How much heat is needed to convert 20 g of ice at -40 C to steam at 300 C

[khadcheemz]

Okay so I get when to use q=mC∆T and Q=mHfusion or Q=mHvaporization

But for this specific problem for the solid phase they use 40 C for ∆T and I have no idea where that is coming from
For the liquid phase its 100 C for ∆T
and for the gas phase its 200 C for ∆T

Does anyone know where these values are coming from?

Also another question, does anyone know how to tell if a molecule is polar or nonpolar from its Lewis structure? I can't seem to understand it. I've been told that if all the atoms around the central atom are the same and if there are no lone pairs its nonpolar and if theres a lone pair its automatically polar. Some problems I've been doing, however, don't follow that pattern. Is there any quick and obvious way to know?
Thank you!

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[bodmon]

ice melts to water at 0 degrees C, and boils at 100 degrees C. I suggest looking at a phase change diagram to help you set up the different calculations you have to make.

You get better determining whether or not molecules are polar through practice. It takes some time to be able to say if its polar or not without drawing the lewis structure. I just recommend drawing it each time until you start noticing patterns between molecules consisting of atoms from different groups.

 

[nornton]

A molecule is polar if there is a difference in electronegativity between the atoms. For example, H2O is polar because the H and O have a difference in electronegativity (the H will be partially positive and the O will we partially negative) wheras a C-H bond is nonpolar because C and H have similar electronegativities.

 

[khadcheemz]

Thank you both ^^