Kaplan mistake?? (CHEM QUESTION) please help.

Started by dentwannabe
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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
 
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dentwannabe said:
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
Click to expand...


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).
 
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dentwannabe said:
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
Click to expand...

Kaplan's answer is, for sure, correct. MW = 17 g/mol
 
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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 🙂
 
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jaerob said:
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 🙂
Click to expand...


Yup that’s correct. T refers to the increase in the boiling point from original without impurity.
 
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cowsgomoo said:
Yup that’s correct. T refers to the increase in the boiling point from original without impurity.
Click to expand...

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.
 
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dentwannabe said:
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.
Click to expand...

I wish you the best of luck.
 
Upvote 0 Downvote
dentwannabe said:
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.
Click to expand...


Wish you all the best...
 
Upvote 0 Downvote
dentwannabe said:
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.
Click to expand...


Go ROCK that test. You can dooooo it.
 
Upvote 0 Downvote
dentwannabe said:
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.
Click to expand...

Wow, I hope you ROCK it!
 
Upvote 0 Downvote
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