Question

Explain the following observations using chem principles:

When table salt and sugar are dissolved in water, it is observed that:

1. Both solutions have a higher boiling point than pure water.

2. the boiling point of .1 M NaCl solution is higher than that of .1 M C12H22O11 solution.

---------------------------------------
1. I was thinking the salt and the sucrose have a higher vapor pressure which causes the vapor pressure of the solution to increase and decrease the volatility.

2. not sure about number 2

Answer 1

Consider on number 2 that NaCl breaks into two particles when dissoved, which doubles the effect on colligative properties.

Answer 2

1 is in the right ball park but the wrong direction.
For #1, both salt and sugar lower the vapor pressure of the solution compared to that of pure water. Lowering the vapor pressure of water means that it must be raised to a higher temperature before it reaches 1 atmosphere vapor presure (which is the definition of boiling point for a liquid).
2. The increase in boiling point, the decrease in freezing point, and the change in vapor pressure when a non-volatile solute is dissolved in a volatile solvent, depends solely upon the number of dissolved particles. Mole for mole, sucrose has 1 particle per molecule while NaCl has two particles per molecule. Therefore, a 0.1 M NaCl will increase the boiling point of water almost twice as much as 0.1 M sucrose.

Answer 3

Ok, for number 2: NaCl breaks into 2 particles when dissolved whereas sucrose remains as one. This doubles the van't Hoff factor which means a higher temperature is required for it to vaporize.

Does that make sense?

Re: number one. Is that answer ok?

Answer 4

Your revised number 2 answer is great. No, your original answer to number 1 is not correct. Take another look at my resonse to your number 1.

Answer 5

I hadn't even seen your response DrBob. Thanks! Making more sense to me now.