Question

Dissolving 120g of urea in 100g of water gave a solution of density 1.15g/mL. The molarity of the solution

Answer 1

Answer:Molarity = Number of moles of solute/Volume of solution in Litres

Moles of urea = Given mass/Molecular mass = 12060 = 2 mol

Volume of solution = Mass of solution/Density of solution= (120+1000) g/1.15 g.mL−1= 974 ml or 0.974 L

Hence, Molarity = 2 mol/0.974 L = 2.05 mol L−1 

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Explanation:

Answer 2

Explanation:

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

1 L

of solution.

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

1 L

=

10

3

mL

of solution.

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

120

g

⋅

1 mole urea

60

g

=

2 moles urea

Now, you know that your solution contains

120 g

of urea, the solute, and

1000 g

of water, the solvent. This implies that the 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

1.15 g mL

−

1

, which means that every

1 mL

of solution has a mass of

1.15 g

.

Use the density of the solution to calculate its volume

1120

g

⋅

1 mL

1.15

g

=

973.9 mL

Now, your goal is to figure out the number of moles of solute present in

10

3

mL

of solution, so use the known composition of the solution as a conversion factor to get

10

3

mL solution

⋅

2 moles urea

973.9

mL solution

=

2.0536 moles urea

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

molarity = 2.1 mol L

−

1

−−−−−−−−−−−−−−−−−−−− this is answer and explanation both