[Baksu888]

Can someone verify my work for problem number 2 and 3? I either did it right or I completely messed it up. If I did it wrong, please lead me in the right direction. Thanks!

https://imgur.com/a/LSuo6Jk

[Borek]

Please read the forum rules. We prefer questions and answers to be typed, makes it easier to read and available for search.

[Baksu888]

Sorry about that! I should have read the forum rules more thoroughly the first time. I'm not sure if editing your own post or deleting your post is an option, but I'm going to type out the problem and my attempt at it in this reply.

Here's the problem:

A volume of 2.0 L of He at 46°C and 1.2 atm pressure was added to a vessel that
contained 4.5 L of N₂ at STP. What is the total pressure and partial pressure of each
gas at STP after the He is added?

First, I solved for the number of moles of He and N₂ using the ideal gas law.

[tex]nHe = \frac {(1.2 atm)(2.0 L)}{(0.08206 \frac {atm*L}{mol*K})(273 K)} = 0.11mol  \; He[/tex]
[tex]nN_2 = \frac {(1 atm)(4.5 L)}{(0.08206 \frac {atm*L}{mol*K})(273 K)} = 0.20mol  \; N_2[/tex]

Using the moles of He and N₂, I then calculated the partial pressures.

[tex]P_{He} = \frac {(0.11mol)(0.08206 \frac {atm*L}{mol*K})(273 K)}{6.5L} = 0.38atm \; He [/tex]
[tex]P_{N_2} = \frac {(0.20mol)(0.08206 \frac {atm*L}{mol*K})(273 K)}{6.5L} = 0.69atm \; N_2 [/tex]

I added the volume of He and the volume of N₂ together to get 6.5 L of gas in the vessel.

Finally, I used Dalton's law to get P_Total

[tex]P_{Total} = P_{He} + P_{N_2} = 0.38atm \; He + 0.69atm \; N_2 = 1.07atm[/tex]
So, how badly did I mess this up?

[AWK]

I understand that He was moved (added, pumped) to a larger vessel, not that the vessels were connected and the pressure was equalized.

[AWK]

Only the final results should be rounded to two significant digits.

[Baksu888]

Ok, so my setup for this problem was incorrect. What should the first step be towards solving this problem? Can you lead me in the right direction? Is the ideal gas law supposed to be used in this problem?

[AWK]

Moles are almost OK  (significant digits).
Change volume from 6.5 to 4.5 in the next two calculations (significant digits, if they are intermediate results for p_total)
or p_total you can calculate from the sum of moles.

[Baksu888]

If I understood this correctly, it's supposed to be 4.5 L instead of 6.5 L because gas takes up the volume of the container. Originally, there was 4.5 L of N₂ in the vessel, meaning that even if you add He to the vessel, the volume of the gases would still be 4.5 L?

Also, am I not supposed to round to two significant figures when calculating the moles and the atm? When you're dividing, aren't you supposed to round to the smallest sig figs in the calculation? Why should only the results be rounded to two sig figs?

[AWK]

The vessel is not stretchy like a balloon.

Approximate calculation rules - we are only rounding the final results