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I was reading salt hydrolysis of a salt made from strong acid and weak base. I came across the hydrolysis of $\ce{NH_4Cl}$. These reactions were mentioned

$$\ce{NH_4Cl + H_2O<=>NH_3 + HCl \tag{R1}}$$ $$\ce{NH_4+ + Cl- + H_2O<=>NH3 + H+ + Cl-}\tag{R2}$$ $$\ce{NH_4+ + H_2O<=>NH3 + H3O+\tag{R3}}$$

$\ce{H+}$ and $\ce{Cl-}$ will not combine appreciably and concentration of $\ce{HCl}$ cam be neglected.

But in the second reaction, why did they assume that $\ce{NH_4Cl}$ dissociated completely into ions in water? Is that practically true too?

Edit: I also have another question; when calculating $\text{pH}$ of a salt like $\ce{NH_4Cl}$, is the concentration of hydrogen ions, $\ce{[H+]}, $ inclusive of the contributions from water?

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  • $\begingroup$ Is this supposed to be in aqueous solution? $\endgroup$ – MaxW Oct 20 '20 at 18:54
  • $\begingroup$ A weak acid NH4+(aq) reacts with the strong base OH-, passing to it its proton, forming NH3(aq) and H2O. $\endgroup$ – Poutnik Oct 20 '20 at 19:36
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    $\begingroup$ NH4OH does not exist. It is a solution of NH3 in water. The formula NH4OH is a wrong formula invented in the 19th century, and still printed by merchants, because the customers (caretakers and contractors) prefer this name to the name of ammonia solution . $\endgroup$ – Maurice Oct 20 '20 at 20:18
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Reactions that are in equilibrium will have both reactants and products present. However, the interesting thing about dissolution reactions is that a solution that is undersaturated (below its solubility limit) will not be in equilibrium (i.e. no solid exists). So unless the example you are talking about specified that the concentration of salt is above the solubility limit and both solid and dissolved salt exist, you can assume full dissolution.

Here is a link to a ChemLibreTexts article that further discusses this topic.

For calculating the pH of a salt solution, an ICE chart can be used which accounts for the concentrations of the ammonium ion, ammonia, and the hydronium ion.

Here is a link that gives an example for calculating the pH of an ammonium chloride solution.

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