Solution concentration - IGCSE Chemistry Revision Notes

Calculate Concentrations of Solutions

A solute is a solid substance that dissolves into a liquid
- The amount of solute can be expressed in grams (g) or moles (mol)
A solvent is the liquid that a solute dissolves in
- The amount / volume of a solvent is measured in cm³ or dm³
Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent
A solution is the mixture formed when a solute dissolves in a solvent
- The amount / volume of a solution measured in cm³ or dm³
Concentration 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, the greater the concentration
- Concentration is sometimes commonly referred to as strength
  - For example, dissolving more coffee in hot water results in a stronger coffee

Typically, concentration is expressed in terms of the amount of substance per dm³
- Therefore, the units of concentration are:
  - g / dm³
  - mol / dm³
It is more useful to a chemist to express concentration in terms of moles per unit volume rather than mass per unit volume
To calculate concentration in mol / dm3 we use the following equation:

$concentration (mol / {dm}^{3}) = \frac{number of moles of solute (mol)}{volume of solution ({dm}^{3})}$

Concentration moles formula triangle, downloadable IB Chemistry revision notes

The concentration-moles formula triangle

Volumes are often expressed in cm³, but dm³ must be used when calculating concentration
- To convert cm³ to dm³, divide by 1000
- To convert dm³ to cm³, multiply by 1000

How to convert from cm3 to dm3 and back

Converting between cm³ and dm³

Worked example

Calculate the amount of solute, in moles, present in 2.5 dm³ of a solution whose concentration is 0.2 mol / dm³.

Answer:

Write down the information you are given in the question:
- Concentration of solution: 0.2 mol / dm³
- Volume of solution: 2.5 dm³
Calculate the number of moles:
- Moles = concentration x volume
- Moles = 0.2 x 2.5 = 0.5 mol

Worked example

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol / dm³, when 80 g is dissolved in 500 cm³of water.

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

Answer:

Calculate the M_r of NaOH:
- 23 + 16 + 1 = 40
Determine the number of moles of NaOH:
- 40 g = 1 mole
- So, 80 g = 2 moles
Convert cm³ to dm³:
- $\frac{500}{1000}$ = 0.5 dm³
Calculate the concentration:
- Concentration = $\frac{moles}{volume}$
- Concentration = $\frac{2}{0.5}$ = 4 mol / dm³

Worked example

25.0 cm³ of 0.050 mol / dm³ sodium carbonate was completely neutralised by 20.00 cm³ of dilute hydrochloric acid. Calculate the concentration in mol / dm³ of the hydrochloric acid.

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Answer:

Calculate the moles of sodium carbonate:
- Moles of Na₂CO₃ = concentration x volume
  - Remember: The volume needs to be in dm³
- Moles of Na₂CO₃ = 0.05 x $\frac{25.0}{1000}$ = 0.00125
Calculate the moles of hydrochloric acid:
- The balanced symbol equation shows that 1 mole of Na₂CO₃ reacts with 2 moles of HCl
- So, 0.00125 moles of Na₂CO₃ reacts with 0.00250 moles of HCl
Calculate the concentration of hydrochloric acid:
- Concentration =
  - Remember: The volume needs to be in dm³
  - 20 cm³ ÷ 1000 = 0.02 dm³
- Concentration = $\frac{0.00250}{0.02}$ = 0.125 mol / dm³

Examiner Tip

Don't forget your unit conversions:

To go from cm³ to dm³: divide by 1000

To go from dm³ to cm³: multiply by 1000

Calculate Concentrations of Solutions (Edexcel IGCSE Chemistry)

Revision Note

Calculate Concentrations of Solutions

Worked example

Worked example

Worked example

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Author: Stewart