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Concentration in g/dm3 (OCR GCSE Chemistry A (Gateway))

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Philippa Platt

Last updated

Concentration in g/dm3

Higher Tier Only

  • Concentration is a measure of how much of a substance is present in a given volume

  • The simplest way to show a calculation is in grams per decimetre cubed, g/dm3

  • This is calculated using the following equation:

concentration space equals space fraction numerator mass space of space solute space left parenthesis grams right parenthesis over denominator volume space of space solution space left parenthesis dm cubed right parenthesis end fraction

Worked Example

A solution contains 2.0g of sodium hydroxide in 200cm3 of water.

What is the concentration in g/dm3 ?

Answer

  • mass space of space solute space equals space 2.0 space straight g

  • volume space of space solution space in space dm cubed space equals space fraction numerator 200 space cm cubed over denominator 1000 end fraction space equals space 0.20 space dm cubed

  • concentration space equals space fraction numerator mass space of space solute space over denominator volume space of space solution end fraction space equals space fraction numerator 2.0 space straight g over denominator 0.20 space dm cubed end fraction equals space bold 10 bold space bold g bold divided by bold dm to the power of bold 3

Converting between g/dm3 and mol/dm3

  • You may have to convert from g/dm3 into mol/dm3 and vice versa depending on the question

    • To go from g/dm3 to mol/dm3:

      • Divide by the molar mass in grams

    • To go from mol/dm3 to g/dm3:

      • Multiply by the molar mass in grams

Worked Example

A solution of sulfuric acid, H2SO4, has a concentration of 0.1 mol/dm3

What is the concentration in g/dm3?

Answer

  • Both units are per decimetre cubed, so you just need to convert moles to mass, using the molar mass

  • Molar mass of H2SO4 = 98 g/mol

  • Mass of H2SO4 = moles (mol) x molar mass (g/mol)

  • Mass of H2SO4 =  0.1 mol x  98 g/mol = 9.8 g

  • This is the mass present in 1dm3 so the concentration = 9.8 g/dm3

Examiner Tips and Tricks

Don't forget your unit conversions:

To go from cm3 to dm3 : divide by 1000

To go from dm3 to cm3 : multiply 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