Chem Q

[211183]

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25.5 g of nonelectrolyte dissolvd in 500g h2o. Boiling Pt at 1atm = 101.56
Kb of H20 = .52 k*g/mol

what is the molecular weight of the solute?

Help..

 

[211183]

no one know?

 

[doc3232]

Is it 4.25 g?

 

[DDS13]

ok, dt=kb m

first of all, i think bp is 105 degree, and u mistyped. so supposing that, it is (105-100= 5 degree)

so,

5=.52 (moles/0.5kg)

moles= (5/.52) * .5kg

then divide the original mass by the number of mole, am i clear enough??

 

[211183]

sorry my brain must be dead... BP = 101.56.. i don't know what i did with the rest of that number?

A; 17g/mol

 

[doc3232]

SOLVE: 1.56 = ((25.5/X) x (0.52)) / (0.5)

Where X = MW.
You will find that X equals 17

1.56 is the temp change. 25.5/X is the number of mols. and you divide by half because there is only 0.5 kg of water. If it was 1 kg then you would divide by 1.

Hope this explains it well.

 

[tranv117]

Also, the Kb should be C*g/mol.

 

[211183]

Also, the Kb should be C*g/mol.

i dunno, its what the prob says in kaplan?

 

[tranv117]

hmm, im probably wrong. =) I guess it has to do with change in temperature and a 1.56 change is the same in Kelvin or C.

 

[211183]

yea i think i remember thinking the same thing as you when i took that test. 🙂 at least you are on your toes