Question

What is the osmolarity of a solution that contains `"25 g"` of urea in `"0.4163 L"` of solution?

Answer 1

`"0.42 Osmol L"^(-1)`

Explanation:

A solution's osmolarity basically tells you the number of moles of particles that contribute to the solution's osmotic pressure present in one liter of solution.

A particle that contributes to a solution's osmotic pressure is called an osmole.

Now, urea, `"CO"("NH"_2)_2`, is a non-electrolyte, which means that it does not dissociate into ions when dissolved in water. Simply put, every molecule of urea dissolved in solution will not dissociate to produce additional particles of solute.

This implies that every mole of urea dissolved in solution will produce one osmole of particles of solute.

Use the molar mass of urea to calculate how many moles you have in that sample

`25 color(red)(cancel(color(black)("g"))) * "1 mole urea"/(60.055color(red)(cancel(color(black)("g")))) = "0.4163 moles urea"`

Assuming that the volume of the solutionThis means that the solution's osmolarity will be equal to

`0.4163 color(white)(a)color(red)(cancel(color(black)("moles")))/"1 L" * "1 Osmol"/(1color(red)(cancel(color(black)("mole")))) = color(green)(|bar(ul(color(white)(a/a)color(black)("0.42 Osmol L"^(-1))color(white)(a/a)|)))`

I'll leave the answer rounded to two sig figs, but keep in mind that you only have one sig fig for the volume of the solution.