AQA Chemistry (8462) 4.3.2.5 · 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 · Foundation tier
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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
Explain5.0 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.
Equations
Learn it · no chemistry equation sheetc = 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.
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.
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 (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).
Photograph this card. Tomorrow, cover it and say each line first.
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.