Calculating Concentrations of Solutions

A solute is a substance that dissolves in a solvent. When a solvent and solute are mixed together, they form a solution. Water is a commonly used solvent in many chemical reactions.

The concentration of a solution refers to how much solute is dissolved in a specific volume of solvent. For a given volume, the higher the amount of solute, the greater the concentration of the solution.

If we know the volume and concentration of a solution, we can calculate the mass of the solute. The concentration can be found during reactions between acids and alkalis. The equation for calculating concentration is:

The triangle below can help you learn the equation and how to rearrange it.

Equation triangle linking mass, concentration and volume.

Each part of the equation can be rearranged to find the answer in the desired unit. For example, to convert a volume in cm³ to a volume in dm³, we divide by 1000. This is because 1 dm³ is equal to 1000 cm³.

Here are some examples of how to use the triangle to solve for each of the three variables:

  • To find the concentration: concentration = amount of solute / volume of solution
  • To find the volume of solution: volume of solution = amount of solute / concentration
  • To find the amount of solute: amount of solute = concentration × volume of solution

Calculating Concentration

Let’s consider an example to understand how to calculate concentration. Suppose we dissolve 20 g of solute in 250 cm³ of solution. We want to find the concentration of this solution.

First, we need to convert the volume from cm³ to dm³, so we divide 250 by 1000:

250 cm³ ÷ 1000 = 0.25 dm³

Now, we can use the equation for concentration:

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

= 20 g ÷ 0.25 dm3

Concentration (g/dm³) = 80 (g/dm³)

This means that the concentration of the solution is 80 g of solute per dm³ of solution.

Concentrations can also be expressed in moles per decimetre cubed (mol/dm³). To convert between the two units of concentration, we need to use the relative formula mass (Mr) of the solute.

Calculating the Amount of Solute

Let’s look at an example to understand how to calculate the amount of solute dissolved in a solution. Suppose we have 2 dm³ of a solution with a concentration of 0.1 g/dm³. We want to find the mass of the solute dissolved in this solution.

We use the equation for calculating the mass of solute, which is:

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

= 0.1 g/dm³ × 2 dm³

Mass of solute (g) = 0.2g

Therefore, there are 0.2 g of solute dissolved in 2 dm³ of a 0.1 g/dm³ solution.

Calculating Volume

Let’s consider an example to understand how to calculate the volume of a solution. Suppose we have a solution with a concentration of 24 g/dm³, which contains 12 g of solute. We want to find the volume of the solution.

We use the equation for calculating the volume of solution, which is:

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

= 12 g ÷ 24 g/dm³

Volume of solution (dm³) = 0.5 dm³

Note that 0.5 dm³ is the same as 500 cm³ (0.5 × 1000). Therefore, the volume of the solution is 500 cm³.

You’ve used 0 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up