# Dissolving 120g of urea (mol.wt 60) in 1000g of water gave a solution of density 1.15 g/mL. What is the molarity of the solution?

##### 1 Answer

#### Explanation:

The idea here is that a solution's **molarity** tells you the number of moles of solute present in

So in order to calculate a solution's **molarity**, you essentially need to know the number of moles of solute present in exactly

Start by using the **molar mass** of urea to calculate the number of moles present in your sample

#120 color(red)(cancel(color(black)("g"))) * "1 mole urea"/(60color(red)(cancel(color(black)("g")))) = "2 moles urea"#

Now, you know that your solution contains **total mass** of the solution

#"mass solution = mass solute + mass solvent"#

will be equal to

#"mass solution" = "120 g + 1000 g" = "1120 g"#

You also know that this solution has a **density** of **every** **of solution** has a mass of

Use the density of the solution to calculate its volume

#1120 color(red)(cancel(color(black)("g"))) * "1 mL"/(1.15color(red)(cancel(color(black)("g"))) ) = "973.9 mL"#

Now, your goal is to figure out the number of moles of solute present in *conversion factor* to get

#10^3 color(red)(cancel(color(black)("mL solution"))) * "2 moles urea"/(973.9color(red)(cancel(color(black)("mL solution")))) = "2.0536 moles urea"#

You can thus say that the molarity of the solution is equal to

#color(darkgreen)(ul(color(black)("molarity = 2.1 mol L"^(-1))))#

I'll leave the answer rounded to two **sig figs**.