General Chemistry

Question

Assuming equal concentrations, rank these solutions by pH. Highest to lowest:

NH3(aq)

KOH(aq)

Ca(OH)2(aq)

HCN(aq)

HI(aq)

Answer 1

From the list, highest pH will be most basic which will be

Ca(OH)₂ followed by KOH then NH3. These are bases and since both Ca(OH)₂ and KOH are strong , pH of the former is greater than the later because of 2 OH VS 1 OH. Next comes NH3 which is a weak base. Finally, consider the two acids, HCN and HI. Since HCN is a weak acid and HI is a strong acid, HCN will be more basic. So, in summary, pH from highest to liwrst will be

Ca(OH)₂ > KOH > NH₃ > HCN > HI

Really no need to consult pKa or pKb nor Ka or Kb values. You can determine rank order based on strong vs wesk base, number of OH and weak acid vs strong acid.