Thanks EulerFan,
I'm still a bit confused. What should I be reading off the graph with the method you suggested? So I went n/(Mr*V) = 0.250/31 /(31*250E-3), then *1000 which gave me 0.104? If I had to read off the graph, I got C = 0.1M -> g/L, multiply by 31. This wouldn't be correct either?
I have a feeling that this is beyond what should be expected of VCE...
Reverse engineering the answer they got, I find that they saw a concentration of 0.41 mM, which is obviously wrong looking at the graph - in fact, this is four times (and three magnitudes) what you thought it should be. Now, assuming they accidentally put mg instead of g, this is only four times what you'd expect. But, it turns out that in a molybdenum phosphate complex, there are actually 4 phosphate ions, which would mean that the concentration of phosphate IS 4*0.10 (as the concentration is by complex, not by phosphate). You are not expected to know this, because nobody would know this without further information (or looking it up like I did/they just happen to know the compound beforehand).
tl;dr - I wouldn't worry, this seems beyond VCE level.