Concentration in mol/dm3 (OCR GCSE Chemistry A (Gateway))
Revision Note
Concentration in mol/dm3
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 therefore 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:
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
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 relationship:
The concentration-moles formula triangle can help you solve these problems
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/dm3
Answer
Worked Example
Example 2
Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol/dm3, when 80 g is dissolved in 500 cm3 of water. (Na= 23, H= 1, O= 16)
Answer
Examiner Tips and Tricks
Set your calculations out set by step and remember the units. It is much easier for you and anyone else to check calculations if they are not just lots of random numbers.
Convert mass to moles first
Convert volume into dm3 if necessary
Write out the calculation to convert moles and volume into concentration
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?