3.4.1 Using Concentrations in mol/dm3

infoNote

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.

infoNote

Concentration can be expressed in two ways:

Grams per dm³ (g/dm³): This tells us how many grams 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).

infoNote

Calculating Concentration in mol/dm³

To calculate the concentration of a solution in mol/dm³, you use the formula:

Concentration (mol/dm^3)= \frac{Moles of Solute}{Volume of Solution (dm^3)}

lightbulbExample

Example: If you have 2 moles of magnesium sulfate (MgSO₄) dissolved in 500 dm³ of water, the concentration is:

\frac{2 mol}{500 dm^3} - 0.004 mol/dm^3

infoNote

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:

Moles= \frac{Mass (g)}{Mr (Relative Formula Mass)}

Once you have the moles, you can then use the concentration formula above to find the concentration in mol/dm³.

infoNote

Worked Example

Problem: You dissolve 100 grams of potassium chloride (KCl) in 200 dm³ of water. Calculate the concentration in mol/dm³.

Calculate moles of KCl:

Mr of KCl=39 (K)+35.5 (Cl)=74.5

Moles of KCl= \frac{100g}{74.5}=1.34 mol

Calculate concentration:

Concentration= \frac{1.34 mol}{200 dm^3}=0.0067 mol/dm3

infoNote

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:

C1×V1=C2×V2

where C is the concentration and V is the volume.

lightbulbExample

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:

1×500=C_2×350

C_2= \frac{500}{350}=1.43 mol/dm^3

Using Concentrations in mol/dm3 Simplified Revision Notes for GCSE AQA Chemistry

3.4.1 Using Concentrations in mol/dm3

500K+ Students Use These Powerful Tools to Master Using Concentrations in mol/dm3 For their GCSE Exams.

Other Revision Notes related to Using Concentrations in mol/dm3 you should explore