Questions & Answers

Question

Answers

A. 0.200 m

B. 0.192 m

C. 0.100 m

D. 1.200 m

Answer
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We will first find the moles of urea, which is the ratio of mass to molar mass.

- As we know that density is the ratio of mass to volume.

- We are provided with density = 1.052 g/ml, so we can say that 1500 ml of solution corresponds to $1.052 g/ml\times 1500 ml = 1578 g$

- Molar mass of urea is 60 g/mol

- We will find the number of moles of urea, which is the ratio of mass to molar mass. It is:

\[\begin{align}

& \frac{18g}{60g/mol} \\

& =0.3mol \\

\end{align}\]

- We know that molality is the ratio of number of moles of solute to the mass of solvent. Hence, we can write, mass of water = 1575 g - 18 g = 1560 g

We will convert 1560 gram to kilogram= 1.560 kg, as (1 kg = 1000 g)

-Now, we will calculate molality as:

\[Molality=\frac{number\text{ }of\text{ }moles\text{ }of\text{ }urea}{mass\text{ }of\text{ }water\text{ }in\text{ }kg}\]

\[Molality=\frac{0.3} {\begin{align}

&1.560 \\ \end{align}}\] $= 0.192 m$