Question #8baf6

1 Answer
Feb 17, 2017

#"0.255 moles K"_2"Cr"_2"O"_7#


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 #"15.00 g"# of potassium dichromate for every #"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 color(red)(cancel(color(black)("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.