Boiling Point Elevation (quick q)

[Transformers]

In a boiling point elevation graph of %NaCl (x-axis) vs. Boiling Point (y-axis), what would the graph look like...is it exponential or logarthmic?

Can you explain why too..?

thanks.

 

[unDRdog]

why would it be log?...

 

[basolateral]

In a boiling point elevation graph of %NaCl (x-axis) vs. Boiling Point (y-axis), what would the graph look like...is it exponential or logarthmic?

Can you explain why too..?

thanks.

boiling point elevation = molality x constant
it's a linear equation, with boiling point being directly proportional to concentration so if i had to guess i'd say it's a linear graph.

 

[stockraider]

yep linear, remember constant*molality*vant hoff factor will give you a straight line, however eventually there will be a plateau cause you can only put so much NaCl until the solution becomes saturated and can't take any more dissolved salt.

 

[Transformers]

a kaplan topical begged to differ otherwise...i too thought otherwise, but their answer indicated log...i was pissed because i thought linear too.

I think their logic was something along the lines that initially, the NaCl that you disrupts the H2O-H2O interactions significantly but with increasing amounts of NaCl, those solute-solvent interactions become less significant so it essentially plateaus out.

WEIRD

 

[movax]

I saw the OP and immediately recognized a Kaplan question...why am I accidentally remembering questions instead of material? Gaaaah! (that question popped up in their FL.)

(also guessed wrong)

 

[Transformers]

well whats the answer

 

[stockraider]

wait, so a kaplan topical showed that for every increase in the amount of NaCl, there is a TEN fold (logarithmic) increase in the disruptions of H2O-H2O interactions? i find this hard to believe because the molecules act to physically separate only 2 H2O molecs, unless they break up a pair then move onto a next pair before the original pair has the ability to get back together?? i know i may be thinking too much at a molecular level, but i need to justify this 'logarithmic' change somehow...anybody have a better explanation as to why its logarithmic or does mine make any sense?

 

[kentavr]

I would make this approach
1. The solubility of NaCl in water is not infinite. It reaches the saturation point, beyond which the boiling point is not affected.

2. In small concentartions that will be a straight line according to c_b*molality*vant_hoff.

3. So we have raising straight line at the beginning and flat line (parallel X-axis) in the end. It looks more like log rather then exponential.

In reality it probably much more complicated (with higher BP we get higher solubility, some type of equilibrium should be established at high concentration that bring square root dependency, equilibrium constant also depend on temperature etc). But with given choices the answer close to Log, not exp.

 

[stockraider]

my god that was stupid of me and an important mcat lesson, when a graph goes from linear to plateu it is a LOG graph...NOT linear. i completely forgot what a log graph looks like and that's why i was confused. a log graph has an x intercept at 1 and rises, but levels off after a certain point...thanks for clearing this!!