AQA GCSE FOUNDATION Chemistry: Calculating Concentrations in Solutions

Calculating Concentrations in Solutions (g/dm³)

Introduction

Concentration tells us how much solute is dissolved in a given amount of solvent. In this tutorial, we will focus on expressing concentration in grams per cubic decimetre (g/dm³).

What is a Solute and a Solvent?

• Solute: The substance that dissolves in a solution (e.g., sugar in water).
• Solvent: The substance that dissolves the solute (e.g., water in a sugar solution).

Calculating Concentration

Concentration (g/dm³) = Mass of solute (g) / Volume of solution (dm³)

Example 1: Calculating Concentration

• Problem: A solution is made by dissolving 25g of sodium chloride (NaCl) in 250 cm³ of water. What is the concentration of the solution in g/dm³?

• Step 1: Convert volume to dm³

  • 1 dm³ = 1000 cm³
  • 250 cm³ = 250/1000 dm³ = 0.25 dm³

• Step 2: Apply the formula

  • Concentration (g/dm³) = 25 g / 0.25 dm³ = 100 g/dm³

• Answer: The concentration of the solution is 100 g/dm³.

Example 2: Calculating Mass of Solute

• Problem: A chemist needs to make 500 cm³ of a 0.5 g/dm³ sugar solution. How much sugar is needed?

• Step 1: Convert volume to dm³

  • 500 cm³ = 500/1000 dm³ = 0.5 dm³

• Step 2: Rearrange the formula to find mass

  • Mass of solute (g) = Concentration (g/dm³) x Volume of solution (dm³)

• Step 3: Substitute the values

  • Mass of solute (g) = 0.5 g/dm³ x 0.5 dm³ = 0.25 g

• Answer: The chemist needs 0.25 g of sugar.

Tips for Success

• Always remember to convert volumes to dm³ before applying the formula.
• Pay close attention to the units in the problem and make sure your answer has the correct units.
• Practice with different examples to build your confidence.

Key Concepts

• Concentration is a measure of how much solute is dissolved in a given amount of solvent.
• Concentration can be expressed in grams per cubic decimetre (g/dm³).
• We can use the formula: Concentration (g/dm³) = Mass of solute (g) / Volume of solution (dm³) to calculate concentration, mass of solute, or volume of solution.

Remember: Practice makes perfect!