Question

Which will have the lowest boiling point?

A. 

pure water

B. 

0.5 M NaCl (aq)

C. 

1.0 M NaCl (aq)

D. 

2.0 M NaCl (aq)

​

Answer 1

Some of the solutions exhibit colligative properties. These properties depend on the amount of

solute dissolved in a solvent. These properties include freezing point depression, boiling

point elevation, osmotic pressure and vapor pressure lowering. From the given choices, the correct answer is the first option. Pure water will have the lowest boiling point because there are no solute particles to change the boiling point of water while other options will have an elevated boiling point because of the solute.

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Answer 2

Answer: A.   pure water

Explanation:

$\Delta T_b=i\times k_f\times m$

$\Delta T_b$ = change in boiling point

i = Van'T Hoff factor  

$k_f$ = freezing point constant

m = molality

1. For pure water

, i= 1 as it is a non electrolyte and does not dissociate.

2. For $0.5M NaCl$

$NaCl\rightarrow Na^+Cl^-$

, i= 2 , as it is a electrolyte and dissociate to give 2 ions and concentration will be (0.5+0.5)=1M[/tex]

3. For $1M NaCl$

$NaCl\rightarrow Na^+Cl^-$

, i= 2 , as it is a electrolyte and dissociate to give 2 ions and concentration will be (1+1)=2M[/tex]

4. For $2M NaCl$

$NaCl\rightarrow Na^+Cl^-$

, i= 2 , as it is a electrolyte and dissociate to give 2 ions and concentration will be (2+2)=4M[/tex]

Thus as vant hoff factor is lowest for pure water and thus the boiling point will be lowest.