Question

Question #c79f3

Answer 1

`"8.7 mol L"^(-1)`

Explanation:

For starters, you know that an `"85% m/v"` phosphoric acid solution contains `"85 g"` of phosphoric acid for every `"100 mL"` of solution.

As you know, molarity is a measure of the number of moles of solute, which in your case would be phosphoric acid, present in

`1 color(red)(cancel(color(black)("L"))) * (10^3color(white)(.)"mL")/(1color(red)(cancel(color(black)("L")))) = 10^3` `"mL"`

of this solution. Use the mass by volume percent concentration to calculate the mass of phosphoric acid present in `10^3` `"mL"` of solution.

`10^3 color(red)(cancel(color(black)("mL solution"))) * ("85 g H"_3"PO"_4)/(100color(red)(cancel(color(black)("mL solution")))) = "850 g H"_3"PO"_4`

To convert the sample to moles, use the compound's molar mass

`850 color(red)(cancel(color(black)("g H"_3"PO"_4))) * ("1 mole H"_3"PO"_4)/(97.99color(red)(cancel(color(black)("g H"_3"PO"_4)))) = "8.67 moles H"_3"PO"_4`

Since this value represents the number of moles of phosphoric acid present in `10^3` `"mL"` of solution, you can say that its molarity is equal to

`color(darkgreen)(ul(color(black)("molarity = 8.7 mol L"^(-1))))`

The answer is rounded to two sig figs, the number of sig figs you have for the percent concentration of the solution.