An aqueous solution of 0.080 M KCN is made. At equilibrium this solution will be

Question

An aqueous solution of 0.080 M KCN is made. At equilibrium this solution will be?

Basic

Neutral

Acidic

Suppose you need to calculate the pH of an aqueous KCN solution. How many of these statements should you be thinking about in advance?

pH should be < 7

don't forget pH = 14 - pOH

pH should be > 7

The mass action equation will be set equal to Ka

I need an ICE table and a Kb or Kw/Ka

pH = -log[x]

-log[x] = pOH not pH

What reaction would you use in an ICE table to calculate the pH of a KCN solution? CN- + H2O HCN + H3O+

CN- + H2O HCN + OH-

H+ + OH- H2O

KCN (s) K+ + CN-

What is the pH of a 0.080 M KCN solution.

The Ka for HCN is 8.0×10-10

Explanation / Answer

Basic in nature

KCN is a basic salt so the pH should be greater than 7.

KCN + H2O<==>HCN + KOH

net ionic: CN- + H2O<==>HCN + OH-

Kb=[HCN][OH-]/[CN-]
1.0x10^-14/8.0x10^-10=x²/(0.080-x)
ignore x in the denominator and you get
1.0x10^-14/8.0x10^-10=x²/0.080
x=[OH-]=0.001
pOH=-log(0.001)=3
pH=14-3= 11

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