Question

Density of oxygen gas at STP is:
A. 20/7 g/L
B. 1/22.4 g/L
C. 32/22.4 g/L
D. 5 g/L

Answer 1

The general gas law is PV=nRT. It follows that no matter how we change the state of the given amount of gas the ratio PV/T always remains constant.

Complete step by step solution: R is a Rydberg constant which is independent of all variables and popularly known as universal constant .Values of R:
R=0.0821 litre atm /k mol=8.314 J/k mol
We know that density =mass/volume
$\rho = \dfrac{{mass}}{{volume}} \\\ V = \dfrac{{nRT}}{P} \\\$
Substituting the value we get $\rho = \dfrac{{P \times m}}{{R \times T \times n}}$ , m/n =M (molar mass of oxygen=32 g/mol), T=300 K
STP is the standard temperature and pressure condition where the temperature is 300k and pressure is 1 atm or 760 mm of Hg. It is fixed for all the reactions by IUPAC. Substituting the values we get
$\rho = \dfrac{{1 \times 32}}{{0.0821 \times 300}} = \dfrac{{32}}{{22.4}}$ g/L
Hence the correct option is option C.

Additional information: the general gas law has been derived out of so many laws such as
Boyle's law states” at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure”.
Charles law which states that” at constant pressure, the volume of a fixed mass of a gas is proportional to its temperature” .
Gay lussacs law which states that “at constant volume, the pressure of a fixed mass of gas is proportional to the temperature”.
Avogadro’s law states that” the amount of gas in moles is proportional to the volume of the gas”.

Note: Use the value of R after observing the values of pressure you are given with.