TBR CBT 5 PS question #3

[dnovikov]

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A little confused as to the thought process behind answering this question:


3) How many grams of glucose must be added to 0.5 kg of water to raise the boiling point by 0.10 ˚C, given that the value of i is 1 and the value of γ for glucose is roughly equal to that of formic acid?

A. 5 grams
B. 15 grams
C. 90 grams
D. 180 grams

Answer: B <----- Highlight to see

Formula Give: &#8710;Tb = kb • i • m • &#947;
Table for reference

 

[Arctangent]

One thing to try would be to pretend there is a fifth row in the column, for glucose. To fill out this row, you need 1m glucose, or as there is .5kg of water, half a mol of glucose, or 90g (as glucose is 180g/mol). Now considering this scenario, with 90g glucose, we know that kb is the same between our fifth row and the 4th (formic acid), i is 1 for both, m is 1 molal for both, and as stated, &#947; is assumed to be equal. So we know that 90g of glucose corresponds to a .6C increase. We need a .1C increase. As &#8710;Tb is directly proportional to m and thus mass, we need to scale &#8710;Tb by 1/6, and thus we must scale the mass by 1/6. Therefore, 15g would yield a .1C increase.

 

[dnovikov]

Awesome explanation, that clears it up a lot, thanks so much!

 

[pm1]

One thing to try would be to pretend there is a fifth row in the column, for glucose. To fill out this row, you need 1m glucose, or as there is .5kg of water, half a mol of glucose, or 90g (as glucose is 180g/mol). Now considering this scenario, with 90g glucose, we know that kb is the same between our fifth row and the 4th (formic acid), i is 1 for both, m is 1 molal for both, and as stated, &#947; is assumed to be equal. So we know that 90g of glucose corresponds to a .6C increase. We need a .1C increase. As &#8710;Tb is directly proportional to m and thus mass, we need to scale &#8710;Tb by 1/6, and thus we must scale the mass by 1/6. Therefore, 15g would yield a .1C increase.

bump!

I was reviewing this question and I am not sure how do "we know that kb is the same between formic acid and glucose?"

Any help, would be highly appreciated! thank you!!