how would you calculate the molecular weight of a compound if 4.00g of it plus 50.0 grams of water give a solution with a boiling point of 100.41 C the molal boiling constant for water is 0.52 C x kg.mol ?

how would you do this ? IM STUCKKK HELPPPP

Question

how would you calculate the molecular weight of a compound if 4.00g of it plus 50.0 grams of water give a solution with a boiling point of 100.41 C the molal boiling constant for water is 0.52 C x kg.mol ?

how would you do this ? IM STUCKKK HELPPPP

aaronq · Answer

dT=i*m*Kb

dT= change in temp
i=van't off constant (i guess assume it to be 1 because they don't tell you the compound dissociates)
m=molality
Kb=molal boiling point constant

anonymous · Answer

is it like this one ΔTB=KBm  @aaronq

aaronq · Answer

yep its the same thing

anonymous · Answer

0.52 C x kg.mol x ____ = 100.41?

aaronq · Answer

no, it's the change in temperature FROM the normal boiling temp, so:

(0.52 C kg.mol)*m = 0.41

aaronq · Answer

from there you can find the moles, then the molecular mass

anonymous · Answer

0.41?

aaronq · Answer

dT=NEW BP- NORMAL BP= 100.41-100 = 0.41

anonymous · Answer

wait why would you take out the 100? @aaronq

aaronq · Answer

because the formula uses the change (represented by Δ)  in boiling point temperature (caused the solute). So you need to use only the change

anonymous · Answer

how would you know that the change is

aaronq · Answer

you would find the difference between the new boiling point temp from the normal (without solute) bp.

dT=NEW BP- NORMAL BP= 100.41-100 = 0.41

anonymous · Answer

i just don't understand how to find the normal BP

aaronq · Answer

the normal boiling point of water is 100 degrees celsius

anonymous · Answer

so 0.788462 moles/kg os solvent so 0.788462 x (molar mass of H2o? )/mole = 14.2044

anonymous · Answer

?

aaronq · Answer

nope, what did you get from the equation above for molality?

anonymous · Answer

uh isn't molality moles of solute/ kg of solvent so 0.788426m is it not?

aaronq · Answer

yep thats right, why are you using the molecular mass of water if it's in kg?

it should be  0.788426m = n/0.05 kg

anonymous · Answer

n=0.039421? do you add that to the 4.00 grams?

aaronq · Answer

nope, the formula for moles is  n=m/M

so  0.039421=4g/M

anonymous · Answer

wait what des n m and M stand for?

aaronq · Answer

n = moles, m = mass, M= molar mass

anonymous · Answer

so the Molar mass is 0.009855?

aaronq · Answer

M=4 g/0.039421= 101.4687603054209685  g/mol

anonymous · Answer

oh whoops THANKS this finally makes sense tytytyty

aaronq · Answer

good stuff! no problem

anonymous · Answer

0.788426m = n/0.05 kg why would you set up the equation like this though  @aaronq

anonymous · Answer

WAIT ACTUALLY JK I GOT IT

aaronq · Answer

haha it just the definition of molality