remember that in this case pressure is equal to 1.00 atm and temperature is equal to 273.15K. So,
P
V
=
n
R
T
→
n
=
P
V
R
T
=
1.00
a
t
m
⋅
7.0
L
0.082
a
t
m
⋅
L
m
o
l
⋅
K
⋅
273.15
K
=
0.31
Since we know hydrogen's molar mass (
2.0
g
m
o
l
), we can determine the mass
m
H
2
=
n
⋅
m
o
l
a
r
.
m
a
s
s
=
0.31
m
o
l
e
s
⋅
2.0
g
m
o
l
=
0.62
g
If indeed you are dealing with STP, remember that, under these conditions, 1 mole of any ideal gas occupies
22.4
L
. So,
n
=
V
V
m
o
l
a
r
=
7.0
L
22.4
L
=
0.31
moles
And, once again,
m
=
0.31
⋅
2.0
=
0.6
