Question

Prove: V1M1=V2M2 V=volume, M=molarity

Answer 1

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Answer 2

Explanation:

It's based on a direct relationship between the concentration (Molarity) and the volume.

If you remember, n = MV, the number of moles = molarity x volume.

So I've got a certain amount of solute, say 5g of NaCl, I've got a certain number of moles of that substance. If I add this to a litre of water, I've got a certain number of mol l^-1, my concentration, but no matter how much of the solvent I add (in this case, water) the number of moles of NaCl present never changes. It's only the concentration that changes and volume that changes. Number of moles of solute remains constant unless you add more of the solute!

So the relationship is based on the fact the number of moles of the solute will remain constant, but the concentration and volume of the entire solution can change.

This is used for titrations, for example:

In HCl + NaOH --> NaCl + H2O, equivalent moles of HCl and NaOH are needed for it to be neutralised.

Moles = Molarity (moldm-3) x Volume (dm3)

n (moles) = Mv

Equivalent moles, so n = n.

Therefore:

n = n

Mv = Mv

M1v1 = M2v2.