Answer:
1. basic
2. C
Explanation:
1. CN⁻ is the conjugate base of HCN, so the salt is basic.
2. The net balanced chemical equation is:
CN⁻ + H₂O ⇒ HCN + OH⁻
The acid-dissociation constant Kb for this reaction can be found from the Ka for the conjugate acid:
Kb = 10⁻¹⁴/Ka = 10⁻¹⁴/(4.9 x 10¹⁰) = 2.040816...x10⁻⁵
Kb is the equilibrium constant for the above reaction and is expressed in terms of the equilibrium concentrations of the aqueous species in the reaction as follows:
Kb = [HCN][OH⁻ ]/[CN⁻]
The CN⁻ in the solution dissociates by amount "x" into "x" amount of HCN and OH⁻. Therefore, expressions for the equilibrium concentrations of these species can be substituted into the expressions for Kb
Kb = 2.040816...x10⁻⁵ = [HCN][OH⁻ ]/[CN⁻] = (x)(x)/(0.080 - x)
The result is a quadratic equation:
x² - 2.040816...x10⁻⁵x -1.63265...x10⁻⁶ = 0
x = 1.287997...x10⁻³ = [OH⁻]
The pOH is calculated as follows:
pOH = -log[OH⁻] = -log(1.287997...x10⁻³) = 2.890...
The pH is calculated from the pOH as follows:
pH = 14 - pOH = 14 - 2.890 = 11.11
