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)

1 Answer
Stefan V.
Oct 20, 2016

#"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 #"100 g"# of said compound.

In your case, you know that you get #"93.00 g"# of aluminium and #"7.00 g"# of mercury for every #"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

#color(green)(bar(ul(|color(white)(a/a)color(black)("mass of Al " = " 23.3 g")color(white)(a/a)|)))#

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