Notes
The mole and molar mass
- 1 mol $= 6.02 \times 10^{23}$ particles (Avogadro's number).
- Molar mass (g/mol) is the sum of atomic masses in the formula: $\text{H}_2\text{O} = 2(1.01) + 16.00 = 18.02$ g/mol.
The mole map
| From | To | Do this |
|---|---|---|
| grams | moles | divide by molar mass |
| moles | grams | multiply by molar mass |
| particles | moles | divide by $6.02 \times 10^{23}$ |
| moles | particles | multiply by $6.02 \times 10^{23}$ |
| liters at STP | moles | divide by 22.4 L/mol |
| moles | liters at STP | multiply by 22.4 L/mol |
- Always pass through moles. For atoms inside molecules, multiply by atoms per formula.
- STP is 0 °C and 1 atm; 1 mol of any gas fills 22.4 L.
Topics in this unit
Each topic has its own short notes and practice.
Practice
$40.08 + 2(16.00) + 2(1.01) = 74.10$ g/mol. 57.09 forgets the subscript 2.
$\displaystyle \frac{45.0}{180.18} = 0.250$ mol.
$\displaystyle \frac{9.01}{18.02} = 0.500$ mol, and $0.500 \times 6.02 \times 10^{23} = 3.01 \times 10^{23}$ molecules. $9.03 \times 10^{23}$ is the number of atoms.
$\displaystyle \frac{16.0}{32.00} = 0.500$ mol, and $0.500 \times 22.4 = 11.2$ L.
With equal moles, the largest molar mass wins: $\text{CO}_2$ is 44.01 g/mol.
2.00 mol $\text{CO}_2$ contains 4.00 mol O atoms: $4.00 \times 6.02 \times 10^{23} = 2.41 \times 10^{24}$.
Free response
Write your answer first, then open the scoring guide.
Free-response question
An aspirin tablet contains 325 mg of aspirin, $\text{C}_9\text{H}_8\text{O}_4$. (C = 12.01, H = 1.01, O = 16.00)
(a) Calculate the molar mass of aspirin.
Show the answer and scoring guide
$9(12.01) + 8(1.01) + 4(16.00) = 108.09 + 8.08 + 64.00 = 180.17$ g/mol.
Check off the points you earned:
(b) How many moles of aspirin are in one tablet?
Show the answer and scoring guide
$325 \text{ mg} = 0.325$ g. $\displaystyle \frac{0.325 \text{ g}}{180.17 \text{ g/mol}} = 1.80 \times 10^{-3}$ mol.
Check off the points you earned:
(c) How many aspirin molecules and how many carbon atoms are in one tablet?
Show the answer and scoring guide
Molecules: $1.804 \times 10^{-3} \times 6.02 \times 10^{23} = 1.09 \times 10^{21}$. Carbon atoms: $9 \times 1.086 \times 10^{21} = 9.77 \times 10^{21}$.
Check off the points you earned: