How to Calculate Molar Mass
Molar mass is the mass of one mole of a substance in grams per mole. It is the bridge between the grams you can weigh and the moles a chemical equation talks in, which makes it the first step of most calculations.
The method
- Write out the formula and count how many of each atom it contains.
- Look up each element's atomic mass on the periodic table.
- Multiply each atomic mass by its count.
- Add the results. The unit is grams per mole.
Brackets multiply everything inside
In Ca(OH)2 the subscript 2 applies to both the oxygen and the hydrogen, giving two of each. Expanding the bracket before counting is the reliable habit: Ca(OH)2 becomes Ca, O, O, H, H.
The dot in a hydrate
In a formula such as CuSO4 dot 5H2O, the dot means the water is part of the compound. Calculate the molar mass of the salt, then add five times the molar mass of water.
Worked examples
Example 1
Question
Find the molar mass of H2O.
Answer
About 18.02 g/mol
Steps
- 1Count the atoms: 2 hydrogen and 1 oxygen.
- 2Atomic masses: H is 1.008 and O is 16.00.
- 3Hydrogen contribution: 2 x 1.008 = 2.016.
- 4Oxygen contribution: 1 x 16.00 = 16.00.
- 5Total: 2.016 + 16.00 = 18.016, which rounds to 18.02 g/mol.
Example 2
Question
Find the molar mass of Ca(OH)2.
Answer
About 74.10 g/mol
Steps
- 1Expand the bracket: one Ca, two O, two H.
- 2Calcium: 1 x 40.08 = 40.08.
- 3Oxygen: 2 x 16.00 = 32.00.
- 4Hydrogen: 2 x 1.008 = 2.016.
- 5Total: 40.08 + 32.00 + 2.016 = 74.096, which rounds to 74.10 g/mol.
Applying the 2 only to the oxygen gives 57.09 g/mol, which is the single most common error with bracketed formulas.
Example 3
Question
Find the molar mass of CuSO4 dot 5H2O.
Answer
About 249.7 g/mol
Steps
- 1Start with the anhydrous salt: Cu 63.55, S 32.07, and 4 x O at 16.00 giving 64.00.
- 2CuSO4 total: 63.55 + 32.07 + 64.00 = 159.62 g/mol.
- 3Water: one H2O is 18.02 g/mol, so five of them are 5 x 18.02 = 90.10.
- 4Add: 159.62 + 90.10 = 249.72 g/mol.
- 5Rounded, that is about 249.7 g/mol.
Common mistakes
Applying a bracket subscript to only the first element inside it.
It multiplies everything in the bracket. Expand it before counting.
Ignoring the water in a hydrate.
The water is part of the formula mass. Add it, multiplied by its coefficient.
Using the atomic number instead of the atomic mass.
The atomic mass is the decimal value, usually the larger number in the periodic table box.
Rounding each element before adding.
Keep the decimals through the sum and round once at the end.
Practice it
Work these out first, then open the answer. Stuck halfway? Select the problem and ask Solvecoon for the step you are missing.
1. Find the molar mass of CO2.
Show answer
About 44.01 g/mol: 12.011 + 2 x 16.00.
2. Find the molar mass of NaOH.
Show answer
About 40.00 g/mol: 22.99 + 16.00 + 1.008.
3. Find the molar mass of glucose, C6H12O6.
Show answer
About 180.16 g/mol: 72.066 from carbon, 12.096 from hydrogen, and 96.00 from oxygen.
Questions people ask
- What is the difference between molar mass and molecular mass?
- The numbers match. Molecular mass is in atomic mass units per molecule; molar mass is in grams per mole of substance, which is the version calculations use.
- How do I get from grams to moles?
- Divide the mass by the molar mass. Going the other way, multiply moles by molar mass. The units cancelling is your check.
- How many significant figures should I keep?
- Match the data in the question, usually three or four, and carry more through the working than you report.
- Does the state of the substance matter?
- No. Molar mass is a property of the formula, so it is the same for ice, water, and steam.
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