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Topic: Calculate the freezing point  (Read 1344 times)

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Offline Judy

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Calculate the freezing point
« on: April 27, 2021, 03:56:03 AM »
The saturated vapor pressure of a certain concentration of sucrose aqueous solution at 100 °C is 75 cm Hg.
After the sucrose in the solution is completely hydrolyzed by chemical methods, what is the freezing point of the resulting solution?
The given answer is  -2.79  °C.

Here's my calculation:
ΔP = Xb * Pa * i  => 1/=-75/76 = Xb *1*1 => Xb = 1/76
=> Cm = 0.74

ΔTf = Kf* Cm * i = 1.86 * 0.74 * 1 = 1.3764
Therefore, the melting point should be -1.3764

Offline mjc123

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Re: Calculate the freezing point
« Reply #1 on: April 27, 2021, 10:34:45 AM »
What is the result of the hydrolysis of sucrose?

Offline Judy

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Re: Calculate the freezing point
« Reply #2 on: April 27, 2021, 09:28:31 PM »
The hydrolysis of sucrose obtains 1 glucose and 1 fructose, and thus makes the i=2.
However, when can assume the sucrose as i=1, instead of i=2?

Online Borek

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Re: Calculate the freezing point
« Reply #3 on: April 28, 2021, 02:52:42 AM »
The hydrolysis of sucrose obtains 1 glucose and 1 fructose, and thus makes the i=2.
However, when can assume the sucrose as i=1, instead of i=2?

The i for sucrose is always 1. After hydrolysis you have a new solution, in which total concentration of substances responsible for the temperature change is twice as large.
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Offline Judy

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Re: Calculate the freezing point
« Reply #4 on: April 28, 2021, 07:42:59 AM »
The i for sucrose is always 1. After hydrolysis you have a new solution, in which total concentration of substances responsible for the temperature change is twice as large.
[/quote]
Thanks for answering. Could you explain more specifically how exactly the hydrolysis yields a new solution and how it is responsible for twice the temperature change? Thank you.

Offline mjc123

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Re: Calculate the freezing point
« Reply #5 on: April 28, 2021, 08:54:49 AM »
You have said it yourself: "The hydrolysis of sucrose obtains 1 glucose and 1 fructose, and thus makes the i=2." In place of a solution of (say) 1 mol/kg sucrose, you have a solution of 1 mol/kg glucose and 1 mol/kg fructose, i.e. 2 mol/kg total solute.

Offline Judy

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Re: Calculate the freezing point
« Reply #6 on: April 28, 2021, 09:40:04 AM »
You have said it yourself: "The hydrolysis of sucrose obtains 1 glucose and 1 fructose, and thus makes the i=2." In place of a solution of (say) 1 mol/kg sucrose, you have a solution of 1 mol/kg glucose and 1 mol/kg fructose, i.e. 2 mol/kg total solute.
Thanks for you clarification and confirmation.

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