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Why does the mixture of gases $\ce{NH3}$ & $\ce{HCl}$ not obey Dalton's Law of partial pressure at room temperature?
Also, what other mixtures don't obey Dalton's Law of partial pressure

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    $\begingroup$ Your second question could make this post too broad. $\endgroup$ – M.A.R. Aug 30 '16 at 17:00
  • $\begingroup$ Maybe because these gases react, forming solid ammonium chloride? See here: quora.com/… $\endgroup$ – AstronAUT Aug 30 '16 at 17:15
  • $\begingroup$ Dalton's Law of partial pressures is not applicable to mixture of reacting gases. $\endgroup$ – narendra kumar Jan 2 '17 at 6:57
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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.

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Ammonia and hydrochloric acid react spontaneously when put together within the same apparatus. If they react, then the volume obviously changes, which will not result in the pressure that you would predict if the two gases didn't react and remained as separate entities.

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According to dalton's law " total pressure of a gas is sum of partial pressure of individual gases" but he clearly mentioned gases to be "non reacting". As $\ce{NH3 + HCl -> NH4Cl}$ So dalton's law not followed as they react. And also their pressure becomes zeo

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