Concentrations in mol/dm3 | Chemistry for Grade 10 PDF Download

Concentration of Solutions in Moles

• 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:
    
• You may have to convert from g dm-3 into mol dm-3 and vice versa depending on the question
  • To go from g dm^-3 to mol dm^-3:
    • Divide by the molar mass in grams
  • To go from mol dm^-3 to g dm^-3:
    • Multiply by the molar mass in grams

Exam 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

Calculating Concentration from Reacting Solutions

• 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:
  
  The concentration-moles formula triangle can help you solve these problems
• The following examples show how to do this step-by-step

Solved Examples

Example 2: 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)

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

Example 3: 25.0 cm³ of a solution of 0.05 mol dm^-3  sodium carbonate was completely neutralised by 20.00 cm³ of dilute hydrochloric acid. Calculate the concentration of the hydrochloric acid in mol dm^-3.

Exam Tip

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.