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the question says :

Two elements, A and B, combine to form two binary compounds. In the first compound, 3.5 g of A combines with 8.00 g of B. In the second compound, 5.0 g of A combines with 17.1 g of B. If the formula of the first compound is AB2, then the formula of the second compound would be ?

I can't even make the first step :(

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  • $\begingroup$ As for your previous question, a question back to you to get you thinking: If the molecular formula AB2 gives a compound of 3.5g A, and 8.0g B, can you derive a simple A:B mass ratio ie for every 1g of A, how many grams of B? $\endgroup$
    – long
    Commented Jun 19, 2014 at 23:58
  • $\begingroup$ by dividing ; 1g of A will give 2.29 g of B ? $\endgroup$
    – Maher
    Commented Jun 20, 2014 at 0:04
  • $\begingroup$ No, this still represents a ratio of A:2B. You need to bring it back to give you a unity ratio A:B. $\endgroup$
    – long
    Commented Jun 20, 2014 at 0:18
  • $\begingroup$ will it be 1g of A for each 4.57 g of B ? $\endgroup$
    – Maher
    Commented Jun 20, 2014 at 0:23
  • $\begingroup$ Oops. Going the wrong way there. I hope you understand why we try to get you to do some working out before presenting answers to these types of questions. $\endgroup$
    – long
    Commented Jun 20, 2014 at 1:11

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For a molecule AB2, 3.5g of A represents one molar fraction, and 8.00g B represents 2 molar fractions (or 4.0+4.0). Therefore, a direct ratio can be given as 3.5:4.0, or 1:1.14. This means a molecule AnBm will give a mass ratio for A:B of n:1.14xm

For a molecule AB, for every 1g of A, you will have 1.14g of B.

For a molecule AB2, for every 1g of A, you will have 2.28g of B.

For a molecule A2B3, for every 1g of A, you will have (1.14x3/2) 1.71g of B.

etc etc

So for 5.0g of A, in order to have 17.1g of B, you will have 1.14 x (m/n) = 17.1/5

=> m/n = 17.1 / 5 / 1.14 = 3

This means m/n=3, or AB3 (alternatively it could also be A2B6)

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