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Concentrations of Solutions (AQA GCSE Chemistry)

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Stewart Hird

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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 Tips and Tricks

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 Hird

    Author: Stewart Hird

    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 Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.