[Q]

An organic compound contains 21,20% carbon, 5,30% hydrogen and the rest is arsenic and oxygen. From 0,500 grams of that compound was obtained 0,802 grams of magnesium ammonium arsenate, MgNH₄AsO₄. Calculate the mass fraction of arsenic and oxygen in that organic compound and its empirical formula.
Solution: w(As)=66,30%, w(O)=7,20%, C₄H₁₂As₂O
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My attempt:

w(As+O)=73,5%

C_aH_bAs_cO_d + X MgNH₄AsO₄ + Y
M(C_aH_bAs_cO_d):M(MgNH₄AsO₄)= m(MgNH₄AsO₄):m(C_aH_bAs_cO_d)
M(C_aH_bAs_cO_d) = M(MgNH₄AsO₄)*m(MgNH₄AsO₄)/m(C_aH_bAs_cO_d)=290,741

N(C, C_aH_bAs_cO_d)=w(C, C_aH_bAs_cO_d)*M(C_aH_bAs_cO_d)/M(C)=4,846
N(H, C_aH_bAs_cO_d)=w(H, C_aH_bAs_cO_d)*M(C_aH_bAs_cO_d)/M(H)=15,257

I have no clue what should my proceeding step be, even though my calculations aren't correct.
Thank you in advance!

[Borek]

How many grams of As in the produced magnesium ammonium arsenate?

[mjc123]

M(CaHbAscOd):M(MgNH4AsO4)= m(MgNH4AsO4):m(CaHbAscOd)

This is true only if c = 1. Do you understand why?

[Q]

M(CaHbAscOd):M(MgNH4AsO4)= m(MgNH4AsO4):m(CaHbAscOd)

This is true only if c = 1. Do you understand why?

Yes, I do understand, even though it isn't mentioned in the task it was my only idea how to slove it

[Q]

How many grams of As in the produced magnesium ammonium arsenate?

I have translated the question word by word. Even my professor was stumbled with this task, hence I asked about it here

[Borek]

How many grams of As in the produced magnesium ammonium arsenate?

I have translated the question word by word. Even my professor was stumbled with this task, hence I asked about it here

You missed the point - it is a trivial thing to calculate, and it will tell you what is mass percentage of As in the compound.