Thermodynamics- Gauge pressure, moles, molecules

[name_ask17]

Homework Statement

A 4.0 m^3 container of oxygen gas has a gauge pressure of 8x10^5 Pa at 25 degrees c.
a) how many moles of oxy are present in the containe?r ANS: 1292 moles
b)how many oxy molecules are present? ANS: 7.78E26 oxy
(yes, i have the answers but i don't know how to get to them correcly.)

known:
oxy formula weight= 32

Homework Equations

moles=mass/formula weight
ALso, can you explain to me what gauge pressure has to do with moles.

The Attempt at a Solution

i tried to find how many moles by using the denisty of oxygen, but that doesn't work. please help. i am very confused

Nevermind. i got it(:

 

[member 553083]

moles = pressure x volume / R x temperatureR= 8.3144598moles= 8x10^5 Pa x 4 m^3 / 8.3144598 x 298.15 K= 1292 moles molecules= moles x 6.022E23 = 7.78E26 oxy molecules