There is a question which I don't fully understand:-
A student carried out a titration to find the concentration of phosphoric acid in a solution.
50cm3 of phosphoric acid solution was neutralised by 28cm3 of a sodium hydroxide solution of concentration 0.50 mol dm-3.
The equation for this reaction is:
3NaOH(aq) + H3PO4(aq) -> Na3PO4(aq) + 3H2O(l)
(a) Calculate the number of moles used.
(b) Calculate the concentration of phosphoric acid in moles per cubic decimetre (mol dm-3)
This is what I would do:-
(a) Moles = Concentration x Volume
Moles = 0.50 x (28 / 1000) - This would convert the volume into dm3 from cm3.
Moles = 0.50 x 0.028
Moles = 0.014 -> Which is correct.
(b) Concentration = Moles / Volume
Concentration = 0.014 / (50 / 1000)
Concentration = 0.28 -> Which is wrong.
The correct working is as follows (for part (b)):-
0.33 moles H3PO4 reacts with 1 Mole NaOH
Moles H3PO4 in 50cm3 = 0.014 x 0.33
Therefore moles H3PO4 in 1 dm3 = (0.014 x 0.33 x 1000) / 50
= 0.09
I don't get it.