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It depends on the units of the constants in the problem. If it's a problem with molar heat capacity, use kelvin. If specific heat capacity, you would use celsius. Furthermore, usually you'd do delta T, which either would work. The ideal gas law, however, is always in kelvin, and usually if they ask for just "T" it'd be in kelvin.
Usually the question will specify. If you are not sure whether to use Kelvin or Celsius, an easy way of checking is to see what the units are for the other numbers or constants in your equation to see if they have Kelvin or Celsius in them. Depending on which one they use, you may or may not have to convert.
most equations in chemistry tend to use Kelvin instead of Celcius or Farenheit. This is important for these calculations because without conversion, the answers to some questions may come out incorrectly. Usually, if the question does not want you to use Kelvin, it will let you know.
Most of the time, temperature has to be changed to Kelvin to be plugged into a formula. However, when an formula requires the change in temperatures (such as in q = mcdeltaT), the use of Celsius for the initial and final temperatures are acceptable because the scaling factor from Celsius to Kelvin cancels out in the subtraction. However, you should convert the temperature you get from the formula into whatever units the question wants you to put your answer in.
Most questions will require you to convert to Kelvin, since this is the S.I. unit. However, if the question asks for the change in temperature, delta T, then it's unnecessary to change the units. This is because Celsius and kelvin have the same magnitude, so the change in temperature will be the same.
As many students started above, K is usually your best bet. The reason for this, however, is that K is an SI unit and therefore is usually used in multiple occasions/circumstances unless it is specifically stated that the answer should be in C or even F. My old high school chemistry teacher ingrained in our minds that SI units are always the go to standard unless started otherwise
Most of the time yes, because you have to cancel the units in the gas constant R. But when you are calculation the difference between temperatures, for example, in the equation involving heat capacity, you don't have to worry about that.
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