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## A gas in a balloon expands from 2L to 3L against 2 atm of pressure, what is the change in temperature relative to the initial temperature?

# 2 Answers

Since the pressure is constant, V/T = constant

V1/T1 = V2/T2

T2/T1 = V2/V1 = 3/2

(T2-T1)/T1 = 3/2 - 1 = 1/2

It means that the temperature is increased by 1/2 of its initial temperature.

The general equation governing *changes* in fixed-quantity gasses is

` P`

_{1}V_{1} P_{2}V_{2}

--------- = -------

T_{1} T_{2}

This is what is known as the "combined gas law" because it covers the other known laws with separate names:

**Boyle's Law**- constant temperature - P_{1}V_{1}= P_{2}V_{2}**Gay-Lussac's Law**- constant volume - P_{1}T_{2}= P_{2}T_{1}**Charles's Law**- constant pressure - V_{1}T_{2}= V_{2}T_{1}

In this case, since the pressure is constant (2 atm) but volume and temperature are changing, we can use Charles's Law:

`V`

_{1}T_{2} = V_{2}T_{1}

(2)(T_{2}) = (3)(T_{1})

T_{2} = 3/2 T_{1}

So **the final temperature is 1.5x the initial temperature**. Note specifically that both the initial and final temperature in this equation must be measured in Kelvins (absolute temperature) - you cannot use Fahrenheit or even
Celsius, as they are not the natural temperature scales. If the problem gives initial temperatures in either °F or °C then you must convert to Kelvin first:

K = (5/9)(F - 32) - 273.15

= C - 273.15

where F is degrees Fahrenheit and C is degrees Celsius.

## Comments

You didn't answer "

the changein temperature relative to the initial temperature".I disagree.

At worst, I think that is a matter of interpretation on an ambiguous question, and a matter of semantics. Does "the change" refer to an additive change or a multiplicative change. Both are "relative to" the initial temperature. IMO, most questions of this sort expect a multiplicative factor, not an additive one.

I saw that you had posted an additive answer, so I chose to go with the other option. But even so, when you say "it increased by 1/2 of it's original temperature" that is no different than when I say "the final temperature is 1.5x the initial temperature". It's just you stated the change as an additive term, and I stated it as a multiplicative factor. It's the same thing, just the same as saying "my bank account went up by 50%" vs "my bank account is 1.5 time what it used to be".... the only difference is in the semantics, but it's the same answer.

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