Question

To which of the following the Dalton’s law of partial pressure is not applicable?
(A) ${\text{H}}_{\text{2}}\text{and}\text{He}$
(B) $\text{N}{\text{H}}_{\text{3}}\text{and}\text{HCl}$
(C) ${\text{N}}_{\text{2}}\text{and}{\text{H}}_{\text{2}}$
(D) ${\text{N}}_{\text{2}}\text{and}{\text{O}}_{\text{2}}$

Answer 1

Hint: Dalton law of partial pressure states that total pressure of a mixture of gases is equal to the sum of the partial pressure of the constituent gases at the same temperature and volume.
${\text{p}}_{\text{t}}\text{=}{\text{p}}_{\text{1}}\text{+}{\text{p}}_{\text{2}}\text{+}{\text{p}}_{\text{3}}\text{+}.....$
Where ${\text{p}}_{\text{t}}$ is the total pressure of gaseous mixture and ${\text{p}}_{\text{1,}}{\text{p}}_{\text{2,}}{\text{p}}_{\text{3}}$ are the partial pressure of the gaseous mixture.
The partial pressure of a constituent gas is defined as the pressure it would be exerted by the gas molecule, if alone occupied the entire volume of the vessel at the same temperature.
Partial pressure of a constituent gas $\text{=}{\displaystyle \frac{\text{mole}\text{of}\text{constituent}\text{gas}\text{molecule}}{\text{total}\text{moles}\text{of}\text{the}\text{gaseous}\text{mixture}}}\times \text{total}\text{pressure}$
Nitrogen and oxygen gases are not present in the atomic form in normal temperature and pressure.
All the noble gases are chemically unreactive.

Complete answer:
Dalton’s law of partial pressure is applicable for gases which are non- reacting with each other.
(A) $\text{He}$(Helium) is noble gases which will not react with hydrogen gas, so this gaseous mixture follows Dalton’s law.
(B) Ammonia $\text{(N}{\text{H}}_{\text{3}}\text{)}$ reacts with hydrochloric acid in normal temperature and gives ammonium chloride$\text{(N}{\text{H}}_{\text{4}}\text{Cl)}$. So, this gaseous mixture does not follow Dalton's law.
$\text{N}{\text{H}}_{\text{3}}\text{+}\text{HCl}\to \text{N}{\text{H}}_{\text{4}}\text{Cl}$
(C) Nitrogen gas is not reactive in normal temperature because breakdown of nitrogen into atomic form is a highly endothermic process. So, nitrogen and hydrogen gas mixtures follow Dalton's law.
(D) Nitrogen and oxygen gas are not found in atomic form in normal temperature, so these gases will not react with each other. This gaseous mixture will follow Dalton's law.

Note:
Dalton’s law of partial pressure is applicable only for non- reacting gases not for the liquids.
Partial pressure of a gas is measured in atm. If a gas is collected over the liquid its partial pressure will equal to the difference between total pressure of the gaseous mixture and the partial pressure of the vapour of the liquid.