6.3 Converting Mass and Moles
Notes
Molar mass is the bridge between grams and moles.
- Grams to moles: $\displaystyle n = \frac{\text{mass}}{\text{molar mass}}$.
- Moles to grams: $\text{mass} = n \times \text{molar mass}$.
- Example: 36.04 g of $\text{H}_2\text{O}$ is $\displaystyle \frac{36.04}{18.02} = 2.000$ mol.
- Example: 0.250 mol NaCl is $0.250 \times 58.44 = 14.6$ g.
- Set it up with units so grams cancel: $\displaystyle 36.04 \text{ g} \times \frac{1 \text{ mol}}{18.02 \text{ g}}$.
Practice
How many moles are in 88.02 g of $\text{CO}_2$? (molar mass 44.01 g/mol)
$\displaystyle \frac{88.02}{44.01} = 2.000$ mol.
What is the mass of 0.250 mol of NaCl? (molar mass 58.44 g/mol)
$0.250 \times 58.44 = 14.61 \approx 14.6$ g.
How many moles are in 10.0 g of $\text{CaCO}_3$? (Ca = 40.08, C = 12.01, O = 16.00)
Molar mass $= 40.08 + 12.01 + 48.00 = 100.09$ g/mol. $\displaystyle \frac{10.0}{100.09} = 0.0999$ mol.
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