[Mr.2007]

The pressure question is

If 4.50g of methane gas (CH₄) is in a 2.00-L container at 35^oC, What is the pressure in the container?

Can someone give me the formula i need to use to solve that.

The molar mass question is

What is the molar mass of an unknown gas if a 1.25g sample occupies 1.00L at 26.5^oC and .990atm

Whats the formula i need to use to solve this question

[Borek]

pV=nRT

n=m/M

[Mr.2007]

^^^ How do i rearrange pv=nRT to solve for P

[Borek]

Using exactly the same approach you were told to use when solving for other unknowns. Enahs and Yggdrasil already gave you hints and links.

[Mr.2007]

Alright so its

p = nRT
       V

[Mr.2007]

Alright so i got the pressure on i still need help with the molar mass one I know i solve for n then convert moles to grams but im having trouble solving for n this is what i did

N = P *V / R * T

.990atm * 1.00L = 0.00001332866606
8.31 * 299.5K

is this  right or do i need to convert atm to kpa

[enahs]

You solved for n correct.

Yes you either have to convert something, or use the appropriate gas constant.

In this case, you have units of atm, L mol and K, so you could use the gas constant of:

0.08206 L atm mol^-1 K^-1

If you look in the front cover (or back cover) of your book, you should see a lot of constants, and you should see a couple of versions of the gas constant.

But you should also be adept at converting units, so when it comes to test you can get the correct answer if you only remember the one main version of the gas constant. And yes, all you would have to do with the one you used is convert atm to kPa.

You do not need anybody to tell you if your answer is correct or if you need to convert. Do your units cancel? With how you have it written (and include the units in the gas constant) do all the units cancel and leave you with moles (what is on the other side of the equation)? If the units do not cancel/simplify to be the same on both sides of the equation it is wrong.

[Mr.2007]

^^^ alright enahs thanks now can you help me with this one

What mass of nitrogen (N₂), is contained in a 7.25 tank at 37^oC and 2.50atm pressure.

this work sheet has the answers on it so i know the answer is 19.9g

I did

n = P * V / R * T then i converted from moles to grams but i keep getting the wrong answer heres my work

253.3125kpa * 7.25L = 0.02388808045005mol
8.31 * 310K

Then i converted to grams

0.02388808045005mol * 14g N₂/1 mol N₂ = 0.3344331263007gN2

but  i know thats not right because the answer is 19.9g can you help me

[Yggdrasil]

1)  for the calculation of moles, I get 0.712 moles

2)  The molar mass of nitrogen gas (N₂) is 28g/mol, not 14g/mol.

[Mr.2007]

^^^^can you show me your work for finding moles

[enahs]

^^^^can you show me your work for finding moles

Your work is correct. You just did the arithmetic wrong/ typed it in the calculator wrong. Use brackets on your calculator so it does the correct order of operation.

[Mr.2007]

^^^ oh alright i dont have a chemstry calculator of my own but they have them in my class for us to use i've been using this at home

http://www.creative-chemistry.org.uk/alevel/calculator.htm