# Question 8baf6

Feb 17, 2017

${\text{0.255 moles K"_2"Cr"_2"O}}_{7}$

#### Explanation:

The first thing you need to do here is to figure out how many grams of potassium dichromate you have in your sample.

You know that your solution is 15.00%, presumably by mass, which tells you that you get $\text{15.00 g}$ of potassium dichromate for every $\text{100 g}$ of solution.

This means that your sample will contain

500. color(red)(cancel(color(black)("g solution"))) * ("15.00 g K"_2"Cr"_2"O"_7)/(100color(red)(cancel(color(black)("g solution")))) = "75.0 g K"_2"Cr"_2"O"_7#

Now all you have to do is use the molar mass of potassium dichromate to convert the number of grams to moles

$75.0 \textcolor{red}{\cancel{\textcolor{b l a c k}{{\text{g"))) * ("1 mole K"_2"Cr"_2"O"_7)/(294.185color(red)(cancel(color(black)("g")))) =color(darkgreen)(ul(color(black)("0.255 moles K"_2"Cr"_2"O}}_{7}}}}$

The answer is rounded to three sig figs, the number of sig figs you have for the mass of the solution.