Question

What is the density of hydrogen at STP conditions?
A. 0.1 g/L
B. 0.2 g/L
C. 0.09 g/L
D. 2 g/L

Answer 1

Hint: Density of any substance is the value of its mass divided by volume. STP conditions are standard temperature and pressure. The values at STP are $0{}^\circ $C temperature, and 1 bar pressure.
Formula used: Density formula, $density=\dfrac{mass}{volume}or\,\rho =\dfrac{m}{v}$

Complete answer:
We have been given to find the density of hydrogen at STP conditions. As density requires mass divided by volume. We have molar mass of hydrogen as 2 g. Now for mass and volume, we will use the ideal gas equation and take out both mass and volume.
According to the ideal gas equation: $PV=nRT$ , as n = number of moles, it can be written as $n=\dfrac{mass(m)}{molar\,mass(M)}$ .
Therefore, $PV=\dfrac{m}{M}RT$, where M for hydrogen is 2, P is 1 atm, R is 0.0821, T is 273.15K
So, $1\times V=\dfrac{m}{2}0.0821\times 273.15$
V = $\dfrac{m}{2}0.0821\times 273.15$, keeping this value in the formula of density, the mass will get cancelled out and we will get the density as,
Density = $\dfrac{m}{\dfrac{m}{2}0.0821\times 273.15}$
Density = $\dfrac{2}{0.0821\times 273.15}$
Density = 0.09 g/L
Hence the density of hydrogen at STP conditions is calculated to be 0.09 g/L.

So, option C is correct.

Note:
For the ideal gas equation, temperature is taken in Kelvin which is $0{}^\circ C$= 273.15 K, while pressure is taken in atm as, 1 bar = 1 atm pressure. R stands for gas constant which has different values for different units, here it is 0.0821 $L\,atm\,{{K}^{-1}}mo{{l}^{-1}}$.