Mole Conversions Chemistry- Practice Problems with Solutions

What the Mole Actually Is (And Why You Need to Master It)

The mole isn't an animal you dig up in your yard. In chemistry, it's a unit of measurement—just like a dozen means 12, a mole means 6.02 × 10²³ things.

This number is called Avogadro's number, and it's the bridge between the atomic scale and the grams you measure in the lab. Most chemistry classes spend weeks on mole conversions because every single calculation afterward depends on this skill.

You either learn it now, or you drown in every problem that follows. There's no middle ground.

The Two Numbers You Must Memorize

Before touching any problem, these two values need to be tattooed on your brain:

• Avogadro's number: 6.02 × 10²³ particles per mole
• Molar mass: grams per mole (found on the periodic table)

That's it. Two numbers. Everything else is just multiplication and division.

The Three Conversions You Need to Know

Mole conversions only go three directions. Master these three, and you're set:

1. Moles → Particles (multiply by Avogadro's number)
2. Moles → Grams (multiply by molar mass)
3. Moles → Liters (multiply by 22.4 L/mol for gases at STP)

The Golden Rule

Every conversion follows the same pattern: What you have × conversion factor = What you want

Cancel units like they owe you money. If your units don't cancel correctly, you set up the problem wrong.

Step-by-Step: How to Solve Any Mole Conversion Problem

1. Identify what you're starting with. (moles, grams, particles, or liters?)
2. Identify what you need to find. (the target unit)
3. Find the bridge. What's between your starting point and target?
4. Set up the calculation. Units must cancel to leave your target unit
5. Solve. Punch numbers, check your work

Practice Problems with Solutions

Problem 1: Moles to Particles

Question: How many atoms are in 2.5 moles of carbon?

Solution:

2.5 mol C × (6.02 × 10²³ atoms / 1 mol) = 1.505 × 10²⁴ atoms

Simple. Multiply by Avogadro's number. The mole cancels out. You're left with atoms.

Problem 2: Grams to Moles

Question: How many moles are in 50 grams of water (H₂O)?

Solution:

First, find molar mass of H₂O:

• H: 1.01 g/mol × 2 = 2.02 g/mol
• O: 16.00 g/mol
• Total: 18.02 g/mol

Now convert:

50 g ÷ 18.02 g/mol = 2.77 mol H₂O

Notice: when converting from grams TO moles, you divide by molar mass.

Problem 3: Moles to Grams

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

Solution:

Molar mass of NaCl:

• Na: 22.99 g/mol
• Cl: 35.45 g/mol
• Total: 58.44 g/mol

0.75 mol × 58.44 g/mol = 43.83 g NaCl

Problem 4: Moles to Liters (Gas at STP)

Question: How many liters does 3 moles of oxygen gas (O₂) occupy at STP?

Solution:

3 mol × 22.4 L/mol = 67.2 L O₂

This only works at STP (standard temperature and pressure: 0°C and 1 atm). Different conditions require the ideal gas law.

Problem 5: Particles to Grams (Two-Step)

Question: How many grams are in 1 × 10²⁴ molecules of CO₂?

Solution:

Step 1: Convert molecules to moles

1 × 10²⁴ molecules ÷ (6.02 × 10²³ molecules/mol) = 1.66 mol CO₂

Step 2: Convert moles to grams

Molar mass of CO₂:

• C: 12.01 g/mol
• O: 16.00 g/mol × 2 = 32.00 g/mol
• Total: 44.01 g/mol

1.66 mol × 44.01 g/mol = 73.1 g CO₂

When problems go through two units, solve step by step. Don't try to do both at once unless you're confident.

Conversion Cheat Sheet Table

Where Students Actually Screw Up

1. Forgetting to find molar mass first. You can't go from grams to moles without knowing how many grams per mole your substance has. Look it up on the periodic table.

2. Using the wrong molar mass. H₂O is 18 g/mol. Not 2 + 16. You have to account for the subscript.

3. Forgetting to use scientific notation. Avogadro's number is huge. Writing 602000000000000000000000 is useless. Use 6.02 × 10²³.

4. Not canceling units. If you end up with atoms × grams after your setup, you messed up. Units must cancel to leave your target.

5. Mixing up the formulas. Moles to grams = multiply. Grams to moles = divide. Students constantly reverse this.

Quick Practice Drill

Try these without looking at solutions first:

1. Convert 90 grams of glucose (C₆H₁₂O₆) to moles
2. Find how many molecules are in 0.25 moles of sulfur dioxide (SO₂)
3. Calculate the volume of 5 moles of nitrogen gas (N₂) at STP

Check your answers. If you got them wrong, go back and identify which step failed. That's the only way to fix it.

The Bottom Line

Mole conversions are arithmetic with a periodic table. There are no excuses for getting them wrong once you understand the pattern: find your starting point, find your target, use the right conversion factor, cancel your units, calculate.

Students who struggle with this haven't memorized the two key numbers (Avogadro's number and molar mass), or they don't understand how to set up the fraction. Fix those two things, and every mole problem becomes trivial.