\text { 4. What is Kb for } \mathrm{N}_{2} \mathrm{H}_{4} \text { if the pH of a } 0.158 \mathrm{M} \text { solution of } \mathrm{N}_{2} \mathrm{H}_{5} \mathrm{Cl} \text {

is } 4.5 ? \text { (4) }