Homework Help: chem-answer check please

Posted by Samantha on Thursday, January 10, 2013 at 4:41pm.

Compounds like CCl2F2 are known as chlorofluorocarbons, or CFCs. These compounds were once widely used as refrigerants but are now being replaced by compounds that are believed to be less harmful to the environment. The heat of vaporization of CCl2F2 is 289 J/g. What mass of this substance must evaporate to freeze 195 g of water initially at 17°C? (The heat of fusion of water is 334 J/g; the specific heat of water is 4.18 J/g-K.)

first i used q=mcdeltat to get the q for going from 17 degrees to 0. then i found q in q=heat fusion times mass

i added those together then i did the total= heat of vap times m so i divided by mass and got 273.356g as my answer. is that correct? I wasnt sure if i had to switch celcius to kelvin? I didnt but are you suppose to? thanks!

chem-answer check please - DrBob222, Thursday, January 10, 2013 at 6:13pm
That looks good to me; however, you have too many significant figures in your answer. You need not change celsius (note the spelling) to kelvin since the difference in C is the same as the difference in K (try it and confirm that). 290-273 = 17 and 17-0 = 17. :-).
I think you made a typo--total = heat vap x m so I divided by "heat of vap" instead of mass. You are finding mass of the freon.
Finally, if your prof is picky about signs. you should note that the sign of heat from water going from 17 to zero C is negative and the sign for heat vap x m = negative. Mass, of course, still turns out to be positive but some profs want that -289xmass freon = mass H2O x specific heat H2O x (0-17) to show negative on both sides becoming + for mass.

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