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General Chemistry

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Effect of Addition of Ammonium to Ammonia on pH

CHEM-WYB4EU

Consider a solution of $\textrm{NH}_3$.

When $\textrm{NH}_4\textrm{Cl}$ is added (assume no volume change), would the $pH$ increase, decrease or remain constant? Why?

A

Increase. The addition of $\textrm{NH}_4^+$ shifts the equilibrium towards more $\textrm{NH}_3$ by Le Chatelier’s Principle. $\textrm{NH}_3$ is basic, the $pH$ will increase.

B

Increase. $\textrm{NH}_4^+$ is an acid. Adding more acid will increase the $pH$.

C

Remain constant. $\textrm{NH}_3$ is a weak base. The combination of a weak base and conjugate acid is a buffer. A buffer is by definition, a solution in which the $pH$ does not change.

D

Decrease. $\textrm{NH}_4^+$ is a weak acid. Adding more acid will increase the concentration of $\textrm{H}^+$ and thereby decrease the $pH$.

E

Decrease. $\textrm{Cl}^-$ would act as a base would decrease the $pH$.