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IGCSEChemistryEdexcelChemistry (Modular)Unit 1Revision Notes1. Principles of Chemistry: Part 1Chemical formulae, equations, calculationsHow to Calculate Concentrations of Different Solutions

How to Calculate Concentrations of Different Solutions (Edexcel IGCSE Chemistry (Modular))

Revision Note

Stewart Hird

Written by: Stewart Hird

Reviewed by: Lucy Kirkham

Edexcel Chemistry (Modular): Unit 1Edexcel Chemistry

Calculate Concentrations of Solutions

  • A solute is a solid substance that dissolves into a liquid 

    • The amount of solute can be expressed in grams (g) or moles (mol)

  • A solvent is the liquid that a solute dissolves in 

    • The amount / volume of a solvent is measured in cm3 or dm3

  • Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent

  • A solution is the mixture formed when a solute dissolves in a solvent 

    • The amount / volume of a solution measured in cm3 or dm3 

  • Concentration 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

    • Concentration is sometimes commonly referred to as strength

      • For example, dissolving more coffee in hot water results in a stronger coffee

  • Typically, concentration is expressed in terms of the amount of substance per dm3  

    • Therefore, the units of concentration are:

      • g / dm3 

      • 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

  • To calculate concentration in mol / dm3 we use the following equation:

concentration space left parenthesis mol divided by dm cubed 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

Concentration moles formula triangle, downloadable IB Chemistry revision notes

The concentration-moles formula triangle 

  • Volumes are often expressed in cm3, but dm3 must be used when calculating concentration 

    • To convert cm3 to dm3, divide by 1000

    • To convert dm3 to cm3, multiply by 1000

How to convert from cm3 to dm3 and back

Converting between cm3 and dm3

Worked Example

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

Answer:

  • Write down the information you are given in the question:

    • Concentration of solution: 0.2 mol / dm3

    • Volume of solution: 2.5 dm3 

  • Calculate the number of moles:

    • Moles = concentration x volume

    • Moles = 0.2 x 2.5 = 0.5 mol

Worked Example

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol / dm3, when 80 g is dissolved in 500 cm3 of water.

Relative atomic masses, Ar:  Na = 23;   H = 1;   O = 16

Answer:

  • Calculate the Mr of NaOH:

    • 23 + 16 + 1 = 40

  • Determine the number of moles of NaOH:

    • 40 g = 1 mole

    • So, 80 g = 2 moles

  • Convert cm3 to dm3:

    • 500 over 1000 = 0.5 dm3 

  • Calculate the concentration:

    • Concentration = moles over volume

    • Concentration = fraction numerator 2 over denominator 0.5 end fraction = 4 mol / dm3

Worked Example

25.0 cm3 of 0.050 mol / dm3 sodium carbonate was completely neutralised by 20.00 cm3 of dilute hydrochloric acid. Calculate the concentration in mol / dm3 of the hydrochloric acid.

Na2CO3 + 2HCl → 2NaCl + H2O + CO2 

Answer:

  • Calculate the moles of sodium carbonate:

    • Moles of Na2CO3 = concentration x volume

      • Remember: The volume needs to be in dm3 

    • Moles of Na2CO3 = 0.05 x fraction numerator 25.0 over denominator 1000 end fraction = 0.00125

  • Calculate the moles of hydrochloric acid:

    • The balanced symbol equation shows that 1 mole of Na2CO3 reacts with 2 moles of HCl

    • So, 0.00125 moles of Na2CO3 reacts with 0.00250 moles of HCl

  • Calculate the concentration of hydrochloric acid:

    • Concentration = moles over volume

      • Remember: The volume needs to be in dm3 

      • 20 cm3 ÷ 1000 = 0.02 dm3 

    • Concentration = fraction numerator 0.00250 over denominator 0.02 end fraction = 0.125 mol / 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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    Author
    Reviewer
    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.

    Lucy Kirkham

    Author: Lucy Kirkham

    Expertise: Head of STEM

    Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.