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Molality worksheet

Grades 11–12

Solve each problem and show your work. Round answers to 2 decimal places where needed.

1.
Problem 1.
- Solve for: Solute amount
- Measure the solute in: Grams (with molar mass)
- Molality (b): 0.6 mol/kg
- Molar mass of solute: 65 g/mol
- Mass of solvent: 425 g
Find: Mass of solute
Answer
2.
Problem 2.
- Solve for: Molality
- Measure the solute in: Grams (with molar mass)
- Mass of solute: 10 g
- Molar mass of solute: 40 g/mol
- Mass of solvent: 540 g
Find: Molality (b)
Answer
3.
Problem 3.
- Solve for: Solute amount
- Measure the solute in: Moles
- Molality (b): 0.4 mol/kg
- Mass of solvent: 600 g
Find: Moles of solute
Answer
4.
Problem 4.
- Solve for: Molality
- Measure the solute in: Grams (with molar mass)
- Mass of solute: 9 g
- Molar mass of solute: 48 g/mol
- Mass of solvent: 550 g
Find: Molality (b)
Answer
5.
Problem 5.
- Solve for: Solvent mass
- Measure the solute in: Grams (with molar mass)
- Molality (b): 0.5 mol/kg
- Mass of solute: 16 g
- Molar mass of solute: 50 g/mol
Find: Mass of solvent
Answer
6.
Problem 6.
- Solve for: Solvent mass
- Measure the solute in: Moles
- Molality (b): 0.4 mol/kg
- Moles of solute: 0.2 mol
Find: Mass of solvent
Answer
7.
Problem 7.
- Solve for: Solute amount
- Measure the solute in: Grams (with molar mass)
- Molality (b): 0.4 mol/kg
- Molar mass of solute: 55 g/mol
- Mass of solvent: 375 g
Find: Mass of solute
Answer
8.
Problem 8.
- Solve for: Molality
- Measure the solute in: Grams (with molar mass)
- Mass of solute: 11.75 g
- Molar mass of solute: 40 g/mol
- Mass of solvent: 475 g
Find: Molality (b)
Answer
9.
Problem 9.
- Solve for: Molality
- Measure the solute in: Moles
- Moles of solute: 0.3 mol
- Mass of solvent: 275 g
Find: Molality (b)
Answer
10.
Problem 10.
- Solve for: Molality
- Measure the solute in: Moles
- Moles of solute: 0.2 mol
- Mass of solvent: 560 g
Find: Molality (b)
Answer

Answer key: Molality worksheet

1. Mass of solute: 16.58 g
1. Molality uses kilograms, so divide the grams by 1,000
  $\frac{425 g}{1,000}$ $= 0.425 kg$
2. Find the moles you need: molality × kilograms, n = b × m
  $n$ $= 0.6 mol/kg \times 0.425 kg$ $= 0.255 mol$
3. Turn moles into grams you can weigh. Use the molar mass M: mass = n × M
  $mass$ $= 0.255 mol \times 65 g/mol$ $= 16.575 g$
2. Molality (b): 0.46 mol/kg
1. Turn grams into moles: divide by the molar mass, n = mass ÷ molar mass
  $n$ $= \frac{10 g}{40 g/mol}$ $= 0.25 mol$
2. Molality uses kilograms, so divide the grams by 1,000
  $\frac{540 g}{1,000}$ $= 0.54 kg$
3. Molality is moles per kilogram. So divide the moles by the kilograms: b = n ÷ m
  $b$ $= \frac{0.25 mol}{0.54 kg}$ $= 0.462963 mol/kg$
3. Moles of solute: 0.24 mol
1. Molality uses kilograms, so divide the grams by 1,000
  $\frac{600 g}{1,000}$ $= 0.6 kg$
2. Find the moles you need: molality × kilograms, n = b × m
  $n$ $= 0.4 mol/kg \times 0.6 kg$ $= 0.24 mol$
4. Molality (b): 0.34 mol/kg
1. Turn grams into moles: divide by the molar mass, n = mass ÷ molar mass
  $n$ $= \frac{9 g}{48 g/mol}$ $= 0.1875 mol$
2. Molality uses kilograms, so divide the grams by 1,000
  $\frac{550 g}{1,000}$ $= 0.55 kg$
3. Molality is moles per kilogram. So divide the moles by the kilograms: b = n ÷ m
  $b$ $= \frac{0.1875 mol}{0.55 kg}$ $= 0.340909 mol/kg$
5. Mass of solvent: 0.64 kg
1. Turn grams into moles: divide by the molar mass, n = mass ÷ molar mass
  $n$ $= \frac{16 g}{50 g/mol}$ $= 0.32 mol$
2. Find the kilograms of liquid you need: moles ÷ molality, m = n ÷ b
  $m$ $= \frac{0.32 mol}{0.5 mol/kg}$ $= 0.64 kg$
6. Mass of solvent: 0.5 kg
1. Find the kilograms of liquid you need: moles ÷ molality, m = n ÷ b
  $m$ $= \frac{0.2 mol}{0.4 mol/kg}$ $= 0.5 kg$
7. Mass of solute: 8.25 g
1. Molality uses kilograms, so divide the grams by 1,000
  $\frac{375 g}{1,000}$ $= 0.375 kg$
2. Find the moles you need: molality × kilograms, n = b × m
  $n$ $= 0.4 mol/kg \times 0.375 kg$ $= 0.15 mol$
3. Turn moles into grams you can weigh. Use the molar mass M: mass = n × M
  $mass$ $= 0.15 mol \times 55 g/mol$ $= 8.25 g$
8. Molality (b): 0.62 mol/kg
1. Turn grams into moles: divide by the molar mass, n = mass ÷ molar mass
  $n$ $= \frac{11.75 g}{40 g/mol}$ $= 0.29375 mol$
2. Molality uses kilograms, so divide the grams by 1,000
  $\frac{475 g}{1,000}$ $= 0.475 kg$
3. Molality is moles per kilogram. So divide the moles by the kilograms: b = n ÷ m
  $b$ $= \frac{0.29375 mol}{0.475 kg}$ $= 0.618421 mol/kg$
9. Molality (b): 1.09 mol/kg
1. Molality uses kilograms, so divide the grams by 1,000
  $\frac{275 g}{1,000}$ $= 0.275 kg$
2. Molality is moles per kilogram. So divide the moles by the kilograms: b = n ÷ m
  $b$ $= \frac{0.3 mol}{0.275 kg}$ $= 1.090909 mol/kg$
10. Molality (b): 0.36 mol/kg
1. Molality uses kilograms, so divide the grams by 1,000
  $\frac{560 g}{1,000}$ $= 0.56 kg$
2. Molality is moles per kilogram. So divide the moles by the kilograms: b = n ÷ m
  $b$ $= \frac{0.2 mol}{0.56 kg}$ $= 0.357143 mol/kg$