[yummy]

please can someone help with this question.. what is the volume of gas at -30C and 0.75 atm if it occupies a volume of 255L at 40.0C and 1.25atm? thank you all

[arnyk]

I can help you if you show me some workage.

[Donaldson Tan]

1. convert all temperature to kelvin
2. solve using the general gas law, ie. PV = nRT (assume perfect gas)
3. n is constant since amount of gas remains unchanged.

[arnyk]

Alrighty then, let's see what you are given and what you require.

Given:

P1 = 1.25 atm (convert to kPa)
V1 = 255 L
T1 = 40C (convert to K)

P2 = 0.75 atm (convert to kPa)
V2 = ?
T2 = -30C (convert to K)

Now what formula could you possibly use which utilizes all givens and solves for the variable?

As geodome said, n is constant.

Combined
Gas
Law
=
Combined
Gas
Law

 

[xiankai]

a basically summarised version of arnyk's formula is the combined gas law, visually represented as:

P₁V₁/T₁ = P₂V₂/T₂.

n is the universal gas constant while R is the molar mass. molar mass is also a constant, so u can safely ignore it

[arnyk]

I always thought n = moles and R = universal gas constant and molar mass usually isn't in the gas law formulas?

[xiankai]

yeah thats the problem i thought too. there are websites that say one thing, but there are websites that say the opposite. while both are reputable, i have no choice but to follow up on geo's statement that n is the constant

just to note, these shouldnt be considered too seriously as they can be used alternatively anyway, their positions in the formula are basically the same.

yeah i think molar mass usually isnt in the gas law formulas. i brought it up in case yummy is curious

[Donaldson Tan]

n is the universal gas constant while R is the molar mass. molar mass is also a constant, so u can safely ignore it

The above quote is wrong. Reference to any physical chemistry textbook will tell that R refers to molar gas constant, and n refers to number of moles. Since n is constant, then nR must be constant, and given that PV = nRT, then

P₁V₁/T₁ = P₂V₂/T₂

[arnyk]

Woohoo!  For once in my life, I got something right lol.  

R = 8.31 kPaL/molK

[xiankai]

ok thanks for clarifying that  

hey u've been getting everything right so far