Question #9b1c1

You know that molarity of the solution is defined as the number of moles of solute, which in your case is perchloric acid, `"HClO"_4`, divided by the volume of the solution - expressed in liters.

This means that you're going to have to pick a sample solution to base your calculations on. To keep things simple, pick a `"1.00-L"` sample of perchloric acid.

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 `"70%"`. This means that you get `"70 g"` of perchloric acid for every `"100 g"` of solution.

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 `"1.00 L"`, the solution's molarity will be

`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 want
• calculate the mass of that sample solution
• use the percent cocnentration to find how many moles of solute you have in the sample
• use the molar mass of the solute to determine the number of moles