Celsius to Kelvin conversion problem (Kinetic theory)

1. Feb 19, 2016

Forco

1. The problem statement, all variables and given/known data
Find the temperature T that allows the rms speed of a gas to be equal to another gas with T=47°C.
The molecular mass of the first gas is 64, and the molecular mass of the second gas is 32.

2. Relevant equations
$$v_{rms}= \sqrt{\frac{3RT}{M}}$$

3. The attempt at a solution
The problem is actually very easy. It's actually really simple to conclude that
$$T_1=2T_2$$. However, my problem arises when actually replacing the given temperature.
If I take the second temperature to equal 47°C, then the first temperature is equal to 94°C. And converting that to kelvin gives 367.15 K.
However, if instead I use directly the temperature in K (47+273.15), then my answer becomes 640.3 K.
Which one is right? I assume the second one because in order for the equation to make sense, T needs to be expressed in K. I'd like to be sure, however.

2. Feb 19, 2016

Staff: Mentor

Well, that depends which gas you label 1, and which you label 2.

Think about this: what if the temperature was 0 °C instead of 47 °C.

3. Feb 19, 2016

Forco

That would make the other temperature zero. Understood! Thank you very much.