Highest Boiling Point

[Umyo]

Which solution has the HIGHEST boiling point? (Berkeley Review Gen Chem. Part 2, page 90, example 7.16)

A.) 0.10 moles magnesium chloride in 100 mL water

B.) 0.15 moles lithium bromide in 150 mL water

C.) 0.20 moles sodium iodide in 100 mL water

D.) 0.25 moles potassium fluoride in 150 mL water

The answer is "C'. I put "A". I used the formula for finding the increase in boiling point when solute is added to a solution: Delta Tb = kb*i*m

I calculated the (i*m) values for each of the solutions, since (kb) is constant, and the highest value came out to be 9, for choice A.

How is the answer C when the (i*m) value for "C" is 8?

Any and all help would be much appreciated!

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[jcb123]

Ok, so if you just calculate the concentrations of the ions for each you can see that C has the highest concentration
A) 0.1 moles/100 mL = .001 M x 3 ions per molecule = .003
B) .15/150 = .001 x 2 ions = .002
C) 0.2/100 = .02 x2 = .04
D) .25/150 = aprox .002 x 2= .004

hope that helps, I literally just got done working on this type of problem from the EK chem book

 

[Umyo]

Thanks for the help. I made a clumsy error by thinking that molality involves counting the number of ions that an ionic compound splits up into.

 

[SweetBurger]

I thought you just calculated the molality of each substance then multiplied the molality times the number of ions that the substance broke up into and then multiplied by the boiling point constant and that would give you your temperature change

 

[jcb123]

yeah thats what I was doing just sort of skipping the last steps. and yeah I used molarity instead of molality because we didn't need any exact number solutions we just needed to know which had the highest concentration of ions to determine the largest change in boiling point

 

[MDhopeful74]

Ok, so if you just calculate the concentrations of the ions for each you can see that C has the highest concentration
A) 0.1 moles/100 mL = .001 M x 3 ions per molecule = .003
B) .15/150 = .001 x 2 ions = .002
C) 0.2/100 = .02 x2 = .04
D) .25/150 = aprox .002 x 2= .004

hope that helps, I literally just got done working on this type of problem from the EK chem book

I'm not sure if it's too late to comment on this or if I need to make another thread asking this, but why can we do this calculation in this example with molarity? Can't we only use the molality for when dealing with van hoff/highest amount of impurities sorta of questions? Aren't we supposed to use molality for concentration in these kinds of solutions?

 

[aldol16]

I'm not sure if it's too late to comment on this or if I need to make another thread asking this, but why can we do this calculation in this example with molarity? Can't we only use the molality for when dealing with van hoff/highest amount of impurities sorta of questions? Aren't we supposed to use molality for concentration in these kinds of solutions?

That's not the point of the problem. The point of the problem is recognizing which one of those solutions has the greatest concentration of electrolytes. You're not calculating the exact boiling point elevation here so you don't need the exact concentrations.

 

[BerkReviewTeach]

I'm not sure if it's too late to comment on this or if I need to make another thread asking this, but why can we do this calculation in this example with molarity? Can't we only use the molality for when dealing with van hoff/highest amount of impurities sorta of questions? Aren't we supposed to use molality for concentration in these kinds of solutions?

For aqueous solutions, molality is only slightly higher than molarity, so on the MCAT many questions will use them interchangeably. And as Aldol pointed out, you are looking for a relative boiling point, as opposed to an exact value. For an exact value type of question, more common in college chemistry than on the MCAT, you would need molality. But in this question, using molarity gets you the same relative boiling points for the solutions as you'd get using molality.

 

[MDhopeful74]

That's not the point of the problem. The point of the problem is recognizing which one of those solutions has the greatest concentration of electrolytes. You're not calculating the exact boiling point elevation here so you don't need the exact concentrations.

For aqueous solutions, molality is only slightly higher than molarity, so on the MCAT many questions will use them interchangeably. And as Aldol pointed out, you are looking for a relative boiling point, as opposed to an exact value. For an exact value type of question, more common in college chemistry than on the MCAT, you would need molality. But in this question, using molarity gets you the same relative boiling points for the solutions as you'd get using molality.

Thank you! So if I was asked for exact concentrations, I could be use molarity or molality interchangeably?

 

[aldol16]

Thank you! So if I was asked for exact concentrations, I could be use molarity or molality interchangeably?

What do you mean? If you're asked for exact concentrations, you will be asked to give the answer in either molars or molals. These will be (slightly) different numbers. For instance, if you were doing an actual boiling point elevation calculation, you would need molality since the constant is usually provided in terms of molals.