Question #9c19a

Every time you're looking for a solution's molarity, you must determine how many moles of solute you get in one liter of solution.

That is what molarity essentially tells you - how many moles of solute you'd get if you had exactly one liter of solution.

#color(blue)(|bar(ul(color(white)(a/a)"molarity" = "moles of solute"/"one liter of solution"color(white)(a/a)|)))#

Notice that the problem provides you with the volume of the solution and with the mass of the solute, which in your case is potassium fluoride, #"KF"#.

Your first goal here will be to use potassium fluoride's molar mass to determine how many moles you get in that sample

#116.2color(red)(cancel(color(black)("g"))) * "1 mole KF"/(58.097color(red)(cancel(color(black)("g")))) ~~"2.00 moles KF"#

So, you know that this solution contains #2.00# moles of potassium fluoride in a volume of #"3.00 L"#. In order to find the solution's molarity, you must determine how many moles you get in #"1.00 L"# of solution.

This means that you can use the number of moles of solute present in this sample as a conversion factor to help you find the number of moles of solute in #"1.00 L"#

#1.00color(red)(cancel(color(black)("L solution"))) * "2.00 moles KF"/(3.00color(red)(cancel(color(black)("L solution")))) = "0.667 moles KF"#

The solution will thus have a molarity of #"0.667 mol L"^(-1)#.

In other words, every liter of this solution will contain #0.667# moles of potassium fluoride.

Notice that you can find the solution's molarity by dividing the number of moles of solute by the total volume of the solution

#color(blue)(|bar(ul(color(white)(a/a)c = n_"solute"/V_"solution"color(white)(a/a)|)))#

In this case, you would have

#c = "2.00 moles"/"3.00 L" = color(green)(|bar(ul(color(white)(a/a)"0.667 mol L"^(-1)color(white)(a/a)|)))#

The answer is rounded to three sig figs, the number of sig figs you have for the volume of the solution.