What is Stoichiometry?
Stoichiometry (pronounced stoy-kee-AH-meh-tree) is the math of chemistry. It allows you to predict:
- •Exactly how many grams of product a chemical reaction will make.
- •Exactly how many grams of reactant you need to mix together so nothing is wasted.
Think of a recipe: If 2 slices of bread + 1 slice of cheese makes 1 sandwich, stoichiometry lets you calculate how many sandwiches you can make from 20 slices of bread.
The Master 3-Step Stoichiometry Bridge
In chemistry, you cannot directly compare grams of one substance to grams of another because different molecules have different atomic weights. You must convert through MOLES!
- Grams to Moles: Divide by molar mass of A.
- Mole to Mole Ratio: Multiply by (Coefficients from balanced equation: Want BHave A).
- Moles to Grams: Multiply by molar mass of B.
Worked Example 1: Full Grams-to-Grams Problem
Problem:
In the synthesis of ammonia gas (NH_3):
N_2 + 3H_2 \longrightarrow 2NH_3
How many grams of ammonia (NH_3) are produced when 28.02 grams of nitrogen gas (N_2) reacts completely with excess hydrogen?
Step 1: Molar masses from periodic table
- •Molar mass of N_2 = 2 × 14.01 = 28.02 g/mol
- •Molar mass of NH_3 = 14.01 + (3 × 1.01) = 17.04 g/mol
Step 2: Bridge Step 1 (Grams N_2 to Moles N_2)
Step 3: Bridge Step 2 (Mole Ratio: N_2 → NH_3)
Look at the balanced coefficients: 1 N_2 produces 2 NH_3:
Step 4: Bridge Step 3 (Moles NH_3 to Grams NH_3)
Final Answer: You will produce 34.08 g of ammonia gas.
Worked Example 2: How to Solve Limiting Reactant Problems
In real life, reactants rarely match the exact mole ratio. One reactant will run out first—this is the Limiting Reactant. The limiting reactant decides the maximum amount of product that can be formed (Theoretical Yield).
Problem:
You react 2.0 moles of Mg with 3.0 moles of HCl:
Mg + 2HCl \longrightarrow MgCl_2 + H_2
Which is the limiting reactant, and how many moles of H_2 gas will be formed?
Step 1: Calculate product yield from EACH reactant separately
- •From Mg:
- •From HCl:
Step 2: Compare results
- •HCl produces only 1.5 mol H_2, while Mg could produce 2.0 mol H_2.
- •The smaller number wins!
- •Limiting Reactant: HCl (runs out first).
- •Excess Reactant: Mg (some remains unreacted).
- •Theoretical Yield of H_2: 1.5 moles.
Percent Yield Formula
On chemistry lab reports, you compare what you actually measured on the lab scale to what stoichiometry predicted on paper:
Never Use Coefficients When Calculating Molar Mass!
In 2NH_3, the molar mass of NH_3 is 17.04 g/mol, NOT 34.08 g/mol. The 2 is accounted for strictly in the mole ratio step, never inside the molar mass calculation!