Question #15857

The idea here is that you need to figure out how much oxygen is present in #"100 g"# of carbon dioxide #-># this number will give you the percent composition of oxygen in carbon dioxide.

Since you know that every mole of carbon dioxide contains

• one mole of carbon, #1 xx "C"#
• two moles of oxygen, #2 xx "O"#

you can say that a mass of carbon dioxide that is equivalent to #1# mole of this compound will contain the mass of #2# mole of oxygen.

Now, carbon dioxide has a molar mass of #"44.0095 g mol"^(-1)#, which means that #1# mole of this compound has a mass of #"44.0095 g"#. Oxygen has a molar mass of #"15.9994 g mol"^(-1)#, and so #1# mole of oxygen has a mass of #"15.9994 g"#.

Therefore, you can say that #2# moles of oxygen will have a mass of

#2 color(red)(cancel(color(black)("moles O"_2))) * "15.9994 g"/(1color(red)(cancel(color(black)("mole O"_2)))) = "31.9988 g"#

Since this is how much oxygen you get in #"44.0095 g"# of carbon dioxide, you can say that #"100 g"# of carbon dioxide will get you

#100color(red)(cancel(color(black)("g CO"_2))) * "31.9988 g O"/(44.0095color(red)(cancel(color(black)("g CO"_2)))) = "72.71 g O"#

This means that the percent composition of oxygen in carbon dioxide is

#color(darkgreen)(ul(color(black)("% composition CO"_2 = "72.71% O")))#

I'll leave the answer rounded to four sig figs.