Solutions – Moles and Concentration

Solutions

A solution is a mixture of different elements or compounds and is made when a solute is dissolved into a solvent. 

Solute – a substance that can be dissolved in a solvent to form a solution. This can be a solid, liquid or gas.

Solvent – a substance that dissolves a solute to form a solution. They are usually liquids (but can be a solid or gas).

In a solution there is a greater amount of the solvent than the solute. There can be more than one solute in a single solution.

Solutions are homogeneous mixtures, meaning that that the composition is uniform (the solutes are distributed evenly throughout the solvent).

Let’s take salt water as an example – salt is the solute and water is the solvent. The salt is dissolved in the water and distributed evenly to form the salt water solution.

Moles and Concentration

Concentration – the amount of a solute in moles per unit volume.

This can be calculated using the following equation:

Equation to calculate the concentration of a solution

Where n is the amount of the solute in moles and V is the volume of the solution. The unit commonly used in chemistry for volume is decimetres cubed (dm3).

The most common unit for concentration is moles per decimetres cubed – mol dm-3 (this is the same as mol/L).

This is the molar concentration (or molarity) of a solution.

NOTE: If a volume is given in cm3 it can be converted to dm3 by dividing by 1000.

The concentration of a solution tells us how much of the solute is dissolved in the solvent. For example, a concentration of 5mol dm-3 means that 5 moles of the solute are dissolved in every 1 dm-3 of the solution.

Worked Examples

Example 1

A solution has a concentration of 6 mol dm-3. Calculate the amount of solute in moles that is dissolved in 15 dm3 of the solution.

Solution to Example 1

Example 2

4 moles of a solute are dissolved in 125 cm3 of a solution. Calculate the concentration of the solution in mol dm-3.

Solution to Example 2

Example 3

5g of HCl are dissolved into 0.5 dm3 of a solution. Calculate the concentration of the solution in mol dm-3. The relative atomic masses of hydrogen and chlorine are given below:

H = 1.0u
Cl = 35.5u

This question requires knowledge of Moles and Atomic Masses.

Solution to Example 3

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