TBR Chapter 3 Passage 8

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MrNeuro

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there are two questions that both relate here one of the questions here says that

as temperature increases density will decrease as the volume of the solution will increase...im all good here

Then a question in the same passage asks

Which solution has the HIGHEST boiling point? (side note molar mass of MX < MY)
A. 1.0 grams MX with 10 mL water at 25 C
B. 1.0 grams MY with 10 mL water at 25 C
C. 1.0 grams MX with 10 mL water at 50 C
D. 1.0 grams MX with 10 mL water at 50 C

Answer: C
"the solution with the highest boiling point is the solution with the highest molality of impurities. Molality is defined as moles solute per kg solvent. MX has a lower molecular mass than MY, thus 1.0 grams of MX generates a greater moles of solute than 1.0 grams of MY. Because the density of water decreases as the temperature of water increases, the mass of 10 mL water is greater at the lower temperature. This means that the largest molality is found with 10 mL water at 50 C, because the mass solvent is least.

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Must be an interesting passage. Not sure how temperature would affect mass. Even if it did, you'd think that if M increases, D would increase as well. D=M/V and all...

Incidentally, because volume does change slightly as temperature changes, the molarity of a solution also changes slightly with temperature. That's why, for exact work, scientists use molality (moles of solute per kg of solvent) as a temperature-independent measure of concentration. But I'm off topic.
 
Must be an interesting passage. Not sure how temperature would affect mass. Even if it did, you'd think that if M increases, D would increase as well. D=M/V and all...

Incidentally, because volume does change slightly as temperature changes, the molarity of a solution also changes slightly with temperature. That's why, for exact work, scientists use molality (moles of solute per kg of solvent) as a temperature-independent measure of concentration. But I'm off topic.

Maybe I'm getting this wrong heres what they say

"because the density of water decreases as the temperature of water increases the mass of 10 mL of water is greater at the lower temperature. This means that the largest molality is found with 10 mL of water at 50 degrees Celsius, because the mass solvent is least."

Heres what i think they're trying to say

D= m/v
as T increases V of water increases = D decreases
as T decreases V of water decreases = D increases meaning that mass of water should also be greater???? can you say that?
 
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Your bolded text says that there is more mass of the solvent water per unit volume at lower temperatures.

Your original post says that there is more mass of the solvent water per unit volume at higher temperatures.

I am confused.

I don't have the original passage, but it sounds like they are dumping a constant amount of solute into a constant volume of solvent at various temperatures. While the molarities (mol solute per liter soln) would be the same, the molalities (mol solute per kg solvent) would be different because the mass of the solvent per unit volume would be different at different temperatures. At a high temperature you would have the same solute in a smaller mass of solvent (because its density is lower), making a larger molality.
 
there are two questions that both relate here one of the questions here says that

as temperature increases density will decrease as the volume of the solution will increase...im all good here

then they go on to say in another question that density is decreasing due to an increase in temperature BUT instead of attributing it to the volume increase they're saying its due to an increase in the mass of the water in the solution....Can you have it both ways??? more specifically questions 54 and 56...

Could you clearly state the problem?
 
Could you clearly state the problem?

Your bolded text says that there is more mass of the solvent water per unit volume at lower temperatures.

Your original post says that there is more mass of the solvent water per unit volume at higher temperatures.

I am confused.

I don't have the original passage, but it sounds like they are dumping a constant amount of solute into a constant volume of solvent at various temperatures. While the molarities (mol solute per liter soln) would be the same, the molalities (mol solute per kg solvent) would be different because the mass of the solvent per unit volume would be different at different temperatures. At a high temperature you would have the same solute in a smaller mass of solvent (because its density is lower), making a larger molality.

i guess I'm having difficulty understanding how the mass of water is greater at a lower temperature....

I'm having difficulty wrapping my head around how you can say that a mass has more mass at lower temperatures

water = 1 g/mL
Density = mass / volume

mass= volume * Density

according to TBR
volume increases as temperature increases thus density decreases
and
volume decreases as temperature decreases thus density increases

both cases would yield the same MASS of water....is this better?

sorry if my last posts didn't make sense I'm sorta sleep deprived.

one last note did some digging through the internet and found some people on physics forums saying that the mass of water does not change w/ temperature.

oh and I've also edited the original post to include the full explanation for the question.
 
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That's what I figured. The mass of a mass of water doesn't change with temperature. The mass of a volume of water does vary, depending on its temperature.

It isn't like they took four equal volumes of water, and heated them to different temperatures, and then added solute. They had four buckets of water, each at different temperatures (and different densities), and they grabbed a constant volume of water from each.

Then sure, the volume extracted from the bucket of water at the higher temperature has less mass, because it also has the lowest density.
 
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