What is the Total Pressure in a Cylinder Containing Nitrogen and Oxygen?

[ravadongon]

Question 1: 5L of nitrogen at 100kPa and 298K and 5L of oxygen at 200kPa are pumped into a evacuated 50L cylinder. Temperature remains constant, what is the total pressure in the cylinder?

Correct Answer: 30kPa
My answer: 300 kPa

Question 2: A 10L flask containing nitrogen at 500mmHg is connected via a closed tap to a 4L flask containing oxygen at 800mmHg pressure, if the tap is opened what will be the final pressure assuming temperature remains constant?

Answer: 586mmHg
My answer: 1300mmHg

Formulas I used:

pV = nRT (to find mols)
PA = (nA/nA+nB) x PT

How did they get these answers...cos I can't seem to get them..?? Any help would be appreciated...

 

[Bystander]

Question 1: 5L of nitrogen at 100kPa and 298K and 5L of oxygen at 200kPa are pumped into a evacuated 50L cylinder. Temperature remains constant, what is the total pressure in the cylinder?

Correct Answer: 30kPa
My answer: 300 kPa

Question 2: A 10L flask containing nitrogen at 500mmHg is connected via a closed tap to a 4L flask containing oxygen at 800mmHg pressure, if the tap is opened what will be the final pressure assuming temperature remains constant?

Answer: 586mmHg
My answer: 1300mmHg

Formulas I used:

pV = nRT (to find mols)
PA = (nA/nA+nB) x PT

How did they get these answers...cos I can't seem to get them..?? Any help would be appreciated...

What are the volumes given in the questions?

 

[ravadongon]

What are the volumes given in the questions?

what do u mean? I gave the volumes...

 

[quetzalcoatl9]

Question 1: 5L of nitrogen at 100kPa and 298K and 5L of oxygen at 200kPa are pumped into a evacuated 50L cylinder. Temperature remains constant, what is the total pressure in the cylinder?

Correct Answer: 30kPa
My answer: 300 kPa

Question 2: A 10L flask containing nitrogen at 500mmHg is connected via a closed tap to a 4L flask containing oxygen at 800mmHg pressure, if the tap is opened what will be the final pressure assuming temperature remains constant?

Answer: 586mmHg
My answer: 1300mmHg

Formulas I used:

pV = nRT (to find mols)
PA = (nA/nA+nB) x PT

How did they get these answers...cos I can't seem to get them..?? Any help would be appreciated...

So what is it exactly that you are having a hard time with? Let's go through #2:

$$n_A = \frac{P_AV_A}{RT} = \frac{(500 mm Hg)(10 L)}{RT}$$

and for the other flask:

$$n_B = \frac{P_BV_B}{RT} = \frac{(800 mm Hg)(4 L)}{RT}$$

so when the valve is opened, the total # of molecules of gas in the system will be:

$$n = n_A + n_B$$

so let's use the ideal gas law for the whole system:

$$P_{A+B} = \frac{(n_A + n_B)(RT)}{V_{A+B}} = \frac{(500 mm Hg)(10 L) + (800 mm Hg)(4 L)}{RT} \frac{(RT)}{V_{A+B}}{$$

notice that RT drops out completely, and we are left with:

$$P_{A+B} = \frac{(500 mm Hg)(10 L) + (800 mm HG)(4 L)}{14 L} = 586 mm Hg$$

go forth and do likewise..