Kaplan mistake?? (CHEM QUESTION) please help.

[dentwannabe]

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Ok, this is on the kaplan test on the cd that comes with the blue book.

When 25.5 g of nonvolatile nonelectrolyte are placed ito 500 g H20, with boiling point of the solution at 1 atm being 101.56 degrees C, what is the molecular weight of the solute? (Kb of H20 = .52 degrees C).

kaplan answer, D. 17 g/mol

Ok.. I know the equation for boiling boint elevation is.
T = Kb * m
m = mol. of solute/ kg of solvent
here, T = 101.56
Kb= .52
kg of solvent = .5 kg
mol of solute = 25.5/x (x being the mol. weight, wat we're trying to find)
so my set up was
101.56 = ((25.5/x)/.5) * .52
When i solve for x, i get .26

.26 is far off from the other answer choices... i re-did this problem many times.. same answer, .26

sumone please help, i think kaplan is wrong.. cuz in their explanation they say that "T is 3 times Kb, thus molality must be approximately 3". The only way T is 3 times Kb is if T is 1.0156, NOT 101.56

thanks guys

 

[bosox_9]

Ok, this is on the kaplan test on the cd that comes with the blue book.

When 25.5 g of nonvolatile nonelectrolyte are placed ito 500 g H20, with boiling point of the solution at 1 atm being 101.56 degrees C, what is the molecular weight of the solute? (Kb of H20 = .52 degrees C).

kaplan answer, D. 17 g/mol

Ok.. I know the equation for boiling boint elevation is.
T = Kb * m
m = mol. of solute/ kg of solvent
here, T = 101.56
Kb= .52
kg of solvent = .5 kg
mol of solute = 25.5/x (x being the mol. weight, wat we're trying to find)
so my set up was
101.56 = ((25.5/x)/.5) * .52
When i solve for x, i get .26

.26 is far off from the other answer choices... i re-did this problem many times.. same answer, .26

sumone please help, i think kaplan is wrong.. cuz in their explanation they say that "T is 3 times Kb, thus molality must be approximately 3". The only way T is 3 times Kb is if T is 1.0156, NOT 101.56

thanks guys

I may be wrong, but I think the T in the original equation refers to the boiling point elevation, not the actual boiling point. So the T should actually be 1.56 (101.56 - 100).

 

[dat_student]

Ok, this is on the kaplan test on the cd that comes with the blue book.

When 25.5 g of nonvolatile nonelectrolyte are placed ito 500 g H20, with boiling point of the solution at 1 atm being 101.56 degrees C, what is the molecular weight of the solute? (Kb of H20 = .52 degrees C).

kaplan answer, D. 17 g/mol

Kaplan's answer is, for sure, correct. MW = 17 g/mol

 

[jaerob]

As previously mentioned by BoSox, the "T" in the formula T = Kb*m is actually the INCREASE IN BOILING POINT (not the boiling point temperature). I think that's where you went wrong.. Try it out and see if it works for ya 🙂

 

[cowsgomoo]

As previously mentioned by BoSox, the "T" in the formula T = Kb*m is actually the INCREASE IN BOILING POINT (not the boiling point temperature). I think that's where you went wrong.. Try it out and see if it works for ya 🙂

Yup that’s correct. T refers to the increase in the boiling point from original without impurity.

 

[dentwannabe]

Yup that’s correct. T refers to the increase in the boiling point from original without impurity.

ohhhhhhhh

the boiling point of water is 100 degrees Celsius.

the solution is 101.56.. so the change in T is 1.0156 ??

ohhh i get. ****.. these little things i keep looking over.

TOMORROW THE DAY .. WISH ME LUCK EVERYONE.

 

[dat_student]

ohhhhhhhh

the boiling point of water is 100 degrees Celsius.

the solution is 101.56.. so the change in T is 1.0156 ??

ohhh i get. ****.. these little things i keep looking over.

TOMORROW THE DAY .. WISH ME LUCK EVERYONE.

I wish you the best of luck.

 

[LMav]

ohhhhhhhh

the boiling point of water is 100 degrees Celsius.

the solution is 101.56.. so the change in T is 1.0156 ??

ohhh i get. ****.. these little things i keep looking over.

TOMORROW THE DAY .. WISH ME LUCK EVERYONE.

Wish you all the best...

 

[mochafreak]

Yup, me too. 👍

 

[cowsgomoo]

ohhhhhhhh

the boiling point of water is 100 degrees Celsius.

the solution is 101.56.. so the change in T is 1.0156 ??

ohhh i get. ****.. these little things i keep looking over.

TOMORROW THE DAY .. WISH ME LUCK EVERYONE.

Go ROCK that test. You can dooooo it.

 

[jaerob]

ohhhhhhhh

the boiling point of water is 100 degrees Celsius.

the solution is 101.56.. so the change in T is 1.0156 ??

ohhh i get. ****.. these little things i keep looking over.

TOMORROW THE DAY .. WISH ME LUCK EVERYONE.

Wow, I hope you ROCK it!