# A 25.0 g sample of an aluminum/mercury amalgam used for filling cavities is 7.00% mercury by mass. What is the mass of aluminum in the 25.0 g sample? (Use correct numbers of significant figures)

Oct 20, 2016

$\text{23.3 g Al}$

#### Explanation:

The problem tells you that the sample is 7.00% mercury by mass, which automatically implies that it is 93.00% aluminium by mass.

That is the case because the amalgam contains two elements, aluminium and mercury, and so their respective percent concentration by mass must add up to give 100%.

Now, a compound's percent composition tells you how many grams of each of its constituent elements you get for every $\text{100 g}$ of said compound.

In your case, you know that you get $\text{93.00 g}$ of aluminium and $\text{7.00 g}$ of mercury for every $\text{100 g}$ of amalgam. This means that the given sample will contain

25.0 color(red)(cancel(color(black)("g amalgam"))) * "93.00 g Al"/(100color(red)(cancel(color(black)("g amalgam")))) = "23.25 g Al"

Rounded to three sig figs, the answer will be

$\textcolor{g r e e n}{\overline{\underline{| \textcolor{w h i t e}{\frac{a}{a}} \textcolor{b l a c k}{\text{mass of Al " = " 23.3 g}} \textcolor{w h i t e}{\frac{a}{a}} |}}}$