Question

why deltons law in a particular pressure not applicable to a system of ammonia and hydrogen chloride gas?

Answer 1

This is because when you mix HCl and NH3, they react to form NH4Cl which is solid. As it reacts and form solid product it does not obey law of partial pressure.
NH3 + HCl → NH4Cl

Answer 2

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". AsNHX3+HCl⟶NHX4Cl" role="presentation" style="box-sizing: inherit; margin: 0px; padding: 0px; border: 0px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-weight: 400; font-stretch: inherit; line-height: normal; font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 15px; vertical-align: baseline; display: inline; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: 0px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(12, 13, 14); orphans: 2; widows: 2; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); text-decoration-style: initial; text-decoration-color: initial; position: relative;">NH3+HCl⟶NH4ClNHX3+HCl⟶NHX4Cl So dalton's law not followed as they react.