Concentration: how crowded is your solution?

5 minAS Level2.4.1

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can calculate concentrations in $\text{mol dm}^{-3}$ and $\text{g dm}^{-3}$ , find the amount of solute in any volume of solution, and work out concentrations after dilution.

The squash problem

Squeeze the same squirt of orange squash into a glass and into a bucket. Same amount of flavour, wildly different drink. What matters is how much is dissolved per volume.

Chemists do exactly this, but they count the solute in moles and measure the volume in dm³.

Definition

Concentration

The amount of solute, in moles, dissolved in each $1\ \text{dm}^3$ of solution:

$c = \frac{n}{V} \qquad \text{units: mol dm}^{-3}$

You'll also meet mass concentration in $\text{g dm}^{-3}$ : the mass of solute per dm³ of solution.

The toolkit (and the trap)

$n = c \times V \qquad c = \frac{n}{V} \qquad V = \frac{n}{c}$

The trap: $V$ must be in $\text{dm}^3$ . Lab volumes come in $\text{cm}^3$ , so divide by 1000 first:

$25.0\ \text{cm}^3 = \frac{25.0}{1000} = 0.0250\ \text{dm}^3$

Forget this and your answer is out by a factor of 1000, which is the chemistry equivalent of ordering 1000 pizzas instead of one.

Worked example: Two quick conversions

(a) How many moles of NaOH are in 25.0 cm³ of 0.100 mol dm⁻³ NaOH(aq)? (b) 2.65 g of anhydrous sodium carbonate is dissolved and made up to 250 cm³ of solution. What is its concentration? ( $M(\text{Na}_2\text{CO}_3) = 106.0\ \text{g mol}^{-1}$ )

Have a go first?

Quick check

CalculateQuestion 1 · Recall

CalculateQuestion 2 · Apply

Answer the 2 questions above to continue.