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# Acidity or basicity of anions and cations.

## $pH$ of ionic substances

To calculate the $pH$ of a soluble ionic substance one looks up in the →   Table of acid-base couples to find anions and cations within it. - Anions corresponding to strong acids $ClO_4^-$, $NO_3^-$, $Cl^-$, $Br^-$, $I^-$ have no influence on the $pH$ . - Some hydrated metal cations have an acidic $pH$ , the most common (see the →   Table of acid-base couples) are $Fe^{3+}$, $Al^{3+}$, $Zn^{2+}$, $Cd^{2+}$. They are treated as weak acids. - The metal cations that are not included in the table, for example $Li^{+}$, $Na^{+}$, $K^{+}$, $Mg^{2+}$ etc.., have mostly no influence on the $pH$ . - Some anions are acids of one couple ($pK_ {a1}$) and bases from another couple ($pK_{a2}$ ) (see the →   Table of acid-base couples). They are amphoteric (ampholythes) and their $pH$ is calculated using the formula;

Ampholythes: $pH$ $=$ $\frac{1}{2}pK_{a1}$ $+$ $\frac{1}{2}pK_{a2}$

Examples: To calculate the $pH$ of a $0.1\frac{mol}{L}$ solution of sodium cyanide $Na^+CN^-$, $Na^+$ is neglected, and the $pH$ of the weak base $CN^- \;\;0.1\frac{mol}{L}$ is determined. To calculate the $pH$ of a $0.1\frac{mol}{L}$ solution of ammonium bromide $NH_4^+Br^-$,$Br^-$ is neglected, and the $pH$ of the weak acid $NH_4^+ \;\;0.1\frac{mol}{L}$ is determined. To calculate the $pH$ of a $0.1\frac{mol}{L}$ solution of magnesium ethanoate $Mg^{2+}(CH_3COO^-)_2$, $Mg^{2+}$ is neglected, and the $pH$ of the weak base $CH_3COO^- \;\;0.2\frac{mol}{L}$ is determined. To calculate the $pH$ of a $0.1\frac{mol}{L}$ solution of potassium dihydrogenophosphate $K^{+}H_2PO_4^-$, $K^{+}$ is neglected, and the $pH$ of the ampholythe $H_2PO_4^-$ is determined: $pH$ $=$ $\frac{1}{2}pK_{a1}$ $+$ $\frac{1}{2}pK_{a2}$ = $\frac{1}{2}2.12$ $+$ $\frac{1}{2}7.20$ = $4.66$