Answer is: density of methane is 1.176 g/L.
V(CH₄) =
1 L.
T = 75°C = 348.15 K.
p = 2.1 atm.
R = 0.08206 L·atm/mol·K.
Ideal gas law: p·V = n·R·T.
n = p·V / R·T.
n(CH₄) = 2.1 atm · 1 L / 0.08206 L·atm/mol·K · 348.15 K.
n(CH₄) = 0.0735 mol.
m(CH₄) = n(CH₄) · M(CH₄).
m(CH₄) = 0.0735 mol · 16 g/mol.
m(CH₄) = 1.176 g.
d(CH₄) = m(CH₄) ÷ V(CH₄).
d(CH₄) = 1.176 g/L.
