# When MUST you use Kelvin for calculations?

#### TheOneAndOnlyJamesFranco

##### Full Member
7+ Year Member
Recently got this problem from NS 3 wrong...

A balloon has a volume of 3L at 25C, what is its volume at 50C?

I understand Charles law and everything but:

If you use C, you get 6 L

If you use K, you get 3 L

Big difference.

When are you supposed to use K? Is it just best to assume to always convert to Kelvins for gas calculations? Even when there's no mention of standard whatever?

Am I missing something?

Thanks!

#### popopopop

##### Full Member
7+ Year Member
Gas constant R is 8.3144598(48) J mol−1 K−1 per wiki, so yes, always use K when using the gas law unless stated else-wise.

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##### Full Member
2+ Year Member
In Charles’s Law (or Boyle’s Law or Gay-Lussac's Law) it does matter whether you use Celsius or Kelvin, so you have to use Kelvin.

“A balloon has a volume of 3L at 25C, what is its volume at 50C?”

New Volume = 3L * 323K / 298K = 3L * 323 = 3.25L

In the ideal gas law (or any other equation involving the universal gas constant) you have to use the same unit for temperature (or pressure, volume and amount) as the units of the universal gas constant. Usually you will use R = 8.314 L kPa / K mol so those are the units you need to use for the other values.

However when changes of temperature are involved (such as in Q = mcΔT) it doesn’t matter whether you use Celsius or Kelvin. The change in temperature is the same whether it is 50 - 25 or 323 - 293.

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#### TheOneAndOnlyJamesFranco

##### Full Member
7+ Year Member
Gas constant R is 8.3144598(48) J mol−1 K−1 per wiki, so yes, always use K when using the gas law unless stated else-wise.

In Charles’s Law (or Boyle’s Law or Gay-Lussac's Law) it does matter whether you use Celsius or Kelvin. The Celsius scale is not an absolute temperature scale and so you have to use Kelvin.

“A balloon has a volume of 3L at 25C, what is its volume at 50C?”

New Volume = 3L * 323K / 298K = 3L * 323 = 3.25L

In the ideal gas law (or any other equation involving the universal gas constant) you have to use the same unit for temperature (or pressure, volume and amount) as the units of the universal gas constant. Usually you will use R = 8.314 L kPa / K mol so those are the units you need to use for the other values.

However when changes of temperature are involved (such as in Q = mcΔT) it doesn’t matter whether you use Celsius or Kelvin. The change in temperature is the same whether it is 50 - 25 or 323 - 293.

Ah okay thanks guys these made so much sense lol... forgot about R when you're looking at just a couple variables!

Is it safe to assume that we will always be given which ever version of the Gas constant we need ?

Has anyone ever been caught by the AAMC without knowing a particular constant?

#### ElectricNoogie

##### MCAT enthusiast
The AAMC has very few constants they expect you to know, even less so on the new MCAT. As for K vs. C, we primarily use K in calculations when dealing with absolute T. The AAMC would have to provide units of R or T in order for you to know which to use. However, when dealing with T changes, you can use K or C.

Good luck!

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