# Question 9be80

Sep 14, 2015

You have $\text{31.3 g}$ of potassium in that much potassium sulfate.

#### Explanation:

The idea here is that you need to use the number of moles of potassium that you get per mole of potassium sulfate, ${\text{K"_2"SO}}_{4}$.

If you look at the formula unit of potassium sulfate, you'll notice that it contains

• two potassium atoms $\to$ you get that from the $2$ subscript;
• one sulfur atom
• four oxygen atoms $\to$ you get that from the $4$ subscript.

This means that 1 mole of potassium sulfate will contain 2 moles of potassium, 1 mole of sulfur, and 4 moles of oxygen.

In your case, you have 0.400 moles of ptoassium sulfate, which means that you have

0.400color(red)(cancel(color(black)("moles K"_2"SO"_4))) * "2 moles K"/(1color(red)(cancel(color(black)("moles K"_2"SO"_4)))) = "0.800 moles K"

Now just use potassium molar mass to get the number of grams that would contain this many moles

0.800color(red)(cancel(color(black)("moles K"))) * "39.0983 g K"/(1color(red)(cancel(color(black)("mole K")))) = color(green)("31.3 g K")#

Here's a list of videos on percent composition, check them out!

http://socratic.org/chemistry/the-mole-concept/percent-composition/videos