datgirl123

2+ Year Member
May 25, 2016
25
4
Status
Pre-Dental
hello guys,

i keep getting stuck with the fp and bp when it comes to particles. i thought it was given that the more number of particles we have, means theres an increase of freezing point and decrease of boiling point the example being salt on roads.

but then i was confused with this question. which of the following substances produces the highest freezing point when dissolved in a liter of water?

a).10 moles HClO4
b) .10 moles HClO3
c) .10 moles HClO2
d) .10 moles HClO
e). 10 moles HCl

answer is d, not sure. i get that HClO4 will have the most amount of particles bc it dissasociates with having more oxygen. then wouldnt that give it the highest freezing point?

what is a general rule i can follow when it comes to particles and fp/bp. thanks.
 

fit2

2+ Year Member
Aug 7, 2015
1,090
785
Status
Pre-Dental
You are thinking about the lowest boiling. The more particles dissolve the greater you "i" value. Becuase d is the weakest acid you get least amount particles, therefore it is the highest boiling point, or closest to 0. It's like opposite for boiling point...they're highest boiling point would be a and lowest boiling point would be d.
 

fit2

2+ Year Member
Aug 7, 2015
1,090
785
Status
Pre-Dental
Well let's say that you get complete ionization of all your answers a-through d. (c and d aren't really hard acids at all but hypothetically speaking imagine that the first 4 all dissociate). For a you have 6 particles...for d you have 3. If you have a 2m solution then the freezing point for the first one would be -12 and the freezing point would become larger (as it gets closer to 0) as you go down. Answer d would be -6. So d has a higher freezing point than a. Remember the colligative property says that solute lowers the freezing point. (opposite from boiling point) So d is the higher freezing points because -6 is greater than -12...you're talking about negative number that's why it seems opposite from boiling point. Does this make sense? I think you're having a hard time understanding because of the confusing idea of lower/higher freezing point...you have to think negative numbers. Lemme know if this makes sense

P.S. reading your post you might have mixed things up....solute added result in higher boiling point and lower freezing point. They add salt in order for the water on the roads to freeze at lower temperatures....that way if it's -5 or something you don't get ice because it needs a LOWER temperature in order for the water to freeze
 
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