if 3.5 g of element x reacts with 10.5 g of element y to form the compound xy.
what is the percent by mass of element x in the compound? the percent by mass
of element y?

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 %

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation: