Mol to Grams: Formula, Calculator, and Worked Example

Convert an amount in moles to the corresponding mass in grams with this calculator. Enter the number of moles and the substance’s molar mass in grams per mole; the result is the mass in grams. You’ll need the chemical formula or a reliable molar-mass value for the substance.

Formula

Mass in grams equals amount in moles multiplied by molar mass in grams per mole:

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. No conversion factor is needed when the amount is already in moles and the molar mass is in g/mol. If your amount is in millimoles, convert it to moles first by dividing by 1,000.

Worked Example

The calculator’s default values are 0.05 mol and 58.44 g/mol, the approximate molar mass of sodium chloride (NaCl). Substitute those values:

m = n × M

m = 0.05 mol × 58.44 g/mol

m = 2.922 g

The calculator displays the result to three decimal places: 2.922 g. This is a mathematical conversion, not a measurement of a particular sample.

Assumptions

• The substance is identified correctly, and its molar mass matches its chemical formula and stated form.
• The entered amount is in moles, and the molar mass is in grams per mole.
• The result is the calculated mass of pure substance; it does not account for impurities, moisture, purity corrections, or weighing uncertainty.

What You’ll Need

• The amount of substance in moles (mol).
• The substance’s chemical formula, so you can find or calculate its molar mass.
• A calculator, or the calculator above. A periodic table is useful if you need to calculate molar mass yourself.

Step-by-Step

1. Identify the substance. Confirm its complete chemical formula, including subscripts and parentheses. For example, calcium chloride is CaCl₂, not CaCl. If the material includes water of crystallization, such as a hydrate, include that water in the formula.
2. Write down the amount in moles. Use the value provided by your problem, reaction calculation, or measurement. Check whether it is in mol or a smaller unit such as mmol or µmol. Convert before using the formula: 1 mmol = 0.001 mol and 1 µmol = 0.000001 mol.
3. Find the molar mass in g/mol. Use a reliable reference value for the exact substance, or calculate it from the formula. For the calculation, multiply each element’s atomic weight by the number of its atoms, then add the contributions. Use consistent atomic-weight values and keep a few extra digits while calculating.
4. Multiply moles by molar mass. Apply mass (g) = amount (mol) × molar mass (g/mol). For example, 0.20 mol of a substance with molar mass 40.00 g/mol has mass 0.20 × 40.00 = 8.00 g.
5. Check the units and arithmetic. Moles should cancel, leaving grams. A quick scale check can catch errors: for a positive amount, the answer should be positive, and doubling the mole amount while keeping the substance the same should double the mass.
6. Round appropriately. Keep extra digits through the multiplication, then round the final mass to suit the precision of your inputs and the purpose of the calculation. If this is for bench work, follow your lab method, the safety data sheet (SDS) and your supervisor.

Common Mistakes

• Using the wrong molar mass. Similar names can refer to different compounds, and hydrated and anhydrous forms have different molar masses. Check the exact formula before calculating.
• Mixing up mol and mmol. One millimole is one-thousandth of a mole. Entering a mmol value as if it were mol makes the calculated mass 1,000 times too large.
• Using the wrong units for molar mass. The formula expects g/mol. If a reference gives kg/mol, convert it to g/mol before multiplying; 1 kg/mol = 1,000 g/mol.
• Adding atomic weights without counting subscripts. In CaCl₂, there are two chlorine atoms, so the chlorine contribution is twice chlorine’s atomic weight.
• Rounding too early. Rounding atomic weights or intermediate totals can shift the final result. Retain extra digits and round once at the end.
• Confusing theoretical mass with a sample’s actual mass. The calculation gives the mass for the stated amount of pure substance. A real sample may have a different purity or contain moisture.

Tips

• Write the formula with units before substituting numbers. This makes a unit mismatch easier to spot.
• For a compound, calculate molar mass by summing each element’s atomic weight times its atom count. For example, a formula with two hydrogen atoms uses 2 × the atomic weight of hydrogen.
• Use the molar mass for the correct chemical form, including hydrate water, counterions, or other components written in the formula.
• Keep the calculator’s displayed precision in perspective. The digits shown do not make the input values more precise than the data you started with.
• For a solution-preparation calculation, distinguish the mass of solute from the final solution volume. This mol-to-grams conversion alone does not determine the volume of solvent or final solution.

Troubleshooting

• The answer is 1,000 times too large or too small: Check whether the amount was entered in mmol or µmol instead of mol. Convert the amount to mol, or adjust it by the correct factor before calculating.
• The answer seems chemically unreasonable: Recheck the substance’s formula, parentheses, subscripts, and hydration state. Then verify that the molar mass is for that exact form.
• The calculator rejects an entry: Enter numerical values only in the input fields. The amount and molar mass must be nonnegative; do not type units or a comma into a number field.
• Your reference gives a different molar mass: Check the units, atomic-weight convention, and compound form. Small differences can result from rounding; larger differences often indicate a different formula or a salt/hydrate form.
• Your weighed mass does not match the calculated mass: Check balance units, tare, sample purity, and whether the target amount was calculated correctly. If the material is not pure, its total weighed mass and the mass of the pure compound are not the same.

Frequently Asked Questions

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

How do I find molar mass?
Add the atomic weights of all atoms in the chemical formula, counting each atom according to its subscript. The result is in g/mol when atomic weights are used in g/mol.

How many grams are in one mole?
It depends on the substance. One mole of a compound has a mass, in grams, numerically equal to its molar mass in g/mol.

How do I convert millimoles to grams?
Convert millimoles to moles by dividing by 1,000, then multiply by molar mass. Equivalently, grams = mmol × molar mass (g/mol) ÷ 1,000.

Can I use this formula for a mixture or solution?
Use it for the amount of a specified substance. For mixtures or solutions, first determine the moles of that particular component; the total mixture mass is not necessarily its mass.

How many decimal places should I report?
Follow the precision of the input data and the requirements of your calculation or lab method. Keep extra digits during the calculation and round the final answer once.

Related

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

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