Answer:
The percentage by mass of element X is 25 %.
The percentage by mass of element Y is 75 %
Explanation:
X + Y ⇒ XY
3.5 g of element X + 10.5 g of element Y = 14g in total
⇒Element X : 3.5g / 14g = 0.25 ⇒ x 100 % = 25 %
⇒Element Y : 10.5 / 14 = 0.75 ⇒ x 100 % = 75 %
The percentage by mass of element X is 25 %.
The percentage by mass of element Y is 75 %