# Chem multiple choice PV=nRT Q

385 cm? sample of carbon dioxide at 100 ka and 25 °C was mixed with 2.89 × 102 mol of argon. The gas constant. R= 8.31 J KI was)
What is the mole fraction of carbon dioxide in the mixture?

The answer is 0.35 but I keep getting 0.8something?

Any help appreciated
I’m just going to check I am interpreting everything correctly.

385 cm? should read as 385 cm^3, 100 ka should read as 100 kPa and 2.89 × 102 mol should read as 2.89 × 10^-2 mol

Starting from pV = nRT, can you rearrange to make n the subject?

Converting 25°C to K and either p or V to appropriate units, can you now calculate a value for n?

Mole fraction of CO2 = (moles of CO2)/(moles of CO2 + moles of Ar) - does this give the final answer you are expecting?
Original post by TypicalNerd
I’m just going to check I am interpreting everything correctly.

385 cm? should read as 385 cm^3, 100 ka should read as 100 kPa and 2.89 × 102 mol should read as 2.89 × 10^-2 mol

Starting from pV = nRT, can you rearrange to make n the subject?

Converting 25°C to K and either p or V to appropriate units, can you now calculate a value for n?

Mole fraction of CO2 = (moles of CO2)/(moles of CO2 + moles of Ar) - does this give the final answer you are expecting?

sorry yes my phone copy and paste gives strange formatting. Yes, and frustratingly no. This is what i did to get to 0.8 something...
Original post by uhhhh6678987654
sorry yes my phone copy and paste gives strange formatting. Yes, and frustratingly no. This is what i did to get to 0.8 something...

I think I know where you went wrong. I think there was an error in which you incorrectly converted one of the units.

When you have the ideal gas law, you have two combinations of units that work:

(1): p is in Pa and V is in m^3
(2): p is in kPa and V is in dm^3

I tend to use combination (2). How might you convert 385 cm^3 to dm^3?
Original post by TypicalNerd
I think I know where you went wrong. I think there was an error in which you incorrectly converted one of the units.

When you have the ideal gas law, you have two combinations of units that work:

(1): p is in Pa and V is in m^3
(2): p is in kPa and V is in dm^3

I tend to use combination (2). How might you convert 385 cm^3 to dm^3?

I was taught (1) I did 385/10’6

if I had done (2) it would be over 1000
Original post by uhhhh6678987654
I was taught (1) I did 385/10’6

if I had done (2) it would be over 1000

Ok. You are converting the volume correctly. I’d imagine you have converted T to K correctly.

Since you did (1), can you convert 100 kPa to Pa?

Then try plugging in p, V, R and T into pV = nRT.
Original post by TypicalNerd
Ok. You are converting the volume correctly. I’d imagine you have converted T to K correctly.

Since you did (1), can you convert 100 kPa to Pa?

Then try plugging in p, V, R and T into pV = nRT.

X1000. And yes I did that? I’ll try again when I have a chance