The chemical analysis of a compound shows that it contains 30.0% by mass C, 7.5% by mass H, and 62.5% by mass As.

Question

What is simplest formula for a molecule of this compound?

 

Available options:

 

AsC4H7


As2C3H9


AsC3H9


As2C6H18


AsC5H9

Answer 1

If every 100% of compound contain 62.5% of As, 30% of C and 7.5% of H, you can express composition of that compound in grams: 62.5g of As, 30g of C, and 7.5g of H. Next step will be to convert grams to moles using molar mass of each element, so you will have 0.834 moles of As, 2.5 moles of C, and 7.5 moles of H. We divide amounts of moles of each by the smallest number of moles (in order to finally have ratio in whole numbers).

As 0.834/0.834=1

C 2.5/0.834 = 2.99 ≅ 3

H 7.5/0.834 = 9 

So our subscripts will be 1, 3, 9

Simplest formula of that compound is : AsC₃H₉