burgers).

What is a molar ratio?

Coefficients showing mole relationships in a balanced equation.

Example molar ratio 1:3:2 means?

1 mole of first substance reacts with 3 of second and 2 of third.

Step 1 in mass-to-mass stoichiometry?

Convert grams of known
\rightarrow moles using molar mass.

Step 2 in mass-to-mass stoichiometry?

Convert moles known
\rightarrow moles unknown using equation coefficients.

Step 3 in mass-to-mass stoichiometry?

Convert moles unknown
\rightarrow grams using molar mass.

Mass-to-mass general formula?

grams unknown = grams known
\times (1 mol known/molar mass)
\times (X/Y)
\times (molar mass unknown/1 mol).

Role of the mole?

Bridges microscopic particles and macroscopic measurements.

Three key mole conversions?

Moles
\leftrightarrow grams, moles
\leftrightarrow particles, moles
\leftrightarrow liters (gases at STP).

What is a limiting reagent?

Reactant consumed first, stopping the reaction.

How to find limiting reagent?

Compare product possible from each reactant; smaller product = limiting.

Excess reagent calculation?

Initial moles
\text{--} moles reacted (from limiting reagent).

Does limiting reagent depend on mass or moles?

Moles, not mass.

Molarity formula?

M = moles solute / liters solution.

Steps for solution stoichiometry?

1) Calculate moles solute, 2) Use mole ratio, 3) Convert moles
\rightarrow grams.

Reactants?

Starting materials on the left side of an equation.

Products?

Substances formed, on the right side of an equation.

Limiting reagent?

Reactant that runs out first.

Excess reagent?

Reactant left over after reaction stops.

Multi-step stoichiometry involves?

Mole ratios, molarity, and mass-mole conversions.

Balloon reaction example?

Balloon inflates according to limiting reactant moles, not mass.

For HCl + Mg, how to find limiting reactant?

Compare moles and use reaction ratio; fewer moles limits.

Making 150 g ammonium sulfate, what
\text{'}s needed?

Use mass
\rightarrow moles
\rightarrow mole ratio
\rightarrow grams of ammonia.

With 500 g ammonia, sulfuric acid needed?

Convert grams
\rightarrow moles
\rightarrow mole ratio
\rightarrow grams of acid.

Mass-to-mass problem approach?

Known mass
\rightarrow moles
\rightarrow mole ratio
\rightarrow unknown mass.

Molar masses needed for warm-up?

(NH₄)₂SO₄ = 132.14 g/mol
H₂SO₄ = 98.08 g/mol