Moles to Grams: Formula and Worked Example

To convert moles to grams, multiply the amount in moles by the substance’s molar mass in grams per mole: mass (g) = amount (mol) × molar mass (g/mol). Enter those two values in the calculator below to get the mass in grams, along with the molar mass used in the calculation.

Formula

The conversion is:

m = n × M

• m is mass, in grams (g).
• n is amount of substance, in moles (mol).
• M is molar mass, in grams per mole (g/mol).

The units cancel as follows: mol × g/mol = g. The calculator treats molar mass as an input because it depends on which substance is being measured. For a compound, obtain its formula and calculate its molar mass from the atomic weights of its elements, or use a reliable reference value for that exact chemical form.

Worked Example

Using the calculator’s default amount, n = 0.05 mol, and default molar mass, M = 58.44 g/mol:

1. Write the relationship: m = n × M.
2. Substitute the values: m = 0.05 mol × 58.44 g/mol.
3. Multiply: m = 2.922 g.

The calculator displays 2.9220 g at four decimal places. The molar mass used is 58.44 g/mol. These defaults use the approximate molar mass of sodium chloride (NaCl); the example is a calculation, not a preparation instruction.

Assumptions

• The entered amount and molar mass refer to the same chemical substance and formula unit.
• The molar mass is appropriate for the substance’s chemical form, including any waters of hydration or specified salt form.
• The result is a calculated mass; it does not account for sample purity, moisture, or measurement uncertainty.

What a Mole and Molar Mass Mean

A mole is the SI unit for amount of substance. One mole contains exactly 6.02214076 × 10²³ specified entities, such as atoms, molecules, ions, or formula units. Which entities are meant depends on how the substance is defined. For example, a mole of sodium chloride corresponds to a mole of NaCl formula units, while a mole of water corresponds to a mole of H₂O molecules.

Molar mass connects this particle-counting scale to a mass that can be measured. Its numerical value in grams per mole is closely related to the substance’s relative formula or molecular mass. For a molecule, add the atomic weights of every atom in its formula, counting repeated atoms. Using abridged standard atomic weights, water has a molar mass of approximately 2(1.008) + 15.999 = 18.015 g/mol. Thus, 1 mol of water has a mass of about 18.015 g.

For an ionic compound, use its formula unit rather than calling it a molecule. For a hydrate, include the waters of hydration shown in the formula. For example, a hydrated salt has a different molar mass from the anhydrous salt. Likewise, a stated salt form should not be substituted for the molar mass of a different form.

Finding and Using Molar Mass

Before converting, make sure the molar mass matches the substance named in the problem or on the sample label. A practical calculation has three parts:

1. Identify the chemical formula and the entity counted by the mole amount.
2. Calculate or look up the molar mass in g/mol. For a formula, sum each element’s atomic weight multiplied by its subscript.
3. Multiply the amount in moles by the molar mass, retaining units until they cancel.

For carbon dioxide, CO₂, the molar mass is approximately 12.011 + 2(15.999) = 44.009 g/mol. An amount of 0.250 mol therefore has a mass of 0.250 mol × 44.009 g/mol = 11.00225 g, or 11.0 g when rounded to three significant figures. The multiplication works the same way for elements, molecular compounds, and ionic compounds; what changes is the molar mass and the entity being counted.

If a problem gives millimoles rather than moles, convert first: 1 mmol = 0.001 mol. For instance, 25 mmol is 0.025 mol. Keeping the units visible helps prevent a factor-of-1,000 error.

Reading the Result and Avoiding Errors

The first calculator output is the calculated mass in grams. Its precision depends on the precision of the inputs, not merely the number of digits shown by a calculator. In a reported experimental result, avoid implying greater certainty than the measured amount or molar mass supports. The worked example shows the direct product; rounding for a final report may be appropriate.

• Check the units: multiplying moles by g/mol gives grams. Do not multiply by a molar mass stated in kg/mol without converting units.
• Check the formula: a missing subscript, hydrate, or counterion can change molar mass substantially.
• Distinguish mass from amount: grams are mass; moles are amount of substance. They are related but are not interchangeable.
• Do not invert the formula: moles to grams uses multiplication. To convert grams to moles, divide the mass by molar mass.
• Use the right chemical form: purity, hydration, or a different salt form may make the actual sample differ from the simple formula used in a calculation.

For bench work, follow your lab method, the safety data sheet (SDS), and your supervisor. This arithmetic alone does not establish a safe handling method or replace an approved procedure.

When This Conversion Is Useful—and Its Limits

Moles-to-grams conversion is useful when a calculation specifies a chemical amount but a balance measures mass. It appears in general chemistry exercises, stoichiometry, and study or lab-planning arithmetic. In a reaction calculation, first determine the required amount in moles from the balanced equation; then multiply by the molar mass to find the corresponding theoretical mass. A balanced equation’s coefficients express mole ratios, not direct gram ratios, because different substances have different molar masses.

The basic formula assumes the entered mole amount is already known and that the molar mass applies to the substance as represented. It does not calculate reaction yield, sample purity, or how much of a mixture is a particular compound. If the starting material is impure, simply treating its entire measured mass as pure compound can produce an incorrect amount in moles. Those questions require additional information and calculations.

Frequently Asked Questions

How do I convert moles to grams?
Multiply the number of moles by the molar mass in g/mol. The product is the mass in grams.

How do I find molar mass?
Use the chemical formula and add the atomic weights of all atoms in it, multiplying each element’s atomic weight by its subscript. Check whether the formula includes waters of hydration or a particular salt form.

How do I convert grams to moles?
Use the inverse calculation: moles = mass in grams ÷ molar mass in g/mol. The units reduce to mol.

What is the mass of 1 mole?
It depends on the substance. One mole of water has a mass of about 18.015 g, while one mole of carbon dioxide has a mass of about 44.009 g.

Do I need to convert millimoles to moles first?
Yes, if using this formula with molar mass in g/mol. Divide millimoles by 1,000 to obtain moles; for example, 25 mmol = 0.025 mol.

Related

• Molecules to Grams Calculator: Convert with Molar Mass
• How Many Protons Are in Potassium (K)?
• Moles of Electrons: Formula and Calculator

in n | Amount of substance | mol | 0.05 | 0
in mm | Molar mass | g/mol | 58.44 | 0.001
out m | Mass | g | n * mm | 4
out mmv | Molar mass used | g/mol | mm | 3