Converting grams to moles is one of the most fundamental calculations in chemistry. Whether you're a high school student working on your first stoichiometry problems or a lab researcher needing quick conversions, this guide will walk you through everything you need to know.
The Core Formula
The gram-to-mole conversion uses one simple formula:
n = number of moles (mol)
m = mass of substance (g)
M = molar mass of substance (g/mol)
The molar mass (M) is the mass of one mole of a substance, expressed in grams per mole (g/mol). You can find it by summing the atomic masses of all atoms in the molecular formula.
Step-by-Step Method
- Identify the substance and write its molecular formula
- Calculate the molar mass using atomic masses from the periodic table
- Apply the formula: divide the given mass (g) by the molar mass (g/mol)
- State the answer with correct units (mol) and appropriate significant figures
Worked Example 1: Water (H₂O)
Problem: How many moles are in 36 g of water?
Step 2: M = (2 × 1.008) + (1 × 15.999) = 18.015 g/mol
Step 3: n = 36 g ÷ 18.015 g/mol = 1.999 mol
Answer: ≈ 2.00 mol of water
Worked Example 2: Sodium Chloride (NaCl)
Problem: How many moles are in 117 g of NaCl?
Step 2: M = 22.990 + 35.453 = 58.443 g/mol
Step 3: n = 117 g ÷ 58.443 g/mol = 2.002 mol
Answer: ≈ 2.00 mol of NaCl
Worked Example 3: Glucose (C₆H₁₂O₆)
Problem: How many moles are in 90 g of glucose?
Step 2: M = (6×12.011) + (12×1.008) + (6×15.999) = 72.066 + 12.096 + 95.994 = 180.156 g/mol
Step 3: n = 90 g ÷ 180.156 g/mol = 0.4996 mol
Answer: ≈ 0.500 mol of glucose
Common Mistakes to Avoid
- Using atomic mass instead of molar mass: For diatomic elements (O₂, H₂, N₂), remember to double the atomic mass
- Wrong significant figures: Report your answer to the same number of significant figures as the least precise measurement
- Forgetting units: Always include mol in your answer and make sure you're working in grams, not kg or mg
- Using the wrong formula: n = m/M (grams to moles), not m/n or anything else
Quick Reference: Moles Formula Triangle
Use the formula triangle as a memory aid:
Cover the unknown to see the formula: cover n → m/M, cover m → n×M, cover M → m/n
Practice Problems
- How many moles are in 22 g of CO₂? (M = 44.010 g/mol)
- Convert 50 g of NaOH to moles (M = 39.997 g/mol)
- How many moles of H₂SO₄ are in 196 g? (M = 98.079 g/mol)
- Find the moles in 14 g of nitrogen gas N₂ (M = 28.014 g/mol)
- Convert 250 g of glucose C₆H₁₂O₆ to moles (M = 180.156 g/mol)
Show Answers
- 22 ÷ 44.010 = 0.500 mol
- 50 ÷ 39.997 = 1.250 mol
- 196 ÷ 98.079 = 1.999 mol ≈ 2.00 mol
- 14 ÷ 28.014 = 0.500 mol
- 250 ÷ 180.156 = 1.388 mol
Use Our Free Calculator
Rather than doing the math by hand, use our Gram to Mole Calculator. It automatically looks up molar masses for 150+ substances and shows step-by-step solutions — perfect for checking your work.
Try the Calculator Now
Get instant step-by-step solutions for your grams to moles problems.
Use Calculator →