Molar Concentration (DP IB Chemistry): Revision Note

Molar concentration

Volumes & concentrations of solutions

• Concentration tells us how much solute is dissolved in a solvent to make a solution:

  • It is usually measured as the amount of solute in 1 dm³ of solution

  • The solute is the substance being dissolved (e.g. salt or sugar)

  • The solvent is the liquid that does the dissolving (often water)

• A concentrated solution contains a large amount of solute per unit volume

• A dilute solution contains a small amount of solute per unit volume

Units of concentration

• Concentration can be expressed in three common ways:

  • Moles per unit volume

    • Units: mol dm^-3

    • Often written using square brackets, e.g. [NaCl] = 0.25 mol dm^-3

  • Mass per unit volume

    • Units: g dm^-3

  • Parts per million (ppm)

    • Used for very low concentrations, e.g. pollutants in water or air

Moles per unit volume

• The formula for expressing concentration using moles is:

concentration (g dm^-3) =

• You must make sure you change cm³ to dm³  (by dividing by 1000)

• The relationships can be expressed using this formula triangle

Concentration moles formula triangle diagram

Mass per unit volume

• Sometimes it is more convenient to express concentration in terms of mass per unit volume

• The formula is:

concentration (g dm^-3) =

• To convert from mol dm^-3 to g dm^-3  :

  • Multiply the molar concentration by the molar mass

mass of solute (g) = number of moles (mol) x molar mass (g mol^-1)

• To convert from g dm^-3 to mol dm^-3:

  • Divide the mass concentration by the molar mass

concentration (mol dm^-3) =

Parts per million

• When expressing extremely low concentrations a unit that can be used is parts per million or ppm

• This is useful when giving the concentration of a pollutant in water or the air when the absolute amount is tiny compared to the volume of water or air

• 1 ppm is defined as

  • A mass of 1 mg dissolved in 1 dm³ of water

• Since 1 dm³ weighs 1 kg we can also say it is

  • A mass of 1 mg dissolved in 1 kg of water, or 10^-3 g in 10³ g which is the same as saying the concentration is 1 in 10⁶ or 1 in a million