ChemistryGeneral ChemistryHigh school

Moles and Molar Mass

Convert between grams, moles and particles.

Atoms are far too small to count individually, so chemists count them in moles. One mole is $6.022 \times 10^{23}$ particles, Avogadro's number. The molar mass of a substance, in grams per mole, is the sum of the atomic masses on the periodic table, so one mole of water ( $\text{H}_2\text{O}$ ) has a mass of about $2(1.008) + 16.00 = 18.02$ g.

Molar mass is the bridge between grams, which you can weigh, and moles, which tell you how many particles react. Almost every quantitative chemistry problem passes through moles at some point.

Key terms

Mole: Amount of substance; one mole contains Avogadro's number of particles.
$N_A = 6.022 \times 10^{23}\,\text{mol}^{-1}$

Key ideas

moles $= \frac{\text{mass}}{\text{molar mass}}$ ; particles $=$ moles $\times 6.022 \times 10^{23}$ .
Subscripts multiply the atomic mass of the element they follow.
Parentheses multiply everything inside, as in $\text{Ca(OH)}_2$ .
Keep units in every step to catch inverted conversions.

Worked example

How many moles are in 88 g of carbon dioxide, $\text{CO}_2$ ?

Molar mass: $12.01 + 2(16.00) = 44.01$ g/mol.
moles $= \frac{88}{44.01} \approx 2.00$ mol.
Molecules: $2.00 \times 6.022 \times 10^{23} \approx 1.20 \times 10^{24}$ .

Answer: about $2.00$ mol

Common mistakes

Multiplying by molar mass when converting grams to moles.
Ignoring subscripts or parentheses.
Confusing molecules with atoms (one $\text{CO}_2$ molecule has three atoms).

Practice questions

Try each one, then open the answer. Numerical answers on this page are verified by an automated calculation check.

1.Molar mass of $\text{NaCl}$ (Na 22.99, Cl 35.45), in g/mol.
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$58.44$
2.Moles in 36.04 g of water (18.02 g/mol).
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$2$
3.Mass of 0.5 mol of $\text{O}_2$ (32.00 g/mol), in grams.
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$16$

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Quizzes and flashcards

Flashcards: Chemistry Core Formulas