Concentration in mol/dm3 (OCR Gateway GCSE Chemistry)

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Concentration in mol/dm3

  • 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
  • We can modify the concentration formula to include moles
  • The units in the answer can be written as mol dm-3 or mol / dm3:

Examiner Tip

Don't forget your unit conversions:

To go from cm3 to dm3 : divide by 1000

To go from dm3 to cm3 : multiply by 1000

  • Solving problems on concentrations involves carefully working out moles and volumes in the correct units and applying the concentration formula
  • Some students find formula triangles help them to understand the relationship:

Concentration moles formula triangle, downloadable IB Chemistry revision notes

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

Worked example

Example 1

Calculate the amount of solute, in moles, present in 2.5 dm3 of a solution whose concentration is 0.2 mol/dm3

Answer

Calculating Concentrations WE1, downloadable IGCSE & GCSE Chemistry revision notes

Worked example

Example 2

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol/dm3, when 80 g is dissolved in 500 cm3 of water. (Na= 23, H= 1, O= 16)

Answer

Calculating Concentrations WE2, downloadable IGCSE & GCSE Chemistry revision notes

Examiner Tip

Set your calculations out set by step and remember the units. It is much easier for you and anyone else to check calculations if they are not just lots of random numbers.

  • Convert mass to moles first
  • Convert volume into dm3 if necessary
  • Write out the calculation to convert moles and volume into concentration

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Jennifer

Author: Jennifer

Expertise: Chemistry

Jenny graduated in 'Chemistry for Drug Discovery' from the University of Bath in 2006, followed by her PGCE in secondary science, and has been teaching chemistry to 11-18 year olds ever since. She has taught GCSE and A-level chemistry for over 16 years and been a Director of Science for over 6 years, as well as tutoring and writing science books. Jenny loves helping students to understand the core concepts in chemistry and the links between topics, so is now happily working at Save My Exams to support more students to succeed in their learning.