Ideal gas law and temperature units

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7+ Year Member
Dec 30, 2015
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When using the equation P1V1/T1 = P2V2/T2 for instance in an example where we are looking for V2, why can't we just use the temperature in Celsius since the units will cancel out?

I know that if I am using the equation PV = nRT, then I have to use the temperature in kelvins to cancel out the units in R.

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Because Celsius and Kelvin aren't related to each other by some multiplication factor. The reason you can use whatever units of, say, volume you want is because those are related to each other in a multiplicative fashion. So, for example, 1 mL is 1/1000th of a liter and 2 mL is 2 times 1/1000th of a liter. It's not the same for temperature. Kelvin is 273 plus whatever temperature you have in Celsius so it's a constant that's added to the Celsius temperature, not a factor that the latter is multiplied by.