Molar Concentration

Section 1 of 6What concentration means

Amount of solute in a volume

• Amount of solute in a volume The concentration of a solution tells you how much solute is dissolved in a particular volume of solution.
  • A more concentrated solution has more solute in the same volume.
• Chemists usually measure it in moles per decimetre cubed, mol/dm³.

Units of volume

• Units of volume 1 dm³ is the same as 1 litre and equals 1000 cm³.
• To convert cm³ to dm³, divide by 1000. To convert dm³ to cm³, multiply by 1000.

Section 2 of 6Equations to learn

Quantity	Equation	Units
Concentration	concentration = moles of solute ÷ volume	mol/dm3
Moles	moles = concentration × volume	mol
Volume	volume = moles ÷ concentration	dm3
Mass concentration	concentration (g/dm3) = mass of solute ÷ volume	g/dm3
Changing units	g/dm3 = mol/dm3 × Mr	g/dm3

Section 3 of 6Method for any calculation

1

Write down the values you are given, with units.

2

Convert any volume in cm³ to dm³ by dividing by 1000.

3

Choose the equation that has the unknown on its own, or use a formula triangle with moles at the top.

4

Substitute, calculate, and give the answer with the correct unit and a sensible number of significant figures.

Section 4 of 6Worked examples

1Finding moles

• Finding moles How many moles are in 2.5 dm³ of a 0.2 mol/dm³ solution? Moles = 0.2 × 2.5 = 0.5 mol.

2Converting volume

• Converting volume What is the concentration if 0.050 mol is dissolved to make 250 cm³?
  • Volume = 250 ÷ 1000 = 0.250 dm³.

3Finding concentration

• Finding concentration Concentration = 0.050 ÷ 0.250 = 0.20 mol/dm³.

4Changing to g/dm³

• Changing to g/dm³ For sodium hydroxide, M_r = 40, so 0.20 mol/dm³ × 40 = 8.0 g/dm³.

Section 5 of 6Key terms

Term	Meaning
Concentration	The amount of solute dissolved in a given volume of solution.
Solute	The substance that dissolves in a solvent to make a solution.
Mole	The unit for amount of substance; one mole contains the relative formula mass in grams.
dm3	Cubic decimetre; equal to one litre or 1000 cm3.
Relative formula mass (Mr)	The sum of the relative atomic masses of the atoms in a formula.

Section 6 of 6Exam technique

1

Putting a volume in cm³ straight into the equation. Divide by 1000 first.

2

Multiplying instead of dividing when converting cm³ to dm³; a litre is the larger unit, so the number gets smaller.

3

Mixing up mol/dm³ and g/dm³. One counts moles, the other counts grams, and M_r links them.

4

Leaving out units in the final answer.