An ideal gas in a 1.25-gallon container is at a temperature of 125 degrees celsius and pressure of 2.5 atmospheres. if the gas is oxygen(formula: o2 molecular weight = 32 grams per mole), what is the mass of gas in the container in units of grams?

You will need the equation PV = nRT

P = Pressure in kPa
V = Volume in L
n = moles
R = 8.314 (constant)
T = Temperature in Kelvin 

First convert 2.5 atm into kPa:

2.5 X 101.3 = 253.25 kPa

Convert 125 Celsius into Kelvin:

125 + 273 = 398 K

Convert Gallons to Litres:

1.25  X 3.79 = 4.74 L

Plug your values into the equation to solve for n:

(253.25)(4.74) = n(8.314)(398)

n = (253.25)(4.74)/(8.314)(398)

n = 0.362 moles

Now use M = m/n to solve for the mass of O2

M = Molar Mass 
M = mass
n= moles

32 = m/(0.362)

m = (32)(0.362) 

m = 11.58g