Moles and Mass Conversion Practice Problems

What the Hell Is a Mole and Why Do You Need to Know It?

A mole isn't an animal digging in your yard. In chemistry, a mole is a counting unit — like a dozen, but massive. One mole equals 6.02 × 10²³ things. Atoms, molecules, electrons, whatever you're counting.

This number has a name: Avogadro's number. You'll forget it exactly once, then write it on every sheet of paper you own.

Chemists use moles because atoms are stupid small. You can't weigh one carbon atom on a lab balance. But you can weigh 6.02 × 10²³ of them.

The Molar Mass Trap Most Students Fall Into

Molar mass is the mass of one mole of a substance, measured in grams per mole (g/mol). This number sits right under each element on the periodic table.

Here's what trips people up: molar mass is numerically equal to the atomic mass on the periodic table — but it's in grams, not atomic mass units.

Carbon has an atomic mass of 12.01 amu. Its molar mass is 12.01 g/mol. Same number, different unit. That's it.

The Conversion Chain You Must Memorize

Moles sit at the center of three common conversions:

• Moles ↔ Grams (using molar mass)
• Moles ↔ Particles (using Avogadro's number)
• Grams ↔ Particles (chaining the two above)

Every stoichiometry problem is just one of these conversions or a chain of them.

The Three Formulas That Actually Matter

Mass (g) = Moles × Molar Mass (g/mol)

Particles = Moles × 6.02 × 10²³

Moles = Mass (g) ÷ Molar Mass (g/mol)

That's the entire foundation. Memorize these three. Everything else is just math.

How to Actually Do These Problems

Step 1: Identify what you're starting with and what you need to find.

Step 2: Find the conversion factor you'll need. Get molar mass from the periodic table. Memorize Avogadro's number (or keep it on a card until you do).

Step 3: Set up the calculation so units cancel. If you want moles and you have grams, divide by molar mass. If you want grams and you have moles, multiply by molar mass.

Step 4: Do the math. Check your work. Double-check your significant figures.

Practice Problems with Solutions

Problem 1: Grams to Moles

How many moles are in 36 grams of water (H₂O)?

Solution:

First, find the molar mass of water. Hydrogen is 1.01 g/mol and oxygen is 16.00 g/mol.

H₂O = (2 × 1.01) + 16.00 = 18.02 g/mol

Now convert:

36 g ÷ 18.02 g/mol = 2.00 moles of H₂O

Problem 2: Moles to Grams

What is the mass of 0.75 moles of sodium chloride (NaCl)?

Solution:

Na = 22.99 g/mol, Cl = 35.45 g/mol

NaCl molar mass = 22.99 + 35.45 = 58.44 g/mol

0.75 mol × 58.44 g/mol = 43.83 g of NaCl

Problem 3: Moles to Particles

How many molecules are in 3.5 moles of CO₂?

Solution:

3.5 mol × 6.02 × 10²³ molecules/mol = 2.11 × 10²⁴ molecules of CO₂

Problem 4: Grams to Particles

How many atoms of iron are in 55.85 g of iron?

Solution:

55.85 g is exactly one mole of iron (molar mass = 55.85 g/mol).

1 mol × 6.02 × 10²³ atoms/mol = 6.02 × 10²³ atoms of Fe

Problem 5: Mixed Conversion

How many oxygen atoms are in 90 grams of glucose (C₆H₁₂O₆)?

Solution:

Find molar mass of glucose:

C₆ = 6 × 12.01 = 72.06

H₁₂ = 12 × 1.01 = 12.12

O₆ = 6 × 16.00 = 96.00

Total = 180.18 g/mol

Convert grams to moles: 90 g ÷ 180.18 g/mol = 0.50 mol of glucose

Convert moles to molecules: 0.50 mol × 6.02 × 10²³ = 3.01 × 10²³ molecules

Count oxygen atoms: each glucose has 6 oxygen atoms

3.01 × 10²³ × 6 = 1.81 × 10²⁴ oxygen atoms

Quick Reference: Conversion Cheat Sheet

Mistakes That Will Cost You Points

• Forgetting to find molar mass first. You can't convert grams to moles without it. The periodic table exists — use it.
• Multiplying when you should divide. More grams doesn't mean more moles if the substance is heavier. Check your setup.
• Dropping Avogadro's number entirely. If particles are involved, this number must appear somewhere in your calculation.
• Ignoring coefficients in formulas. H₂O has two hydrogens. O₂ has two oxygens. If your formula says it, count it.
• Rounding too early. Keep extra digits through the calculation. Round only your final answer.

Getting Started: Your First 10 Problems

Don't read more. Go solve these now:

1. Convert 50 g of NaOH to moles
2. Find the mass of 2.5 moles of H₂SO₄
3. Calculate molecules in 0.25 moles of O₂
4. How many moles in 100 g of CaCO₃?
5. Atoms in 1 mole of gold (Au)
6. Mass of 1 × 10²⁴ atoms of carbon
7. Moles in 250 g of glucose (C₆H₁₂O₆)
8. Oxygen atoms in 1 mole of CO₂
9. Mass of 4.5 × 10²³ molecules of N₂
10. Convert 2 moles of HCl to grams

Work through these without looking at answers first. Check your setup, not just your final number. The process matters more than getting it right by accident.

When you can solve these five types of problems consistently — grams to moles, moles to grams, moles to particles, particles to moles, and chained conversions — you know this material.