Is there an error in this practice question? Or am I being silly?

[BoneMental]

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Stumbled upon a practice MCAT question that goes something like this:

Gas at temperature of 27C has a volume of 60mL.
What is the temperature change needed to get the gas to 90mL?

The correct answer is an increase of 150C....! Or so they say.

I keep wanting to do a ratio, but it doesn't seem to work out correctly.

Can someone help me out?

Thanks!👍

 

[loveoforganic]

V1/T1 = V2/T2

T1*V2/V1 = T2

27*90/60 = 40.5

40.5-27 = 13.5 oC

Error

 

[Rabolisk]

Just to clarify, loveoforganic showed you the erroneous way of doing this problem.

 

[loveoforganic]

Sorry, I was on my iPod and forgot the end! The error is in the units you're choosing for temperature

 

[BoneMental]

Wait, so, how do you do this?

 

[Rabolisk]

You probably messed up because you used units of celsius, which is not correct. Disregard the fact that the question gave you the temperature in celsius. You have to convert it.

 

[BoneMental]

It still doesn't work though...!

V1/T1 = V2/T2
(.06L)/(273+27) = (.09L)/(T2)
T2 = 450K

Change in temp = T2 - T1 = 150K

The answer is 150C though...why don't you have to convert back to Celsius from Kelvin?

 

[loveoforganic]

dT = (273+23)(90/60) - (273+23) = 150

Talking about a change in temp, so celsius and kelvin are the same

 

[apumic]

It still doesn't work though...!

V1/T1 = V2/T2
(.06L)/(273+27) = (.09L)/(T2)
T2 = 450K

Change in temp = T2 - T1 = 150K

The answer is 150C though...why don't you have to convert back to Celsius from Kelvin?

It's a ∆T (i.e., K=ºC). Therefore, no change in units necessary on the second half of the problem. 150ºC=150K when talking about a difference (∆T). Think conceptually, not formulaically.

 

[BoneMental]

I am struggling with this concept.

Can you please elaborate why the Kelvin and Celsius are equal here?

EDIT: I got it! Thanks!

 

[Rabolisk]

To convert from Celsius to Kelvin, you add 273. So let's say T1 was 0 degrees Celsius and T2 was 100 degrees Celsius. T2 - T1 = deltaT = 100 degrees Celsius. Now imagine I asked you to calculate deltaT in Kelvin instead of Celsius. T1 = 273K and T2 = 373K. DeltaT is the same.

Celsius and Kelvin are equal in scale. One degree difference in Celsius is the same as one Kelvin difference.

I don't mean to sound harsh here, but this is easy high school chemistry. If you're struggling with this, I suggest that you have a long look in the mirror.