10.03.07 Concentration in mol/dm3 (HT)

Syllabus

- The concentration of a solution can be measured in mol/dm³ .
- The amount in moles of solute or the mass in grams of solute in a given volume of solution can be calculated from its concentration in mol/dm³ .
- If the volumes of two solutions that react completely are known and the concentration of one solution is known, the concentration of the other solution can be calculated.
- Students should be able to explain how the concentration of a solution in mol/dm³ is related to the mass of the solute and the volume of the solution.

We dilute squash with water until it tastes right.

We would say that squash with too much water was 'Too weak' and squash without enough water is 'too strong'.

But Strong and Weak have specific meanings in Chemistry which have nothing to do with the amount of water.

Concentrated Vinegar has very little water but is still weak; Dilute Hydrochloric Acid is still strong.

And it is the words Dilute and Concentrated that tell us how much water is involved.

Dilute = lots of water

Concentrated = not much water.

All we need now is a way to measure concentration

One way to measure a concentration is the number of moles of solute (dissolved stuff) in each litre of water

Unfortunately, chemists write litres as dm³

So, the unit would be moles per dm³ (mol/dm³) rather than mol/L which would be easier.

Your examiner will stick to mol/dm³but at A level it is written as mol dm ^-3and sometimes just M.

If 50 cm³ of HCl with a concentration of 2 mol/dm³ reacts exactly with 40 cm³of NaOH, what is its concentration?

Moles of HCl = concentration x (volume/1000) = 2 x 50/1000 = 0.1 mol

HCl:NaOH 1:1

Moles of NaOH = 0.1

Concentration = Moles / (volume/1000)

= 0.1 / (40/1000) = 2.5 mol/dm³

If 60 cm³ of H₂SO₄ with a concentration of 3 mol/dm³ reacts exactly with 50 cm³of NaOH, what is its concentration?

Moles of H₂SO₄ = concentration x (volume/1000) = 3 x 60/1000 = 0.18 mol

H₂SO₄:NaOH 1:2

Moles of NaOH = 0.18 x 2 = 0.36

Concentration = Moles / (volume/1000)

= 0.36 / (50/1000) = 7.2 mol/dm³

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