Which of the following statements about the density $d$ of an ideal gas are correct?

A $d = \frac{PM}{RT}$
BAt constant $P$ , a plot of $d$ versus $1/ T$ is a straight line through the origin
CAt the same $T$ and $P$ , the ratio of densities of two gases equals the ratio of their molar masses
DVapour density of a gas equals its density in $g L^{- 1}$ at STP

Answer: A, B, C

From

P V = (w / M) RT

d = w / V = PM / RT

(A), which is linear in

1/ T

at constant

P

through the origin (B) and proportional to

M

at fixed

T

P

(C). Vapour density is a dimensionless ratio relative to

H X_{2}

(

= M /2

), while density at STP is

M /22.4 g L^{- 1}

; they are not equal (D wrong).