Chemical Calculations- Practice Problems & Solutions

What Chemical Calculations Actually Are

Chemical calculations are the math problems that make chemistry work. They're how you figure out how much reagent you need, what concentration you're dealing with, or how much product you'll get from a reaction.

If you're taking any chemistry course, you'll face these. If you're working in a lab, you will use them every day. There's no way around it.

This guide cuts through the theory. You'll get practice problems with real solutions, the formulas you actually need, and the mistakes most students make.

The Core Calculations You Must Know

Molar Mass Calculations

Molar mass is the mass of one mole of a substance. You get it by adding up the atomic masses from the periodic table.

Example: Water (H₂O)

• Hydrogen: 1.01 g/mol × 2 = 2.02 g/mol
• Oxygen: 16.00 g/mol × 1 = 16.00 g/mol
• Total: 18.02 g/mol

Mole Conversions

The mole is your bridge between grams and number of particles.

Formula: moles = mass (g) ÷ molar mass (g/mol)

Molarity

Molarity (M) tells you concentration — moles of solute per liter of solution.

Formula: M = moles of solute ÷ liters of solution

Stoichiometry

Stoichiometry is the math of chemical reactions. It answers: "If I have X grams of reactant, how much product will I get?"

You use mole ratios from the balanced equation to convert between substances.

Dilution Calculations

Formula: M₁V₁ = M₂V₂

This is the dilution equation. M₁ is initial concentration, V₁ is initial volume, M₂ is final concentration, V₂ is final volume.

Practice Problems with Solutions

Problem 1: Finding Molar Mass

Question: Calculate the molar mass of sodium hydroxide (NaOH).

Solution:

• Sodium (Na): 22.99 g/mol
• Oxygen (O): 16.00 g/mol
• Hydrogen (H): 1.01 g/mol

Total = 22.99 + 16.00 + 1.01 = 39.99 g/mol

Problem 2: Converting Grams to Moles

Question: How many moles are in 50 grams of glucose (C₆H₁₂O₆)?

Solution:

Step 1: Find molar mass

• C: 12.01 × 6 = 72.06
• H: 1.01 × 12 = 12.12
• O: 16.00 × 6 = 96.00
• Total: 180.18 g/mol

Step 2: Convert

moles = 50 g ÷ 180.18 g/mol = 0.278 mol

Problem 3: Molarity Calculation

Question: You dissolve 0.5 moles of KCl in 2 liters of water. What is the molarity?

Solution:

M = 0.5 mol ÷ 2 L = 0.25 M

Problem 4: Stoichiometry Problem

Question: How many grams of water can you produce from 4 grams of hydrogen gas (H₂)?

Reaction: 2H₂ + O₂ → 2H₂O

Solution:

Step 1: Convert grams H₂ to moles

4 g ÷ 2.02 g/mol = 1.98 mol H₂

Step 2: Use mole ratio

2 mol H₂ produces 2 mol H₂O

So 1.98 mol H₂ produces 1.98 mol H₂O

Step 3: Convert moles H₂O to grams

1.98 mol × 18.02 g/mol = 35.7 g H₂O

Problem 5: Dilution Calculation

Question: You have 100 mL of 6 M HCl. You need 2 M HCl. How much water do you add?

Solution:

Use M₁V₁ = M₂V₂

(6 M)(100 mL) = (2 M)(V₂)

V₂ = 600 ÷ 2 = 300 mL total

Water to add = 300 - 100 = 200 mL

Quick Reference: Common Formulas

Getting Started: How to Solve Any Chemical Calculation

Follow this sequence. Every time. No exceptions.

Step 1: Identify What You're Solving For

Grams? Moles? Molarity? Know your target before you touch the numbers.

Step 2: Write Down What You Know

List your given values. Mass? Volume? Concentration? Put them on paper.

Step 3: Pick the Right Formula

Match your situation to the formula. If you have grams and need moles, that's mass ÷ molar mass. If you're diluting, that's M₁V₁ = M₂V₂.

Step 4: Plug In and Solve

Substitute your numbers. Cancel units that cancel. Do the math.

Step 5: Check Your Work

Are your units right? Does your answer make sense? If you're getting 500 liters from 1 mL of solution, something went wrong.

Common Mistakes That Will Cost You Points

• Forgetting to balance the equation before doing stoichiometry. The mole ratios come from the balanced equation. If it's not balanced, your answer is wrong.
• Confusing mass and moles. Students mix these up constantly. Mass is in grams. Moles is a count of particles.
• Not converting units. If you have mL and the formula needs liters, convert first. Doing it wrong or forgetting it will kill your answer.
• Using atomic mass wrong. Remember: average atomic mass from the periodic table. Use the value with the most decimal places for accuracy.
• Rounding too early. Keep extra digits through your calculation. Round only at the end.

Tools That Actually Help

• Periodic table — You need one. Physical or digital. Know how to read it.
• Scientific calculator — Learn the functions you'll use: exponent, log, inverse. Don't rely on your phone in an exam.
• Unit conversion practice — Get comfortable converting between g, mg, kg, L, mL. This comes up constantly.

That's it. Practice the problems above until you can do them without looking at the solutions. Work through similar problems from your textbook or class materials. The only way to get fast at this is repetition. 🔬