Molar Calculations
Applied Science: BTEC Level 3 National Extended Diploma, topic 1.1.13
Molar Calculations of Solids
There is a specific equation we can use to determine the amount of moles of a sample if we know its mass.
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Mole calculations
- We can calculate the number of moles present in a sample if we know its mass, and its Mr:
- Moles = mass ÷ Mr
A worked example
- Benzene has an Mr of 78. How many moles of benzene are in 7.8g of pure benzene?
- Moles = mass ÷ Mr
- Moles = 7.8g ÷ 78
- Moles = 0.1mol
Molar Calculations of Solutions
We can calculate the number of moles of a liquid or solution using the volume and concentration.
Solutions
- It's much easier to look at a liquid and measure its volume than to evaporate any water and measure the mass of the remaining solid.
- So we need a value that will let us know how much of a solid is dissolved in a solution simply from its volume.
- If we know the concentration of a solution, we can calculate the number of moles in any sample volume of the solution.
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The equation
- Concentration is defined as moles per unit volume. The usual units of concentration are moles per litre.
- Litre is often written as dm3.
- So moles per litre is mol ÷ dm3.
- This is often written as moldm-3.
An example
- Three moles of NaCl are dissolved in half a litre of water.
- Concentration = number of moles ÷ volume
- Concentration = 3 mol ÷ 0.5 dm3
- Concentration = 6 moldm-3
Test yourself on Molar Calculations
If 58.5g of sodium chloride dissolves in half a litre of water, what is the concentration of the solution?
1 mole of a liquid weighs 18g and has a volume of 36cm3. What is the density of the liquid?
What are the units of concentration?
Which of the following is true of concentration?