Using Concentrations in mol/dm3 (Edexcel GCSE Chemistry): Revision Note
Exam code: 1CH0
Using Concentrations 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 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
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
The Concentration 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 dm-3.
Answer:
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:
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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