How to Use This Molar Mass Calculator
- Molar mass: type a formula to get its molar mass, the mass each element contributes, and each element’s mass percent.
- Grams, moles, and particles: type a formula and an amount in grams, moles, or particles to convert to the other two, including the number of atoms of each element.
How to Calculate Molar Mass by Hand
- Count the atoms of each element. A subscript after parentheses multiplies everything inside: Ca(NO3)2 has 1 Ca, 2 N, and 6 O.
- Multiply each count by the element’s atomic mass from the periodic table.
- Add the subtotals and give the answer in g/mol.
For glucose, C6H12O6: 6(12.01) + 12(1.008) + 6(16.00) = 72.06 + 12.096 + 96.00 = 180.16 g/mol. The sum is reported to the hundredths place because most of the masses added only go to the hundredths, which is the addition rule for significant figures. Parentheses, hydrates, and a full set of practice problems are worked through in AP Chemistry molar mass.
Molar Mass, Molecular Weight, and Formula Mass
| Term | Meaning | Units | Water |
|---|---|---|---|
| Molar mass | Mass of one mole of a substance | g/mol | 18.02 g/mol |
| Molecular weight (molecular mass) | Mass of one molecule | amu | 18.02 amu |
| Formula mass | Mass of one formula unit of an ionic compound | amu | NaCl: 58.44 amu |
The number is the same for all three; only the units and the kind of particle change. That is why a molecular weight calculator and a molar mass calculator give the same answer.
How to Convert Grams to Moles and Particles
- Grams to moles: divide by the molar mass.
- Moles to grams: multiply by the molar mass.
- Moles to particles: multiply by Avogadro’s number, 6.022 × 1023.
For 25.0 g of water: 25.0 ÷ 18.02 = 1.39 mol, and 1.39 mol × 6.022 × 1023 = 8.35 × 1023 molecules. Each molecule has two H atoms, so there are 1.67 × 1024 hydrogen atoms. The full conversion map is in AP Chemistry mole conversions, and the meaning of the mole itself in AP Chemistry Avogadro’s number.
How to Find Percent Composition From a Formula
Divide the mass each element contributes by the molar mass and multiply by 100. In glucose, carbon contributes 72.06 of 180.16 g/mol, or 40.00% by mass; hydrogen is 6.71% and oxygen 53.29%. The calculator shows this for every formula. Working backward, from percent composition to a formula, is covered in AP Chemistry empirical and molecular formulas.
Which Atomic Masses the Calculator Uses
The calculator uses the atomic masses printed on the periodic table in College Board’s current AP Chemistry Course and Exam Description: two decimal places for most elements, such as 32.06 for sulfur, plus 1.008 for hydrogen and 207.2 for lead. Some older tables and textbooks list sulfur as 32.07, so answers for sulfur compounds can differ from those sources in the last digit. For elements with no printed mass, such as radon, the calculator uses the mass number of the longest-lived isotope and says so. Once you have a molar mass, the percent yield calculator carries it through a full reaction, and to see how stronger free-response answers could move your overall estimate, try the AP Chem Score Calculator.
Related Resources
- AP Chem Score Calculator
- AP Chemistry Molar Mass
- AP Chemistry Mole Conversions
- AP Chemistry Avogadro’s Number
- AP Chemistry Empirical & Molecular Formula
- Percent Yield Calculator
- Unit 1 Review: Atomic Structure and Properties
Frequently Asked Questions
How do you calculate molar mass?
Multiply the atomic mass of each element by the number of atoms of that element in the formula, then add the results. For water, H2O: 2(1.008) + 16.00 = 18.02 g/mol.
What is the difference between molar mass and molecular weight?
They have the same number. Molar mass is the mass of one mole in grams per mole (g/mol); molecular weight, or molecular mass, is the mass of one molecule in atomic mass units (amu). For ionic compounds the same sum is called the formula mass.
How do you convert grams to moles?
Divide the mass in grams by the molar mass. For example, 25.0 g of water ÷ 18.02 g/mol = 1.39 mol. To go from moles to grams, multiply by the molar mass instead.
How many particles are in one mole?
Avogadro’s number, 6.022 × 10^23, the value printed on the AP Chemistry equations sheet. Multiply moles by this number to get molecules, formula units, or atoms.
How do you find the molar mass of a hydrate?
Add the molar mass of the water molecules to the rest of the formula. For CuSO4·5H2O, add 159.61 g/mol for CuSO4 and 5 × 18.016 = 90.08 g/mol for the water, giving 249.69 g/mol. Type the formula with a dot, such as CuSO4.5H2O, and the calculator includes the water.
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