Concentration of solutions in moles per cubic decimetre

Science · Matter & Materials · ages 15-16

Name ______________________ Date ____________

A calculation divides a number of moles by a volume in dm³. What unit does the answer carry?
- grams
- cm³ per mole
- g/dm³
- mol/dm³
A cubic decimetre is the same volume as a litre.
Circle one: True False
Convert 750 cm³ into cubic decimetres.
Answer: ______________
Convert 0.4 cubic decimetres into cubic centimetres.
Answer: ______________
0.2 moles of solute are dissolved and made up to 0.5 dm³. What is the concentration in mol/dm³?
Answer: ______________
Which of these makes a solution more concentrated?
- adding more water to it slowly
- dissolving more solute in it
- stirring it for longer
- pouring half of it away
A solution is 0.25 mol/dm³ of a substance with an Mr of 60. What is that in g/dm³?
- 240 g/dm³
- 0.25 g/dm³
- 15 g/dm³
- 60.25 g/dm³
Asked for the concentration of 0.1 moles in 200 cm³, Theo writes 0.1 / 200 = 0.0005 mol/dm³. Where is the slip?
- he should have multiplied rather than divided
- the moles should have been converted, not the volume
- 0.1 moles is too small an amount to have a concentration
- the volume had to be turned into dm³ first, so 0.2
100 cm³ of a 0.6 mol/dm³ solution has 200 cm³ of water added. Priya says the concentration is now 0.3 mol/dm³. Where is the slip?
- the final volume is 300 cm³, not 200, so it is 0.2
- she should have added the two concentrations
- adding water does not change a concentration at all
- the moles change when water is added, so it cannot be worked out
Doubling the volume of a solution by adding water halves its concentration.
Circle one: True False

Answer key

For grown-ups. Fold this page away before handing over the rest.

Concentration of solutions in moles per cubic decimetre W1-mt_vRK78ynnXb-s1

mol/dm³ · The unit follows the calculation. Moles divided by cubic decimetres gives moles for every cubic decimetre, written mol/dm³.
True · Both hold 1000 cm³. Chemists write dm³ and everyone else writes litres, but the volume is identical.
0.75 · Divide by 1000: 750 / 1000 = 0.75 dm³.
400 · Going the other way multiplies by 1000: 0.4 x 1000 = 400 cm³.
0.4 · Concentration equals moles divided by volume in dm³: 0.2 / 0.5 = 0.4 mol/dm³.
dissolving more solute in it · Concentration is solute per dm³. Adding solute raises the top of that fraction. Pouring half away removes solute and water together, so the crowding is unchanged.
15 g/dm³ · Multiply by the Mr to move from moles to grams: 0.25 x 60 = 15 g/dm³.
the volume had to be turned into dm³ first, so 0.2 · 200 cm³ is 0.2 dm³, so the working is 0.1 / 0.2 = 0.5 mol/dm³. Dividing by 200 gives moles per cm³, which is a thousand times too small.
the final volume is 300 cm³, not 200, so it is 0.2 · She has divided by the water added rather than by the final volume. 100 cm³ plus 200 cm³ is 300 cm³, so 0.06 moles over 0.3 dm³ gives 0.2 mol/dm³.
True · The moles stay put and the volume doubles, so the moles per dm³ figure is halved. Concentration and volume move in opposite directions when the solute is fixed.