Question

Assuming equal concentrations, rank these solutions by pH.

Sr(OH)2(aq)

RbOH(aq)

HCN(aq)

HCl(aq)

NH3(aq)

Answer 1

=> Here HCl(aq) is strong acid, so it has lowest pH.

=> Then HCN(aq) is a weak acid it is sparingly soluble in water, so pH is little higher than HCl(aq).

=> NH3 is a weak acid it's pH is nearly 7-8. So it has higher pH than HCl and HCN.

=> Rb(OH) and Sr(OH)2 are strong bases. So these are having higher pH than acids. But in Sr(OH)2 we have 2OH^- ions, so Sr(OH)2 has higher pH than Rb(OH).

The order of pH is

HCl < HCN < NH3 < Rb(OH) < Sr(OH)2

(assuming equall concentrations)

Please let me know if you have any doubt. Thanks.