If a fixed mass of gas has a certain volume at a temperature of 50 Celsius, what temperature is required to double its volume while keeping the pressure constant? 

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According to Charles' law, the volume and Kelvin temperature of a gas are directly proportional. If the pressure and number of moles (or mass) of the gas are constant, then V1/T1 = V2/T2 where V1 and T1 are the initial volume and temperature and V2 and T2 are the new volume and temperature. If the volume is doubled then the temperature must be double as well since the ratio of volume to temperature is constant.

It's necessary to use the Kelvin temperature in gas law calculations because it's a proportional scale with a true zero point.  The Celsius scale isn't proportional because it has negative numbers. Kelvins = degrees C + 273, so the initial temperature is 50+273 = 323 Kelvins. The temperature required to double the gas volume is 2 x 323  = 646 Kelvins. 

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