Concentration 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 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 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
- 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 Tip
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
