In lab you combine 2.5 g of magnesium, #Mg#, with 1.1 g of oxygen, #O#. What is the percent composition by mass of each element in the compound you produced?

In order to determine the percent composition of an element that makes up a specific compound, you need to know the mass of the element and the total mass of the compound.

#color(blue)("% composition" = "mass of element"/"mass of compound" xx 100)#

You know that your compound is made up of two elements, magnesium, #"Mg"#, and oxygen, #"O"#.

Moreover, you know that magnesium contributes #"2.5 g"# and oxygen contributes #"1.1 g"# to the mass of the compound. This means that you will have

#m_"compound" = m_(Mg) + m_(O)#

#m_"compound" = "2.5 g" + "1.1 g" = "3.6 g"#

The percent composition of each element in this compound will thus be

#"% Mg" = (2.5 color(red)(cancel(color(black)("g"))) )/(3.6color(red)(cancel(color(black)("g")))) xx 100 = "69.4%"#

#"% O" = (1.1 color(red)(cancel(color(black)("g"))))/(3.6color(red)(cancel(color(black)("g")))) xx 100 = "30.6%"#

This tells you that every #"100 g"# of this compound will contain #"69.4 g"# of magnesium and #"30.5 g"# of oxygen.

Since you only have two sig figs for the masses of the two elements, you must round the values to two sig figs

#"% Mg" = color(green)(69%)#

#"% O" = color(green)(31%)#