Question

Dissolving 120 g of NaOH in 1000 g of water gave a solution of density 1.12 g/ml. The molarity of the solution is?

Answer 1

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Explanation:

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

120g⋅1 mole urea60g=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

1120g⋅1 mL1.15g=973.9 mL

Now, your goal is to figure out the number of moles of solute present in 103 mL of solution, so use the known composition of the solution as a conversion factor to get

103mL solution⋅2 moles urea973.9mL solution=2.0536 moles urea

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

molarity = 2.1 mol L−1−−−−−−−−−−−−−−−−−−−−

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hope it helps you