Convert Moles to Grams: Formula and Calculator

Convert moles to grams by multiplying the amount in moles by the substance’s molar mass in grams per mole: grams = moles × molar mass. Enter those two values in the calculator below to get the mass in grams, with a worked example and notes on units and accuracy.

Formula

The formula is m = n × M, where m is mass in grams (g), n is amount of substance in moles (mol), and M is molar mass in grams per mole (g/mol). The units cancel as follows: mol × g/mol = g. The calculator’s first result is the mass in grams; its second result converts the input amount to millimoles, where 1 mol = 1,000 mmol.

Molar mass is the mass of one mole of a substance. For sodium chloride, NaCl, an approximate molar mass is 58.44 g/mol: sodium contributes about 22.990 g/mol and chlorine about 35.45 g/mol. The calculator treats molar mass as an input because it changes with the chemical formula and, for some materials, the specified form or composition.

Worked Example

Using the calculator’s default values, convert 0.05 mol of a substance with a molar mass of 58.44 g/mol:

1. Write the relationship: mass = amount × molar mass.
2. Substitute the values: mass = 0.05 mol × 58.44 g/mol.
3. Multiply: mass = 2.922 g, which the calculator displays as 2.922 g to three decimal places.
4. Convert the amount to millimoles: 0.05 mol × 1,000 mmol/mol = 50.0 mmol.

Assumptions

• The amount and molar mass refer to the same chemical substance and its specified chemical form.
• The entered molar mass is appropriate for the material’s formula and composition; the calculator does not identify a compound or look up its molar mass.
• The result is a mathematical mass conversion. It does not account for purity, moisture, impurities, or material lost during handling.

How to Convert Moles to Grams

First determine the amount of substance in moles. It may be given directly in a problem or calculated from other information, such as a concentration and volume. Then find the molar mass for the exact chemical formula. Multiply the two values using the formula above.

• Check the amount unit. If the amount is in millimoles, convert it to moles by dividing by 1,000 before using a molar mass in g/mol. Alternatively, multiply mmol by g/mol and divide by 1,000 to obtain grams.
• Use the correct molar mass. Add the atomic masses in the formula, counting each element according to its subscript. Include waters of crystallization or other parts of a hydrate formula when they are present.
• Multiply and label the result. Moles multiplied by grams per mole gives grams. Keep units in the calculation to catch conversion errors.
• Round appropriately. Do not report more meaningful precision than the input values support. The calculator formats the result to three decimal places, but that display setting does not make the inputs more precise.

For example, the molar mass of water, H₂O, is about 18.015 g/mol using H = 1.008 and O = 15.999. A quantity of 0.20 mol therefore corresponds to 0.20 × 18.015 = 3.603 g before rounding to match the precision of the stated amount.

Finding the Right Molar Mass

For a pure compound with a known formula, calculate molar mass by summing the standard atomic weights for all atoms in one formula unit. For glucose, C₆H₁₂O₆, that calculation is approximately (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 180.156 g/mol. Atomic weights are commonly listed in periodic tables; small differences in rounding can produce slightly different published molar masses.

Be especially careful with salts, hydrates, and substances sold in different forms. An anhydrous compound and its hydrate have different formulas and molar masses. Likewise, a compound’s free-acid form and a salt form are not interchangeable for this calculation. Read the chemical formula and form specified by the problem or material documentation rather than assuming the shortest name identifies the correct mass.

For a mixture or a material with variable composition, one single molar mass may not describe the sample. A calculated mass is only as reliable as the value entered. If the substance’s identity or composition is uncertain, resolve that before treating the numerical answer as a measured quantity.

Common Errors and Limits

• Dividing instead of multiplying. To convert moles to grams, multiply by g/mol. Dividing grams by g/mol is the reverse operation and gives moles.
• Mixing up millimoles and moles. Using 50 mmol as though it were 50 mol makes the calculated mass 1,000 times too large. Convert 50 mmol to 0.050 mol first.
• Using the wrong chemical form. A hydrate, salt, or different molecular formula has a different molar mass, even if its common name is similar.
• Confusing theoretical mass with what is weighed. The equation calculates the mass corresponding to an amount of pure substance. It does not correct for reagent purity or handling losses. If purity matters, use the appropriate documented purity correction separately.
• Overinterpreting decimal places. A result such as 2.922 g is arithmetic output, not proof that a sample can be weighed or known to that precision.

This conversion alone cannot determine moles from a mass: for that, rearrange the equation to n = m ÷ M. Nor does it calculate the amount available in a solution unless concentration and volume are also known. It is a unit conversion based on a supplied amount and molar mass, not a chemical analysis or a measure of reaction yield.

Frequently Asked Questions

What is the formula to convert moles to grams?
Multiply the amount in moles by molar mass in g/mol: mass (g) = moles (mol) × molar mass (g/mol).

How do I convert 2 moles to grams?
You need the substance’s molar mass. Multiply 2 mol by that value in g/mol. For example, if the substance has a molar mass of 18.015 g/mol, 2 mol corresponds to 36.03 g.

Can I convert moles to grams without molar mass?
No. Molar mass depends on the substance, so the amount in moles alone is not enough to find its mass in grams.

How do I convert millimoles to grams?
Either convert millimoles to moles by dividing by 1,000 and then multiply by g/mol, or use grams = millimoles × molar mass ÷ 1,000.

Why is my answer different from another calculator?
Check that both calculations use the same chemical formula, chemical form, atomic-weight rounding, input units, and displayed precision. A hydrate or salt can have a different molar mass from the anhydrous or free-acid form.

Related

• Moles to Grams Calculator: Mass of a Compound From Moles
• Molecules to Moles Calculator: Divide by the Avogadro Constant
• Mass to Mole Calculator: Grams to Moles From the Molar Mass

in n | Amount of substance | mol | 0.05 | 0
in mm | Molar mass | g/mol | 58.44 | 0
out g | Mass | g | n * mm | 3
out mmol | Amount in millimoles | mmol | n * 1000 | 1