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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.

Identify the solute: sodium chloride
Identify the solvent: water
Identify the masses of solute and solvent and convert to the same units if necessary:
mass solute (sodium chloride) = 10g
mass solvent (water) = 90g
Calculate the weight ratio = mass solute ÷ mass solvent
weight ratio = 10 ÷ 90 = 0.11
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.

Identify the solute: sodium carbonate
Identify the solvent: water
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
Calculate the weight ratio = mass solute ÷ mass solvent
weight ratio = 0.250 ÷ 5 = 0.05
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?

Identify the solute: sucrose
Identify the solvent: water
Identify the known quantities:
mass solute (sucrose) = ?g
mass solvent (water) = 200g
weight ratio percentage = 4.2%
Convert the weight ratio percentage to a weight ratio by dividing by 100 :
weight ratio = 4.2 ÷ 100 = 0.042
Re-arrange the weight ratio equation to find the mass of solute:
mass solute = weight ratio x mass solvent
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?

Identify the solute: potassium chloride
Identify the solvent: water
Identify the known quantities:
mass solute (sucrose) = 312mg = 312 ÷ 1000 = 0.312g
mass solvent (water) = ?g
weight ratio percentage = 7.5%
Convert the weight ratio percentage to a weight ratio by dividing by 100 :
weight ratio = 7.5 ÷ 100 = 0.075
Re-arrange the weight ratio equation to find the mass of solute:
mass solvent = mass solute ÷ weight ratio
Substitute in the values:
mass solvent = 0.312 ÷ 0.075 = 4.16g

Practice Questions

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weight ratio concentration drill
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