# Question 94cff

Nov 5, 2017

${\text{0.352 moles Br}}_{2}$

#### Explanation:

Since the problem is providing you with the number of molecules of bromine, ${\text{Br}}_{2}$, and asking for the number of moles present in the sample, a good tool to use here would be the number of molecules needed to have exactly $1$ mole of bromine.

By definition, you can't have $1$ mole of bromine unless you have $6.022 \cdot {10}^{23}$ molecules of bromine. This is Avogadro's constant and represents the definition of a mole.

So right from the start, the fact that you have

2.12 * 10^(23)color(white)(.)"molecules Br"_2 " " < " " overbrace(6.022 * 10^(23)color(white)(.)"molecules Br"_2)^(color(blue)("equivalent to 1 mole of Br"_2))

means that your sample will contain less than $1$ mole of bromine.

More specifically, the sample will contain

2.12 * color(red)(cancel(color(black)(10^(23)))) color(red)(cancel(color(black)("molec. Br"_2))) * "1 mole Br"_2/(6.022 * color(red)(cancel(color(black)(10^(23)))) color(red)(cancel(color(black)("molec. Br"_2)))) = color(darkgreen)(ul(color(black)("0.352 moles Br"_2#

The answer is rounded to three sig figs, the number of sig figs you have for the number of molecules persent in the sample.