Question

Is a solution of sodium acetate acidic, neutral, or basic? Explain. Are solutions of ammonium chloride acidic, basic, or neutral? Explain. Calculate the pH of a 0.100 M KCN solution.

Answer 1

1) The salt sodium acetate, CH,COON2 contains anion of weak acid CH,COOH and the cation from strong base N2OH. The anion of this salt reacts with water to produce basic solution whereas the cation does not react with water. The reaction of the weak base (CH,COO is shown below CH,СОО (ад)+Н,о(1) — СН,СООН (ад)+ОН (аg) Therefore, solution of sodium acetate is basic 2) The salt ammonium chloride NH4C1 contains anion of strong acid HCl and cation of weak base NH4OH The cation of this salt reacts with water to produce acidic solution whereas the anion does not react with water. The reaction of the weak acid (NH) is shown below: (b»).o'н+(р») *нN + ()o'н+(b»):HN Therefore, solution of ammonium chloride is acidic 3) shown below The dissociation of KCN is as KCN(aq)K (aq)+CN (aq) The anion in the above reaction undergoes reaction with water to form hydroxide ions as shown below CN+HO HCN+OH I 0.100 0 C -x 0.100-x E

The equilibrium constant for this reaction is: [HCNOH К, = CN The K of the acid, is used to calculate the K of the conjugate base, CN using the following relation К, 3 К, хК, 1.0x10-146.2 x10-1 xK 1.0x1014 К, 6.2x10-10 =1.61x105 Substitute K shown below in the equation for equilibrium constant as (x)(x) (0.100-x) 1.61x 10 Assume x can be neglected in comparison to 0.100; the above equation becomes: 1.61x10-x)( x) (0.100) x21.61x10x 0.100 =1.61x10 x=1.27x103 is 1.27x 10 [O is 1.27x103 The concentration of hydroxide ions,

The concentration of hydronium ions,HO*is calculated as shown below: [.о'н][но]- "у 1x1014 1.27x10 M[H,0* 1x10-14 1.27x103 M [но]- 7.87x10-12 M The pH of a 0.100 M KCN solution is calculated as shown below -log H,0 - -log (7.87x10-12) рн = 11.1 Therefore, pH of a 0.100 M KCN solution is 11.1.