Molecular weight and boiling point question in salt solution?

[CATSandKILOS]

Hey guys,

My question is from TBR Chp 7 Passage 9 #58.

The HIGHEST boiling point would be associated with which of these solutions in table 1?
I narrowed it down to
A. Solution C (Flask #3- 4.0 grams of NaCl with 100 grams of H20)
B. Solution F (Flask #6- 4.0 grams of KCl with 100 grams of H20).

Now, I understand the more concentrated solution of salt will have the higher boiling point. But why will the NaCl solution have the highest boiling point compared to the KCl solution? Why does NaCl having the lower mass increase its boiling point? I thought it was a higher mass has an increased boiling point.

My thoughts: Because NaCl has less intermolecular forces maybe it is able to overcome the attractive forces between water molecules and can dissolve easier?

Thanks

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[AwayFromReality]

Boiling point elevation depends on the molality which is moles of solute / mass (in Kg) of solution.

NaCl has a molar mass of about 23 + 35 = 58 whereas KCl has molar mass of about 40 + 35 = 75

4 grams of NaCl * 1mol NaCl / 58g = 4/58 = 0.07 mols of NaCl
4 grams of KCl * 1mol KCl / 75g = 4/75 = 0.05 mols of KCl

NaCl has a higher number of moles therefore has a higher molality. We can conclude that NaCl is going to increase the boiling point more than KCl will.

This question was just designed to trick you into using the relationship higher molar mass = higher boiling point which does not apply here. If the question asked which of these have a higher boiling point and gave you NaCl or KCl by themselves (not in water) then the answer would be KCl

Side note: Quick math tip. You don't even have to actually solve the fractions above. Since both fractions have the same numerator (4) and KCl has a bigger denominator we can easily conclude that 4/75 < 4/58 therefore mols of KCl < mols of NaCl

 

[CATSandKILOS]

THANKS!!!

 

[chemtopper]

Hey guys,

My question is from TBR Chp 7 Passage 9 #58.

The HIGHEST boiling point would be associated with which of these solutions in table 1?
I narrowed it down to
A. Solution C (Flask #3- 4.0 grams of NaCl with 100 grams of H20)
B. Solution F (Flask #6- 4.0 grams of KCl with 100 grams of H20).

Now, I understand the more concentrated solution of salt will have the higher boiling point. But why will the NaCl solution have the highest boiling point compared to the KCl solution? Why does NaCl having the lower mass increase its boiling point? I thought it was a higher mass has an increased boiling point.

My thoughts: Because NaCl has less intermolecular forces maybe it is able to overcome the attractive forces between water molecules and can dissolve easier?

Thanks

Just adding more points : This is based on colligative properties and colligative properties depend on the number of particle.Here we will work out the number of particles on the basis of molality and i factor(van't hoff factor) .If the product of van't hoff factor and molality is high then there will be more elevation in boiling point.Like for example if the question is like this :
which solution (in water)has higher boiling point
MgCl2 of 2 m or NaCl of 1m
In this case answer is MgCl2 as i (van't hoff factor) is 3 whereas in case of NaCl it is 2 only.So the product of i and m is more in MgCl2 is more so its boiling point is more.

 

[AwayFromReality]

Just adding more points : This is based on colligative properties and colligative properties depend on the number of particle.Here we will work out the number of particles on the basis of molality and i factor(van't hoff factor) .If the product of van't hoff factor and molality is high then there will be more elevation in boiling point.Like for example if the question is like this :
which solution (in water)has higher boiling point
MgCl2 of 2 m or NaCl of 1m
In this case answer is MgCl2 as i (van't hoff factor) is 3 whereas in case of NaCl it is 2 only.So the product of i and m is more in MgCl2 is more so its boiling point is more.

Quick question, the full equation for boiling point elevation involves (i)(m)(Kb), with Kb being a constant. So between MgCl2 and NaCl isn't the Kb also going to be different and without that information we can't accurately answer the question?

In the original question I noticed that they were using Na and K which are in the same group on the periodic table thus have similar chemical properties. I used this relationship to assume that Kb between NaCl and KCl is going to be very similar thus can be ignored. Is this a correct assumption?

Edit: Nevermind I just looked up Kb. That constant is specific to the solvent not the solute. Therefore if we're just using water in both problems, then Kb will be the same.

 

[chemtopper]

Quick question, the full equation for boiling point elevation involves (i)(m)(Kb), with Kb being a constant. So between MgCl2 and NaCl isn't the Kb also going to be different and without that information we can't accurately answer the question?

In the original question I noticed that they were using Na and K which are in the same group on the periodic table thus have similar chemical properties. I used this relationship to assume that Kb between NaCl and KCl is going to be very similar thus can be ignored. Is this a correct assumption?

Edit: Nevermind I just looked up Kb. That constant is specific to the solvent not the solute. Therefore if we're just using water in both problems, then Kb will be the same.

Thats why I have written solution in water .If solvent is not changing then Kb will be constant.

 

[CATSandKILOS]

Thanks everyone!