Worksheet - Concentration Calculations - Honors | Download Free ... - Free Printable

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Problem 1:

What is the molarity of 10.0 g of NaCl dissolved in water to make 100. mL of solution?

#### Step-by-step Solution:

1. Find moles of NaCl:
- Molar mass of NaCl = 22.99 (Na) + 35.45 (Cl) = 58.44 g/mol
- Moles = mass / molar mass = \( \frac{10.0\ \text{g}}{58.44\ \text{g/mol}} = 0.1709\ \text{mol} \)

2. Convert volume to liters:
- 100. mL = 0.100 L

3. Calculate molarity (M):
- Molarity = moles / volume in L = \( \frac{0.1709\ \text{mol}}{0.100\ \text{L}} = 1.71\ \text{M} \)

✔ Answer: 1.71 M NaCl

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Problem 2:

How many grams of glucose are in 50.0 mL of a 1.58 mol/L solution of glucose?

#### Solution:

1. Find moles of glucose:
- Moles = Molarity × Volume (in L)
- Volume = 50.0 mL = 0.0500 L
- Moles = \( 1.58\ \text{mol/L} \times 0.0500\ \text{L} = 0.0790\ \text{mol} \)

2. Convert moles to grams:
- Molar mass of glucose (C₆H₁₂O₆) = 6(12.01) + 12(1.008) + 6(16.00) = 180.16 g/mol
- Mass = moles × molar mass = \( 0.0790\ \text{mol} \times 180.16\ \text{g/mol} = 14.23\ \text{g} \)

✔ Answer: 14.2 g glucose (rounded to 3 significant figures)

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Problem 3:

How many grams of salt are in 500 g of a gasoline-salt mix that is 5% salt?

#### Solution:

- Percent concentration means:
- \( \frac{\text{mass of solute}}{\text{total mass}} \times 100\% = 5\% \)
- So:
- \( \frac{\text{mass of salt}}{500\ \text{g}} = \frac{5}{100} \)
- Mass of salt = \( 500 \times 0.05 = 25\ \text{g} \)

✔ Answer: 25 g salt

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Problem 4:

The EPA (Environmental Protection Agency) currently limits the concentration of thorium in drinking water to no more than 2 mg per liter of water. What is this concentration in parts per million (ppm)?

#### Solution:

- 1 ppm = 1 mg/L (for dilute aqueous solutions)
- Given: 2 mg/L of thorium

So:
- Concentration = 2 ppm

✔ Answer: 2 ppm

> 🔍 Explanation: In water, 1 mg/L is equivalent to 1 ppm because the density of water is approximately 1 g/mL, so 1 L of water ≈ 1000 g. Thus, 1 mg per 1000 g = 1 part per million.

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✔ Summary of Answers:

| Problem | Answer |
|--------|--------|
| 1 | 1.71 M NaCl |
| 2 | 14.2 g glucose |
| 3 | 25 g salt |
| 4 | 2 ppm |

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If you have any specific questions about these calculations, or if there are additional problems on the worksheet not shown here, feel free to ask!