chemistry (college final)

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How to determine which of the compounds will have the lowest boiling point vs the highest boiling point?

• chemistry (college final) -

another question:

Which of the following solutions will have the largest temperature range for the liquid state?
1) 1M C3H8? (ethanol)
2)1M CH3COOH (acetic acid)
3) 1M KBr(Potassium Bromide)
4) 1M MgBr2 (Magnesium Bromide)

I answered b, since it was the largest molecule, thought. How do i approach this question.

BY the way I have a chem final tomorrow, so i might be asking questions throughout the day.

• chemistry (college final) -

#2 question is one on the colligative properties of liquid; i.e., remember
delta Tf = i*Kf*molality
delta Tb = i*Kb*molality
So the one with the largest range will be the one that i*K*m is the highest.
Since K is constant for each salt and m is constant fo reach salt, the only difference is i.
Which has the largest i value. (By the way, C3H8 is NOT ethanol. C3H8 is propane; C2H5OH is ethanol).

• chemistry (college final) -

the largest I value would be MgBr2 because it has 3 ions, KBr has 2, and the other 2 are - to 1 because they do not split, correct?

What about my first question? About determining BP highest or lowest. Does the same principle apply?

• chemistry (college final) -

You are correct that MgBr2 will have the highest range because i=3.
For #1, the same approach applies.
The largest boiling point will be the one with the highest i.
The lowest boiling point depends upon if C3H8 actually is C2H5OH or not. If so, it's a toss up between 1 and 2. Both ethanol and CH3COOH have i = 1 (theoretical) but we know CH3COOH ionizes about 1% or so and that would make me pick ethanol (which has no ionization) over CH3COOH (which has a little). In terms of i, i = 1 for ethanol but perhaps 1.1 for CH3COOH.

• chemistry (college final) -

ok thank you. It is ethanol, not propane o that would make sense.

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