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Weight Ratio Percentage Concentration

Key Concepts

  • Weight Ratio Percentage is a way of expressing the concentration of a solution.

  • Solubilities are often expressed as a weight ratio, eg, grams of solute per 100g of water

  • Weight Ratio concentration = mass solute ÷ mass solvent

  • Weight Ratio Percentage concentration = mass solute ÷ mass solvent x 100
    OR weight ratio percentage concentration = weight ratio x 100

  • Any units of mass (weight) can be used for the calculation but the mass of solute and the mass of solvent must be in the same units.

  • Weight Ratio Percentage concentration must NOT be confused with Weight (Mass) Percentage concentration (w/w).

Example : Weight Ratio Percentage Concentration Calculation

1. 10g of sodium chloride is dissolved in 90g of water.
    Calculate the weight ratio percentage concentration of the solution.
  1. Identify the solute: sodium chloride
    Identify the solvent: water

  2. Identify the masses of solute and solvent and convert to the same units if necessary:
    mass solute (sodium chloride) = 10g
    mass solvent (water) = 90g

  3. Calculate the weight ratio = mass solute ÷ mass solvent
    weight ratio = 10 ÷ 90 = 0.11

  4. Calculate the weight ratio percentage = weight ratio x 100
    weight ratio percentage concentration = 0.11 x 100 = 11%
2. 250mg of sodium carbonate is dissolved in 5g of water.
    Calculate the weight ratio percentage concentration of the solution.
  1. Identify the solute: sodium carbonate
    Identify the solvent: water

  2. Identify the masses of solute and solvent and convert to the same units if necessary:
    mass solute (sodium carbonate) = 250mg = 250 ÷ 1000 = 0.250g
    mass solvent (water) = 5g

  3. Calculate the weight ratio = mass solute ÷ mass solvent
    weight ratio = 0.250 ÷ 5 = 0.05

  4. Calculate the weight ratio percentage = weight ratio x 100
    weight ratio percentage concentration = 0.05 x 100 = 5%

Example : Mass Solute or Solvent Calculation

1. An aqueous sucrose solution contains 200g water and has a weight ratio percentage concentration of 4.2%.
    What is the mass of sucrose in grams?
  1. Identify the solute: sucrose
    Identify the solvent: water

  2. Identify the known quantities:
    mass solute (sucrose) = ?g
    mass solvent (water) = 200g
    weight ratio percentage = 4.2%

  3. Convert the weight ratio percentage to a weight ratio by dividing by 100 :
    weight ratio = 4.2 ÷ 100 = 0.042

  4. Re-arrange the weight ratio equation to find the mass of solute:
    mass solute = weight ratio x mass solvent

  5. Substitute in the values:
    mass solute = 0.042 x 200 = 8.4g
2. An aqueous potassium chloride solution with a weight ratio percentage concentration of 7.5% contains 312mg of potassium chloride.
    What mass of water in grams is present?
  1. Identify the solute: potassium chloride
    Identify the solvent: water

  2. Identify the known quantities:
    mass solute (sucrose) = 312mg = 312 ÷ 1000 = 0.312g
    mass solvent (water) = ?g
    weight ratio percentage = 7.5%

  3. Convert the weight ratio percentage to a weight ratio by dividing by 100 :
    weight ratio = 7.5 ÷ 100 = 0.075

  4. Re-arrange the weight ratio equation to find the mass of solute:
    mass solvent = mass solute ÷ weight ratio

  5. Substitute in the values:
    mass solvent = 0.312 ÷ 0.075 = 4.16g


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