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Using Concentrations in mol/dm3 (AQA GCSE Chemistry)

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Stewart Hird

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Concentration, moles & volume

Higher tier only

Concentration, moles and volume

  • 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 therfore 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:

concentration (mol dm-3) =  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

  • You may have to convert from g dm-3 into mol dm-3 and vice versa depending on the question

Converting cm3 and dm3

How to convert from cm3 to dm3 and back

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

Examiner Tips and Tricks

One common mistake in these calculations is to forget to convert the units, as appropriate

Calculating concentration from reacting solutions

Higher tier only

  • 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 relaitonship:

Concentration moles formula triangle

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

  • The following examples show how to do this step-by-step

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:

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

    • Concentration of solution: 0.2 mol/dm3

    • 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

Worked Example

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol dm-3, when 80 g is dissolved in 500 cm3 of water

(Na= 23, H= 1, O= 16)

Answer:

  • Step 1: Convert the mass of NaOH to moles:

    • Mr (NaOH) = 23 + 16 + 1 = 40

    • 40 g NaOH = 1 mole

    • So, 80 g NaOH = 2 moles 

  • Step 2: Convert cm3 to dm3 

    • 500 cm3 / 1000 = 0.5 dm3

  • Step 3: Calculate the concentration of the NaOH solution:

    • Concentration = moles over volume

    • Concentration = fraction numerator 2 over denominator 0.5 end fraction = 4 mol dm-3

Titration Calculations

  • If the concentration of one of the reactants is known (either the acid or the base), then the exact volumes from a titration along with the balanced chemical equation for the reaction can be used to calculate the concentration of the other reactant

Worked Example

Calculating concentration

A solution of 25.0 cm3 of hydrochloric acid was titrated against a solution of 0.100 mol/dm3 NaOH. 

12.1 cm3 of NaOH was required for a complete reaction.

Determine the concentration of the acid.

Answer:

  • Step 1: Write the equation for the reaction:

    • HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

  • Step 2: Calculate the number of moles of the NaOH

    • Moles = (volume over 1000) x concentration

    • Moles of NaOH = 0.012 dm3 x 0.100 mol/dm3 = 1.21 x 10–3 mol

  • Step 3: Deduce the number of moles of the acid

    • Since the acid reacts in a 1:1 ratio with the alkali,  the number of moles of HCl is also 1.21 x 10–3 mol

    • This is present in 25.0 cm3 of the solution (25.0 cm3 = 0.025 dm3)

  • Step 4: Find the concentration of the acid

    • Concentration = fraction numerator moles over denominator volume space open parentheses dm cubed close parentheses end fraction 

    • Concentration of HCl = fraction numerator 1.21 cross times 10 to the power of negative 3 end exponent space mol over denominator 0.025 space dm cubed end fraction = 0.0484 mol/dm3

Examiner Tips and Tricks

You are not given the concentration-moles formula triangle in exams so you have to learn it. It is a good idea to write it down before you start a problem, so you get all the parts in the correct place.

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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.