From the graph, you can see that the maximum amount of precipitate was obtained as soon as 8 millimol of phosphate was added into solution (8 ml of 1M solution = 8 millimoles). Adding any more phosphate doesn't change the amount of precipitate. The amount of T ion in the given solution is 12 millimol (= 12ml x 1M). So, the minimum amount needed to get all 12 mmol of T to precipitate is 8 mmol of phosphate. The ratio of T to PO4 is 3:2 giving the formula T3(PO4)2.

dm3 = L

mol/dm3 = mol/L = M

cm3 = ml

Source: Friend of girlfriend of DWSmith