GCSEChemistry: Combined ScienceAQARevision Notes3. Quantitative Chemistry3.2 Calculating Mass of SubstancesConcentrations of Solutions

Concentrations of Solutions (AQA GCSE Chemistry: Combined Science)

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Expressing concentration

  • A solid substance that dissolves in a liquid is called a solute, the liquid is called a solvent and the two when mixed together form a solution
  • Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent
  • Concentration simply refers to the amount of solute there is in a specific volume of the solvent
  • The greater the amount of solute in a given volume, the greater the concentration
  • A general formula for concentration is thus:

concentration space left parenthesis straight g divided by dm cubed right parenthesis equals fraction numerator mass space of space solute space left parenthesis straight g right parenthesis over denominator volume space of space solution space left parenthesis dm cubed right parenthesis end fraction

  • Concentration can be measured in grams per cubic decimetre
  •  1 decimetre cubed (dm3) = 1000 cm3
    • 1 decimetre cubed (dm3) is the same as 1 litre

  • You may be given data in a question which needs to be converted from cm3 to dm3 or the other way around
      • To go from cm3 to dm3 :
        • Divide by 1000

      • To go from dm3 to cm3 :
        • Multiply by 1000

Calculating concentration in mass per volume

  • To calculate the concentration of a solution in terms of mass per unit volume, you need to
    • Identify the solute and solvent
    • Convert the volume units into decimetres cubed
    • Divide the mass of the solute by the volume of the solution in decimetres cubed

Worked example

A student dissolved 10 g of sodium hydroxide, NaOH, in 2 dm3 of distilled water.

Calculate the concentration of the solution.

Answer:

  • Step 1: Write the relevant information down:
    • Mass = 10 g
    • Volume = 2 dm3 
  • Step 2: Write down the relevant equation:
    • Concentration = fraction numerator mass space of space solute space open parentheses straight g close parentheses over denominator volume space open parentheses dm cubed close parentheses end fraction
  • Step 3: Substitute the values and calculate the concentration:
    • Concentration = 10 over 2 = 5 g / dm3 

Examiner Tip

Be careful when doing volume unit conversions as it is easy to multiply instead of dividing by 1000 and vice-versa. Always ask yourself - is the result going to be a bigger or smaller number than I started with? Do I get more or fewer cubic decimetres when I convert from cubic centimetres?

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    Stewart

    Author: Stewart

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.