What is the mass of a sample of O2 gas?

Question

Which has a pressure of 740. mmHg, at a temperature of 25 degree Celsius, in a volume of 250. mL?

Answer 1

Working in SI units:

n = PV/(RT)

R = 8.314 J/(mol K)
T = (25 + 273)K = 298 K
V = 0.250 L = 0.000250 m³
P = 740(133.3) Pa ≅ 98640 Pa

n = (98640)(0.000250)/[8.314(298)] moles = 0.009953 moles

molecular weight of O₂ ≅ 32.00

m = 0.009953(32.00) g ≅ 0.318 g

Answer 2

This is PV=nRT

 

first you must convert your units to get them in standard form.

 

P= 740 mmHg x 1/760 mmHg = 0.974 atm

V=0.250L

n=?

R=0.0821

T=273K +25C = 298K

 

Now rearrange the equation in terms of n:

 

PV=nRT

divide both sides by RT to get n on its own

 n=PV/RT

 

now substitute your values into the equation:

0.974x0.250/298x0.0821

0.2435/24.4658

n=0.00995 moles (3 sig. fig.)

 

now find the mass:

0.00995 moles x atomic mass of O2

0.00995 x 15.99(2)

=0.159g (3 sig. fig)