Chemistry Clarity · chemistryclarity.com

Concentration Units: Quiz

Seven questions on mass percent, ppm, mole fraction, molality and converting between units, with explanations.

7 questionsIntermediate

7 questions

  1. Question 1Easy12.0 g of sugar is dissolved in 188 g of water. What is the mass percent of sugar?
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    Answer: 6.00 %

    Mass of solution = 12.0 g + 188 g = 200. g, so 12.0 g ÷ 200. g × 100 % = 6.00 %. 6.38 % divides by the water only.

  2. Question 2EasyWhich unit uses the mass of the solvent, not the solution?
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    Answer: Molality

    Molality is moles of solute per kilogram of solvent. Mass percent and ppm use the mass of the whole solution; molarity uses the volume of solution.

  3. Question 3MediumA 500. g water sample contains 0.45 mg of fluoride. What is the concentration in ppm?
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    Answer: 0.90 ppm

    0.45 mg = 0.00045 g; (0.00045 g ÷ 500. g) × 10⁶ = 0.90 ppm.

  4. Question 4MediumA solution contains 1.0 mol of ethanol and 4.0 mol of water. What is the mole fraction of ethanol?
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    Answer: 0.20

    x = 1.0 mol ÷ (1.0 mol + 4.0 mol) = 0.20. 0.25 divides by the water only.

  5. Question 5MediumWhat is the molality of 18.0 g of glucose (180.16 g/mol) dissolved in 90.0 g of water?
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    Answer: 1.11 mol/kg

    n = 18.0 g ÷ 180.16 g/mol = 0.0999 mol; m = 0.0999 mol ÷ 0.0900 kg = 1.11 mol/kg. The water mass must be in kg.

  6. Question 6HardConcentrated sulfuric acid is 98.0 % H₂SO₄ by mass, density 1.84 g/mL. What is its molarity? (H₂SO₄ 98.08 g/mol)
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    Answer: 18.4 mol/L

    1.000 L has a mass of 1840 g, of which 0.980 × 1840 g = 1803 g is H₂SO₄; 1803 g ÷ 98.08 g/mol = 18.4 mol in 1.000 L.

  7. Question 7MediumWhy is molality preferred to molarity for boiling-point elevation calculations?
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    Answer: It does not change with temperature

    Molality uses masses, which don't change on heating; molarity uses a volume, which expands as the solution warms.