# Question #9b1c1

##### 1 Answer

#### Explanation:

You know that molarity of the solution is defined as the number of moles of solute, which in your case is *perchloric acid*,

This means that you're going to have to pick a sample solution to base your calculations on. To keep things simple, pick a

The first thing to do here is determine what the **mass** of this solution will be. To do that, use the given density - don't forget to convert the volume of the sample to *mililiters*, or *cubic centimeters*

#1.00color(red)(cancel(color(black)("L"))) * (10^3color(red)(cancel(color(black)("cm"^3))))/(1color(red)(cancel(color(black)("L")))) * "1.67 g"/(1color(red)(cancel(color(black)("cm"^3)))) = "1670 g"#

Now, you know that this solution has a percent concentration by mass of **for every**

In your case, the sample solution will contain a total of

#1670color(red)(cancel(color(black)("g solution"))) * "70 g HClO"_4/(100color(red)(cancel(color(black)("g solution")))) = "1169 g HClO"_4#

To determine how many moles of perchloric acid you have, use its molar mass

#1169color(red)(cancel(color(black)("g"))) * "1 mole HClO"_4/(100.46 color(red)(cancel(color(black)("g")))) = "11.636 moles HClO"_4#

Since the sample solution has a volume of

#c = n/V#

#c = "11.636 moles"/"1.00 L" = color(green)("11.6 M")#

The answer is rounded to three sig figs.

So, to find molarity given density and percent cocnentration by mass you need to

pick a sample solution of whatever volume you wantcalculate the mass of that sample solutionuse the percent cocnentration to find how many moles of solute you have in the sampleuse the molar mass of the solute to determine the number of moles