Question

Boiling point is least for –
1) 0.1M Urea
2) 0.2 M Urea
3) 0.1 M NaCl
4) 0.2 M $MgC{l_2}$

Answer 1

Hint: Boiling point of a solid means the temperature at which a liquid boils. Think about the relation between concentration of liquid with the boiling point. also think about the relation between binding force between molecules with the boiling point.

Complete Step by Step Solution:
Boiling point means the temperature at which a liquid boils and also the temperature at which the vapour pressure is equal to the standard sea-level atmospheric pressure [760 mm of mercury]. converted into vapour. Now let's see which property it depends on. Now higher the attractive force between the molecules of the liquid higher the boiling point because as temperature of the body increases the kinetic energy of the molecules increases.

 When this temperature exceeds a certain amount where it exceeds the binding energy between the molecules, they are going away from each other. That particular temperature is called boiling of the body. So, the stronger binding force means higher boiling point. Let’s now compare the boiling point of the given compounds. We have three elements here, one is urea and the other is $NaCl$ and the last one is $MgC{l_2}$ .

We all know that urea is a covalent compound and $NaCl$ ,$MgC{l_2}$ are ionic compounds. We all know that ionic bonding is much stronger than covalent bonding. So, Urea has less boiling point than $NaCl$ and $MgC{l_2}$ . But Urea is given two options. Let's try to remember some other points that boiling points depend on. Yes! That is the concentration of the solution. That it is more concentrated implies a higher boiling point. So clearly the boiling point of 0.1 M urea is the lowest among all.
Hence option 1 is the correct option.

Note: Concentration of the liquid is directly proportional to the boiling point of the liquid. Also, boiling is higher when the binding force between the molecules is stronger. Remember that ionic bonding is stronger than covalent bonding.