How do you calculate the molarity of a solution prepared by dissolving 15.6 g of solid KBr in enough water to make 1.25 L of solution?

1 Answer
Jun 17, 2017

#"0.105 mol L"^(-1)#


For starters, convert the mass of potassium bromide to moles by using the molar mass of the compound.

#15.6 color(red)(cancel(color(black)("g"))) * "1 mole KBr"/(119.002color(red)(cancel(color(black)("g")))) = "0.1311 moles KBr"#

Now, in order to find the molarity of the solution, you need to figure out how many moles of solute are present for every #"1 L"# of solution.

You already know that #"1.25 L"# of solution contain #0.1311# moles of potassium bromide, the solute, so all you have to do now is to use this known composition as a conversion factor to go from #"1 L"# of solution to number of moles of solute.

#1 color(red)(cancel(color(black)("L solution"))) * "0.1311 moles"/(1.25color(red)(cancel(color(black)("L solution")))) = "0.10488 moles KBr"#

Since this represents the number of moles of solute present in #"1 L"# of solution, you can say that the molarity of the solution is equal to

#color(darkgreen)(ul(color(black)("molarity = 0.105 mol L"^(-1))))#

The answer must be rounded to three sig figs, the number of sig figs you have for your values.