Reversing the calculations
2 marks · EasyExplain why, when rearranging the concentration equation to solve for volume, the number of moles must be divided by concentration, and not the other way around.
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Also accept: The concentration formula is concentration = moles / volume, so to find volume we need to divide moles by concentration.; Because concentration tells us how many moles are in each dm³; dividing the total moles by the concentration gives the number of dm³ needed.
Common mistakes: Inverting the operation; Stating the rearrangement without explanation
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Marking points
B1 · 1 mark · States the original formula is concentration = moles ÷ volume.
B2 · 1 mark · Explains that solving for volume requires dividing the number of moles by concentration to give volume.
Worked solution
The concentration equation is concentration = moles / volume. To find the volume, the equation is rearranged:
Dividing by concentration gives the correct units (dm³), because concentration tells us the amount of moles in each dm³. Dividing the total number of moles by the amount in 1 dm³ tells us how many dm³ are needed.