Concentration of Solutions (OCR GCSE Chemistry A (Gateway Science Suite)): Revision Notes
3.4.1 Using Concentrations in mol/dm3
Concentration is a measure of how much solute (the substance being dissolved) is present in a given volume of solvent (the liquid in which the solute is dissolved). It tells us how "strong" or "weak" a solution is.
Concentration can be expressed in two ways:
- Grams per dm³ (g/dm³): This tells us how many grammes of solute are in each cubic decimetre (dm³) of solution.
- Moles per dm³ (mol/dm³): This tells us how many moles of solute are in each dm³ of solution. A mole is a unit that represents a specific number of particles (6.02 × 10²³ particles, known as Avogadro's number).
Calculating Concentration in mol/dm³
- To calculate the concentration of a solution in mol/dm³, you use the formula:
Example: If you have 2 moles of magnesium sulphate (MgSO₄) dissolved in 500 dm³ of water, the concentration is:
Using Mass to Calculate Concentration Sometimes, instead of knowing the moles directly, you know the mass of the solute. In this case, you can first convert the mass to moles using the formula:
- Once you have the moles, you can then use the concentration formula above to find the concentration in mol/dm³.
Worked Example
- Problem: You dissolve 100 grammes of potassium chloride (KCl) in 200 dm³ of water. Calculate the concentration in mol/dm³.
- Calculate moles of KCl:
- Calculate concentration:
Relationship Between Two Solutions
- When two solutions react completely, the moles of one reactant will be equal to the moles of the other reactant. This can be expressed as:
where C is the concentration and V is the volume.
Example: A 1 mol/dm³ solution of sodium chloride (NaCl) with a volume of 500 cm³ reacts with 350 cm³ of silver nitrate (AgNO₃) solution. To find the concentration of the silver nitrate: