1.5.9 Calculate Concentrations of Solutions | Edexcel IGCSE Chemistry Revision Notes 2019

Calculate Concentrations of Solutions

A solid substance that dissolves in a liquid is called a solute, the liquid is called a solvent and the two when mixed together form a solution
Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent
Concentration simply refers to the amount of solute there is in a specific volume of the solvent
The greater the amount of solute in a given volume then the greater the concentration
A general formula triangle for concentration is thus:

Concentration moles formula triangle, downloadable IB Chemistry revision notes

The concentration-moles formula triangle

It is useful to a chemist to express concentration in terms of moles per unit volume
Concentration can therefore be expressed in moles per decimetre cubed
- The units in the answer can be written as mol dm^-3 or mol/dm³:

You may have to convert from g dm^-3 into mol dm^-3and vice versa depending on the question
- To go from g dm^-3 to mol dm^-3:
  - Divide by the molar mass in grams
- To go from mol dm^-3 to g dm^-3:
  - Multiply by the molar mass in grams

Example 1:

Calculate the amount of solute, in moles, present in 2.5 dm³ of a solution whose concentration is 0.2 mol dm^-3.

Answer:

Calculating Concentrations WE1, downloadable IGCSE & GCSE Chemistry revision notes

Example 2:

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol dm^-3, when 80 g is dissolved in 500 cm³of water.

Relative atomic masses, A_r: Na = 23, H = 1, O = 16

Answer:

Calculating Concentrations WE2, downloadable IGCSE & GCSE Chemistry revision notes

Don't forget your unit conversions:

To go from cm³ to dm³: divide by 1000

To go from dm³ to cm³: multiply by 1000