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Converting to and from moles

There are only three conversions and one rule about how they connect. Almost every mole problem is those three used once or twice in a row.

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Everything goes through moles

Grams on the left, particles on the right, moles in the middle, and litres of gas hanging off the middle. There is no direct route from grams to molecules; you convert to moles and out again. Drawing that map once, at the top of the page, turns most problems into "which arrows do I follow".

Grams and moles: divide or multiply by the molar mass

Moles = grams ÷ molar mass. Grams = moles × molar mass. The molar mass comes off the periodic table — add up the atomic masses in the formula, so H₂O is 2(1.008) + 16.00 = 18.02 g/mol. Getting the formula mass wrong is the single most common source of a wrong answer, so check it before you divide.

Moles and particles: Avogadro's number

Moles → particles, multiply by 6.022 × 10²³. Particles → moles, divide by it. "Particles" means whatever the formula unit is — molecules for CO₂, ions for Na⁺, atoms for Fe. Read the question for which one it wants, because it sometimes wants atoms of a specific element inside the molecule.

Gases at STP: 22.4 litres

One mole of any ideal gas takes up 22.4 L at STP. Litres ÷ 22.4 gives moles, moles × 22.4 gives litres. The catch is that it only holds at standard temperature and pressure — off STP you are into PV = nRT instead.

Let the units check the setup

Write each step as a fraction and cancel: 36.0 g × (1 mol ÷ 18.02 g) leaves mol, which is what you wanted. If the units do not cancel down to the unit in the question, the setup is wrong and no amount of careful arithmetic will rescue it. This is the habit that makes multi-step problems safe.

Two substances means a mole ratio

Crossing from one compound to another is only legal through the coefficients of a balanced equation. Grams of A → moles of A → (mole ratio) → moles of B → grams of B. Every stoichiometry problem you will meet is that sentence, and the balancing has to happen first.

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22 terms Chemistry

Mole Conversions

General chemistry · Stoichiometry

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Questions people ask

How do I convert grams to moles?

Divide the mass in grams by the molar mass in g/mol. 36.0 g of water ÷ 18.02 g/mol = 2.00 mol. The molar mass is the sum of the atomic masses in the formula, read off the periodic table.

How do I convert moles to grams?

Multiply the number of moles by the molar mass. 0.500 mol of CO₂ × 44.01 g/mol = 22.0 g.

How do I convert moles to molecules?

Multiply by Avogadro's number, 6.022 × 10²³. 2.00 mol of water × 6.022 × 10²³ = 1.20 × 10²⁴ molecules. To go the other way, divide.

Can I convert grams straight to molecules?

Not in one step. Divide by the molar mass to get moles, then multiply by 6.022 × 10²³. Everything routes through moles, which is why the mole map is worth drawing before you start.