Question

Confusing question,
A typical solid fertiliser for use with household plants and shrubs contains the elements N, P, and K in the ratio of 15 g : 30 g :15 g per 100 g of fertiliser. The recommended usage of fertiliser is 14 g of fertiliser per 5 dm3 of water.
What is the concentration of nitrogen atoms in this solution?

Answer 1

The answer is 0.03moldm-3
From what I get is
N P K
15 30 15

So per 100g,

25g 50g 25g

Then suppose they use 14g of fertiliser, that means 14% of it
14/100 x25g = 3.5g of N
3.5x1/14=0.25mol of N
0.25mol/5 = 0.05moldm-3

Answer 2

I think I can see the problem with this question:

The recommended amount is 14g in 5l (or 5dm3). This converts to 2.8g per litre. Remember that the fertiliser is only 15% N, so the amount of N per litre is 2.8 times 0.15 or 0.42g. Dividing the 0.42g by 14 g per mole gives a value of 0.03moles per litre (or 0.03 moles per dm3) or 0.03M. I don't know why you changed the proportion of the elements in your calculations but I think this might be why your answer is 66.66% bigger. Hope this helps, check back if you're still stuck.