Question

Suppose there are two known compounds containing the generic elements X and Y. You have a 1.00-g sample of each compound. One sample contains 0.25 g of X and the other contains 0.33 g of X. Identify plausible sets of formulas for these two compounds. Check all that apply.

Answer 1

I think the correct answers are X2Y and X3Y, X2Y5 and X3Y5, and X4Y2 and X3Y, for the following reason: 

If you look at the combining masses of X and Y in each of the two compounds, 

The first compound contains 0.25g of X combined with 0.75g of Y 
so the ratio (by mass) of X to Y = 1 : 3 

The second compound contains 0.33 g of X combined with 0.67 g of Y 
so the ratio (by mass) of X to Y = 1 : 2 

Now, you suppose to prepare each of these two compounds, starting with the same fixed mass of element Y ( I will choose 12g of Y for an easy calculation!) 

The first compound will then contain 4g of X and 12g of Y 
The second compound will then contain 6g of X and 12g of Y 

The ratio which combined the masses of X and the fixed mass (12g) of Y
= 4 : 6 
or 2 : 3 

So, the ratio of MOLES of X which combined with the fixed amount of Y in the two compounds is also = 2 : 3 

The two compounds given with the plausible formula must therefore contain the same ratio.