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## Homework Statement

The problem is that we need to calculate, how much time it takes for nitrogen to evaporate.

Given data: dewar flask is in a room, where is T= 293K temperature; In such flask m=0,2 kg of Hydrogen would evaporate in t= 3600s. Also there is given boiling point temperatures for Hydrogen and Nitrogen T

_{H2}= 20,4K ; T

_{N2}= 77,3K

Heat of evaporations for both are : L

_{H2}= 4,5 x 10

^{5}J/kg

L

_{N2}=2 x 10

^{5}J/kg

## Homework Equations

I don't really know how to do it. Dewar flask don't allow to lose the heat, so no external loss Q=0 J

I found only one way to find the answer, but I think it's not possible. I used this formula -

k(T-T') t = Lm

## The Attempt at a Solution

k(T-T

_{H2}) t = L

_{H2}m

k(T-T

_{N2}) t' = L

_{N2}m

So t' would be t'= L

_{N2}( T-T

_{H2}) t / ( L

_{H2}(T-T

_{N2}))

And I would get the answer of 2022 seconds or about 33,7 minutes

I'm waiting for your opinion and attempt for the answer. :)

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