How can I find the actual percent composition of magnesium oxide?

1 Answer
Jun 28, 2016

Answer:

Here's how you can do that.

Explanation:

A compound's percent composition essentially tells you how many grams of each constituent element you get per #"100 g"# of compound.

#color(blue)(|bar(ul(color(white)(a/a)"% element i" = "mass of i"/"mass of compound" xx 100color(white)(a/a)|)))#

This means that in order to find a compound's percent composition, you need to pick a sample and use

  • the total mass of the sample
  • the mass of each constituent element in this sample

Now, magnesium oxide, #"MgO"#, has a molar mass of #"40.3044 g mol"^(-1)#, which means that one mole of magnesium oxide has a mass of #"40.3044 g"#.

This will be your sample of magnesium oxide. Notice that one mole of magnesium oxide contains

  • one mole of magnesium, #1 xx "Mg"#
  • one mole of oxygen, #1 xx "O"#

The molar masses of the two elements are

#M_("M Mg") = "24.3050 g mol"^(-1)#

#M_("M O") = "15.9994 g mol"^(-1)#

This tells you that one mole of magnesium oxide, i.e. #"40.3044 g"#, will contain

#"24.3050 g Mg" " "# and #" " "15.9994 g O"#

Now you have all that you need to figure out the percent composition of the compound. You will have

#"% Mg" = (24.3050 color(red)(cancel(color(black)("g"))))/(40.3044color(red)(cancel(color(black)("g")))) xx 100 = color(green)(|bar(ul(color(white)(a/a)color(black)("60.304 %")color(white)(a/a)|)))#

#"% O" = (15.9994color(red)(cancel(color(black)("g"))))/(40.3044color(red)(cancel(color(black)("g")))) * 100 = color(green)(|bar(ul(color(white)(a/a)color(black)("39.696 %")color(white)(a/a)|)))#

You can thus say that #"100 g"# of magnesium oxide will contain

#"60.304 g Mg" " "# and #" " "39.696 g O"#