AQA Chemistry (8462) 4.3.2.5 · 4.3.4 · Quantitative

Concentration of solutions

The same 10 g of sugar can make a drink that is sickly sweet or barely sweet. Which number tells you how strong a solution is, and how do you calculate it?

Triple Science · Higher tier

Start here

60 g of sugar in every litre.

A sports drink label reads: carbohydrate 60 g/dm³. That number is a concentration: the mass of sugar dissolved in each dm³ of drink. One dm³ is one litre, or 1000 cm³. You pour yourself a 250 cm³ glass.

If you had to guess, is there about 15 g of sugar in your glass, or about 240 g?

Concentration is the mass of solute dissolved in each dm³ of solution, measured in g/dm³. A concentrated solution has a lot of solute in a given volume; a dilute solution has a little. To calculate it, divide the mass of solute by the volume of the solution: concentration (g/dm³) = mass (g) ÷ volume (dm³). The volume must be in dm³. Lab volumes usually come in cm³, so divide by 1000 first: 500 cm³ = 0.500 dm³.

The solution bench

Pick a mass and a volume. Call the concentration. Then make it.

0 of 3 solutions made

Mass of solute

Volume of solution

Commit first. 10 g of solute is dissolved to make 250 cm³ of solution. What is its concentration?

Think again

“The volume is 250 cm³, so I divide the mass by 250.”

Spot the flaw

Explain

5.0 g of salt is dissolved to make 250 cm³ of solution. A student writes: concentration = 5.0 ÷ 250 = 0.02 g/dm³. What went wrong?

Convert first

How many dm³?

Sort each volume by its value in dm³. 1 dm³ = 1000 cm³ = 1 litre.

Tap a card, then tap the box it belongs in. Tap a placed card to take it back.

Higher

Think again · mass, volume and concentration

“The big beaker has more solution in it, so it must be more concentrated.”

Your call

Compare

Both beakers were filled from the same bottle of 40 g/dm³ salt solution. Which is more concentrated?

Then dilute it

Calculate

Water is added to the 50 cm³ beaker until it holds 200 cm³ of solution. What is its concentration now?

Higher · Triple

Concentration in mol/dm³ · AQA 4.3.4

Concentration can also be measured in mol/dm³: the amount of solute, in moles, in each dm³ of solution. concentration (mol/dm³) = amount (mol) ÷ volume (dm³). Mass and concentration in mol/dm³ are linked by M_r: turn grams into moles first (n = mass ÷ M_r), then divide by the volume in dm³.

Your call

Calculate

4.0 g of sodium hydroxide (Mr = 40) is dissolved to make 250 cm³ of solution. Calculate its concentration in mol/dm³.

Equations

Learn it · no chemistry equation sheet

c = m ÷ V

concentration (g/dm³) = mass (g) ÷ volume (dm³)

m = c × V

V = m ÷ c

mass (g) = concentration (g/dm³) × volume (dm³); volume (dm³) = mass (g) ÷ concentration (g/dm³)

cm³ ÷ 1000 = dm³

Convert every volume before you insert it.

Higher · Triple

c = n ÷ V

concentration (mol/dm³) = amount (mol) ÷ volume (dm³)

Command words in this lesson

Calculate: Convert the volume to dm³, work through the formula, and end with the unit: g/dm³ for a concentration, g for a mass, dm³ or cm³ for a volume.
Describe: The steps in order, with the equipment and the measurements. No reasons needed.
Explain: Link the mass of solute and the volume of solution to the concentration, with a reason for each step.

Key fact

Concentration = mass of solute ÷ volume of solution. With the mass in g and the volume in dm³, the answer is in g/dm³.

Key note · AQA 4.3.2.5, 4.3.4 (8462)

Concentration of solutions

Concentration (g/dm³) = mass (g) ÷ volume (dm³).
Convert cm³ to dm³: ÷ 1000.
Concentrated = lots of solute per volume. Dilute = little solute per volume.
mass (g) = concentration (g/dm³) × volume (dm³). volume (dm³) = mass (g) ÷ concentration (g/dm³).
Dilution: same mass, larger volume, lower concentration.
Concentration (mol/dm³) = amount (mol) ÷ volume (dm³). Turn grams into moles with Mr first.

Photograph this card. Tomorrow, cover it and say each line first.

Previous and next

Connects to

Ask MrBadmus

Not sure when to divide by 1000?

The bench draws the solution in a measuring cylinder with one dot for every 0.5 g of dissolved solute, so the number of dots in a given space stands for the concentration. Real dissolved particles are far too small to see, and an accurate solution is made up in a volumetric flask rather than a measuring cylinder.