Using Concentrations in mol/dm3 (AQA GCSE Chemistry)
Revision Note
Author
StewartExpertise
Chemistry Lead
Concentration, Moles & Volume
Concentration, moles and volume
Higher Tier Only
- 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
- To go from g dm-3 to mol dm-3:
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
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 dm-3.
Answer:
Worked example
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)
Answer:
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
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.
Answer:
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.
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