What is the percentage composition of a compound containing 32.0 g of bromine and 4.9 g of magnesium?

1 Answer
Mar 31, 2016

{("% Br" = 87%), ("% Mg" = 13%) :}

Explanation:

In order to find a compound's percent composition, all you basically have to do is determine how many grams of its constituent elements you get per "100 g" of compound.

So, for a given compound, you can get the percent composition of its constituent element i by using

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

In your case, the compound is said to contain two elements, bromine, "Br", and magnesium, "Mg".

The total mass of the sample will be

m_"sample" = m_(Br) + m_(Mg)

m_"sample" = "32.0 g" + "4.9 g" = "36.9 g"

So, you know that "36.9 g" of this compound contain "32.0 g" of bromine, which means that "100 g"E of this compound must contain

100color(red)(cancel(color(black)("g compound"))) * "32.0 g Br"/(36.9color(red)(cancel(color(black)("g compound")))) = "86.72 g Br"

Likewise, this sample also contains "4.9 g" of magnesium, which means that "100 g" of compound must contain

100color(red)(cancel(color(black)("g compound"))) * "4.9 g Mg"/(36.9color(red)(cancel(color(black)("g compound")))) = "13.28 g Mg"

Since percent composition tells you how many grams of each element you get in "100 g" of compound, you can say that

"% Br" = color(green)(|bar(ul(color(white)(a/a)"87%"color(white)(a/a)|)))

"% Mg" = color(green)(|bar(ul(color(white)(a/a)"13%"color(white)(a/a)|)))

The answers are rounded to sig figs, the number of sig figs you have for the mass of magnesium.