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It depends on the units of the constants in the problem. For example, in a problem that gives molar heat capacity, you would use kelvin, whereas in a problem that gives specific heat capacity, you would use celsius. If no preference is stated, than either or is fine.
when you're doing a problem that calls for the change in temperature it doesn't matter what units you use because the difference in temp will be the same no matter what units you're in, but the problem should specify what units if they are needed.
If the equation requires deltaT, then it doesn't matter whether you use K or C. If the equation requires T and not deltaT, then usually you will need to use K. Make sure to keep track of your units and that after cancelling, your ending units are actually what you are looking for.
In addition, for entropy you have to use Kelvin, because in the equation you aren't using delta T, you are using T, so multiplying the other terms by 25 (degrees celsius) will get a much different result than multiplying the other terms by 298 (degrees kelvin). In contrast, for a problem asking for delta T, 25-0 (degrees celsius) = 25 degrees C, and 298K - 273K = 25K, so in both cases you multiply the other terms by 25.
Everyone posting in this forum is correct. Usually, when using deltaT, it doesn't matter whether you use K or C because a change in temperature for either will result in the same numbers. However, when using just T in an equation, you need to use the other labels in order to determine whether or not the other units use K or C.
The most important thing to consider in these cases are the units of the constants and given values. This will help you determine if you need to use K or C because you might need to get some of the units to cancel out. i think for things like deltaT either should be fine because they are both scaled the same so the difference would be the same
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