Moles to Grams Calculator: Mass of a Compound From Moles

This calculator turns an amount of substance in moles into the mass you need to weigh. Enter the number of moles and the molar mass of the compound; you get the mass in grams, the same mass in milligrams and the number of formula units (molecules, or ion pairs for a salt) in that amount.

Formula

m = n × M, and N = n × N_A.

• m: mass in grams (g)
• n: amount of substance in moles (mol)
• M: molar mass in grams per mole (g/mol), the sum of the atomic weights of every atom in the formula
• N: number of formula units; N_A = 6.02214076 × 10²³ mol⁻¹ (the Avogadro constant, exact)
• 1 g = 1000 mg

Worked Example

0.25 mol of glucose, C₆H₁₂O₆. Its molar mass is 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol.

1. m = 0.25 × 180.156 = 45.039 g
2. In milligrams: 45.039 × 1000 = 45,039 mg
3. Formula units: 0.25 × 6.02214076 × 10²³ = 1.506 × 10²³ molecules

Assumptions

• The molar mass belongs to the exact form you will weigh, including any water of crystallisation (a hydrate has a larger M than the anhydrous salt).
• The substance is pure; for a reagent of known purity p (as a fraction), weigh m ÷ p to get the stated moles of the compound itself.
• Atomic weights are the standard values used on the periodic table page; their last digit can differ slightly between tables.

How to read the result

The mass in grams is the number to put on the balance. Small amounts are easier to read in milligrams: 0.002 mol of glucose is about 360 mg (0.36 g). For the reverse direction, grams to moles, use the mass to mole calculator; if you do not know M, type the formula into the molar mass calculator.

Limits and common mistakes

• Using the molar mass of the anhydrous compound when the bottle holds a hydrate (or the reverse) gives a wrong mass, often by 10% or more.
• Entering millimoles as moles makes the mass 1000 times too large; divide millimoles by 1000 first.
• For a solution of a given molarity, first find the moles as concentration × volume in litres, then convert to grams; the molarity calculator does the reverse step.

Frequently Asked Questions

How many grams are in 0.25 mol of glucose?

About 45.04 g, using M = 180.156 g/mol.

What is the formula for moles to grams?

Grams = moles × molar mass (g/mol).

How do I convert moles to grams for any compound?

Find the molar mass from the formula (add the atomic weights), then multiply it by the number of moles.

Is the number of formula units the same as the number of atoms?

No. A glucose molecule holds 24 atoms, so the number of atoms is 24 times the number of molecules.

in n | Amount of substance | mol | 0.25 | 0
in mm | Molar mass | g/mol | 180.156 | 0.001
out m | Mass | g | n * mm | 3
out mg | Mass in milligrams | mg | m * 1000 | 0
out nn | Number of formula units | - | n * 6.02214076e23 | 3