Question

22.22 gram of urea was dissolved in 300 grams of water. Calculate the number of moles of urea and molality of urea solution. [Given Molar mass of urea = $60\; grams\;mol^{-1}$]

Answer 1

Molality is defined as the number moles of a solute contained in a kilogram of a solvent. The SI unit of molality is mol/kg.

Formula used: $\begin{aligned} & Molality=\dfrac{Mass\,of\,salute}{Mass\,of\,solvent\,in\,kg} \\\ \end{aligned}$

Complete answer:
Given,
Mass of urea = 22.22g
Mass of water = 300gm
Molar mass of urea = 60g/mole
Now, we have to find the moles of urea,
Number of moles of urea

&=\dfrac{Mass\,of\,urea}{Molar\,mass\,of\,urea} \\\ &=\dfrac{22.22}{60} \\\ &=0.370 mol \\\ \end{aligned}$$ Therefore, the number of moles of urea = 0.370 mol We have calculated the moles of urea and mass of solvent is given as, Mass of solvent = 300 grams Applying the formula of Molality, $$\begin{aligned} & Molality(m)=\dfrac{Moles\,of\,urea}{Mass\,of\,water\,(in\,kg)} \\\ & Molality(m)=\dfrac{0.370\times 1000}{300} \\\ & Molality(m)=1.23 mol/kg \\\ \end{aligned}$$ Thus Molality of urea solution = 1.23 mol/kg **Note:** Molality and Molarity both are different so don’t confuse between them. Molarity is the number of moles of a substance per litre of a solution. A capital M signifies solutions labelled with molar concentration. A 1.0M solution contains 1mole of solute per litre of solution. Molality is the number of solvent moles per kilogram.