To solve this, we must remember that the number of moles in each formula must a whole number.
So we are given two X’s:
X = 0.3
X = 0.39
We divide the two by the smallest X, so divide by 0.3:
X = 0.3 / 0.3 = 1
X = 0.39 / 0.3 = 1.3
However the second X is not a whole number yet, therefore multiply the two X’s by 3 to make them whole numbers:
X = 1 * 3 = 3
X = 1.3 * 3 = 4
So the X’s must be: X3 and X4 and based from the choices, the answer is:
X3Y and X4Y
The plausible sets of formulas for these two compounds are:
A. X2Y and X3Y
B. XY3 and XY4
C. X3Y and X4Y
D. XY and X3Y
E. XY and X2Y
F. X2Y5 and X3Y5
G. X4Y2 and X3Y
I think the correct answers are A and F. and G:
If we look at the combining masses of X and Y in each of the two compounds, the first compound contains 0.25 g of X combined with 0.75 g of Y, therefore the ratio (by mass) of X to Y is 1 : 3. Whereas the second compound contains 0.33 g of X combined with 0.67 g of Y, therefore the ratio (by mass) of X to Y is 1 : 2.
Then, suppose prepared each of these two compounds, starting with the same fixed mass of element Y (I will choose 12 g of Y for an easy calculation)
The first compound will then contain 4g of X and 12g of Y. Next 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 (12 g) 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. Then the two compounds given with the plausible formula must therefore contain the same ratio.
Grade: 9
Subject: Chemistry
Chapter: compounds
Keywords: the generic elements X, the generic elements Y, two compounds, XY, X3Y
