Gaseous State(Dalton's Law)
As defined in an excerpt from this Wikipedia entry:  


  "In chemistry and physics, Dalton's law (also called Dalton's law of
  partial pressures) states that in a mixture of non-reacting gases, the
  total pressure exerted is equal to the sum of the partial pressures of
  the individual gases."


Where this assumption falls apart for your question is that $\ce{NH3}$ and $\ce{HCl}$ readily react with each other, both in the gas and aqueous phases.  This alone renders Dalton's Law mute.