[Judy]

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

[mjc123]

What is the result of the hydrolysis of sucrose?

[Judy]

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?

[Borek]

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.

[Judy]

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.

[mjc123]

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.

[Judy]

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.