Question

A 630. mL flask contains O2 at a pressure of 0.75 atm and a temperature of 20°C. What mass of O2 is present?

I know you Use PV = nRT and solve for n = number of mols gas. Then n = grams/molar mass and solve for grams.

But I get the wrong answer

Answer 1

Show your work and let us find the error. A common error is not using kelvin for temperature. Also remember volume must be in L

Answer 2

630 ml= 1l= 1000ml
0.63 L
20=293.15 k

0.75*0.63=n*0.0821*293.15
=1687.13002

Answer 3

630 mL = 0.630 L; right.
20 C is 20+273 = 293K; right.
0.75atm x 0.630L = n*0.08206*293
n = 0.0196 mols. I suspect you just punched in the wrong numbers or hit the wrong button somewhere on your calculator. Also you quit when you had n.
Then n = grams/molar mass
0.0196 = g/32
g = 32*0.0196 = approx 0.64 g