Stoichiometry Problems- Practice and Solutions

What Stoichiometry Actually Is

Stoichiometry is the math behind chemical reactions. It tells you exactly how much of each substance you need or will get. No guesswork, no estimation—just numbers.

If you can't work stoichiometry problems, you can't pass chemistry. Period. This guide cuts through the confusion and gives you the exact steps to solve any stoichiometry problem that comes your way.

The Foundation: What You Need to Know First

Before touching any stoichiometry problem, these concepts must be locked in:

• Mole – 6.02 × 10²³ particles (atoms, molecules, whatever)
• Molar mass – mass of one mole, found on the periodic table
• Balanced equation – coefficients must be correct or your answers will be wrong
• Mole ratio – the conversion factor between substances in the reaction

If any of these terms make you uncomfortable, stop here. Go back and review. Trying to solve stoichiometry problems without this foundation is a waste of your time.

The General Method for Solving Stoichiometry Problems

Every stoichiometry problem follows the same skeleton. Master this sequence and you can solve anything:

1. Balance the chemical equation
2. Convert given information to moles
3. Use mole ratio to find moles of desired substance
4. Convert moles back to the units the problem asks for

That's it. Four steps. Everything else is just variations on this theme.

The Mole Bridge

Think of it as a bridge. Everything must cross through moles to get converted:

Given unit → Moles → Desired unit

Skip the mole conversion and your answer will be garbage every single time.

Practice Problem 1: Simple Mass-to-Mass Conversion

Question: How many grams of water form when 4 grams of hydrogen react with excess oxygen?

First, write and balance the equation:

2H₂ + O₂ → 2H₂O

Step 1: Convert 4g H₂ to moles

4g ÷ 2 g/mol = 2 moles H₂

Step 2: Apply mole ratio from balanced equation

2 moles H₂ : 2 moles H₂O (ratio is 1:1)

2 moles H₂ × (2 mol H₂O / 2 mol H₂) = 2 moles H₂O

Step 3: Convert moles H₂O to grams

2 moles × 18 g/mol = 36 grams H₂O

Answer: 36 grams of water

Practice Problem 2: Limiting Reagent

Question: 10g of sodium reacts with 10g of chlorine. Which reactant limits the reaction? How much sodium chloride forms?

Equation: 2Na + Cl₂ → 2NaCl

Calculate moles of each reactant:

• Na: 10g ÷ 23 g/mol = 0.435 mol Na
• Cl₂: 10g ÷ 71 g/mol = 0.141 mol Cl₂

Find how much NaCl each reactant could produce:

• From Na: 0.435 mol × (2 mol NaCl / 2 mol Na) = 0.435 mol NaCl
• From Cl₂: 0.141 mol × (2 mol NaCl / 1 mol Cl₂) = 0.282 mol NaCl

Chlorine produces less NaCl. Chlorine is the limiting reagent.

Calculate mass of NaCl:

0.282 mol × 58.5 g/mol = 16.5 grams NaCl

Answer: Cl₂ limits, 16.5g NaCl forms

Practice Problem 3: Volume of Gas at STP

Question: How many liters of CO₂ form at STP when 12g of carbon burns completely?

Equation: C + O₂ → CO₂

Step 1: Convert 12g C to moles

12g ÷ 12 g/mol = 1 mol C

Step 2: Mole ratio (1:1 for C to CO₂)

1 mol C × (1 mol CO₂ / 1 mol C) = 1 mol CO₂

Step 3: Convert to liters at STP

1 mol × 22.4 L/mol = 22.4 liters CO₂

Answer: 22.4 liters

Common Mistakes That Destroy Your Answers

• Unbalanced equations – This is the #1 killer. Always balance first.
• Forgetting to convert to moles – Every calculation goes through moles.
• Using atomic mass instead of molar mass – They're the same number, but the units matter.
• Wrong mole ratio – Pull coefficients from the balanced equation, not your memory.
• Ignoring the limiting reagent – When two reactants are given, you must check both.
• Rounding too early – Keep extra digits until the final answer.

Quick Reference: Conversion Factors

How to Actually Get Better

Practice is the only way. Reading about stoichiometry won't make you better at it—solving problems will.

• Do 10 problems minimum before your next test
• Always write out the full solution, even if you think you can do it in your head
• Check your answers by working backwards
• Time yourself—eventually you'll need to be fast

Final Warning

Stoichiometry punishes shortcuts. Students who try to skip steps always get burned. Write out every conversion. Show your work. The method matters more than getting the "right" answer by accident.

When you sit down to solve any problem: balance first, convert to moles, apply ratio, convert out. Every single time. Build the habit until it's automatic.