20.0L of a sample of gas at 25 Celsius is compressed to 5L. What is the final temperature in CELSIUS assuming there is no pressure change?i got...

This is a Charles's law application.  To do problems of
this sort you must first convert the temperature to kelvin.  K = 273.15 + degrees
celsius.  After you finish working the problem then convert the answer which you get
from Kelvin back celsius.

In this case you have an initial
volume of 20 L, a final volume of 5 liters, and an initial temperature of 25 C or 298.15
K.

In equation form you
have:

20/298.15 =
5/T2

rearranging to solve for T2, you
get:

T2 = 298.15  x 5 /20

T2 =
74.54 K

Converting to degrees celsius you get 74.54 -
273.15 = -198.6 degrees celsius.

So you did it correctly.
What this illustrates is how much you have to cool a gas to decrease its volume.  You
may have seen the demonstration where liquid nitrogen (77 K) is poured over an inflated
balloon and the balloon becomes almost flat.  This problem is comparable to that type of
low temperature.

As long as you convert to Kelvin first,
then do the problem, and then convert back to celsius you will continue to get the
correct answer.