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GCSEChemistryWJECRevision Notes2. Chemical Bonding, Application Of Chemical Reactions & Organic Chemistry2.2 Acids, Bases & SaltsConcentrations in mol dm-3

Concentrations in mol dm-3 (WJEC GCSE Chemistry)

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Philippa Platt

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Concentrations in mol dm-3

Higher Tier

  • 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
  • The amount of solute can be expressed in grams or moles
  • Typically, concentration is expressed in terms of the amount of substance per dm3, therefore the units of concentration are either:     
    • g dm–3  
    • mol dm–3

What is concentration?

  • Concentration refers to the amount of solute there is in a specific volume of the solvent
  • A general formula to calculate the concentration in g dm–3 is:

concentration space left parenthesis straight g space dm to the power of negative 3 end exponent right parenthesis space equals space 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) is the same as 1 litre
    • 1 decimetre cubed (dm3) = 1000 cm3
  • You may be given data in a question which needs to be converted from cm3 to dm3 or the other way around

Conversion of cm3 and dm3

unit-conversion

To go from cm3 to dm3 divide by 1000. To go from dm3 to cm3 multiply by 1000

  • It is more useful to a chemist to express concentration in terms of moles per unit volume rather than mass per unit volume
  • Concentration can therefore be expressed in moles per decimetre cubed and calculated using the following equation:

concentration space left parenthesis mol space straight d straight m to the power of negative 3 end exponent right parenthesis space equals space fraction numerator number space of space moles space of space solute space left parenthesis mol right parenthesis over denominator volume space of space solution space left parenthesis dm cubed right parenthesis end fraction

  • We can modify the concentration formula to include moles
    • The units in the answer can be written as mol dm–3
  • Some students find formula triangles help them to understand the relationship:

Diagram to show the relationship between moles, concentration and volume

Concentration moles formula triangle, downloadable IB Chemistry revision notes

The concentration-moles formula triangle can help you solve these problems

Worked example

A student dissolved 10 g of sodium hydroxide, NaOH, in 2 dm3 of distilled water. Calculate the concentration of the solution in g/dm3.

Answer

Step 1: Write down the given information

  • Mass = 10 g
  • Volume of solution = 2 dm3

Step 2: Calculate concentration in g dm–3

  • Concentration = fraction numerator mass space open parentheses straight g close parentheses over denominator volume space open parentheses dm cubed close parentheses end fraction = begin mathsize 14px style fraction numerator 10 space straight g over denominator 2 space dm cubed end fraction end style = 5 g dm–3

Worked example

Calculate the amount of solute, in moles, present in 2.5 dm3 of a solution whose concentration is 0.2 mol dm–3.

Answer

Step 1: Write down the information you are given in the question:

  • Concentration of solution: 0.2 mol dm–3
  • Volume of solution: 2.5 dm3 

Step 2: Calculate the number of moles

  • Moles = concentration x volume
  • Moles = 0.2 x 2.5 = 0.5 mol

Examiner Tip

You may not be given the equation in an exam, so make sure you are able to recall it.

Remember to always convert the units from cm3 to dm3 by dividing by 1000.

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    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.