[lalal2011]

I have performed an experiment with KH2PO4 and I had to calculate the mass of how much g of solid of KH2PO4 I have to use for the lab, “Weigh out enough KH2PO4 sample to contain about 0.1 ± 0.01 g of P. Have this mass calculated”. I found out that I have to use 0.4g
I found it by doing
136g-31g
x-0.1p
Now my question is that instead of the KH2PO4 I will use an unknown fertilizer and assume that it contains ~18% P by weight. How can I calculate how much fertilizer I have to use in the experiment? Thank you in advanced!

[Arkcon]

So if by your calculation (I haven't checked it yet) 0.4 g of KH₂PO₄ is 0.1 g of P, what % is that?  Can you now just sale the amount of fertilizer the same way?

[lalal2011]

I solved it but I am not sure if it is right. I got 0.46g, is it right?

[Dan]

I solved it but I am not sure if it is right. I got 0.46g, is it right?

No, show your working.

[lalal2011]

My calculations are:
82%-x
18%-31g

x= 141.2 g
then,

141.2g-31g
x-0.1g

x=0.46g

is it right? if not please tell me what is wrong.