P Block Elements

P? + 3NaOH + 3H? O → PH? + 3NaH? PO? (A)
Here, NaH? PO? is a reducing agent.
NaH? PO? + AgNO? + H? O → Ag + HNO? + NaH? PO?
Equivalents of NaH? PO? = equivalents of Ag (n-factor of NaH? PO? = 4)
1 mole * 4 = n moles of Ag * 1
So, moles of Ag = 4.

KMnO? oxidise HCl to Cl?

Aqua regia is a mixture of concentrated nitric acid (HNO?) and concentrated hydrochloric acid (HCl), optimally in a molar ratio of 1:3. The reaction between them produces nitrosyl chloride (NOCl), chlorine gas (Cl?), and water. The potent oxidizing nature comes from these products, which can dissolve noble metals like gold and platinum. The overall reaction often shows the formation of nitric oxide (NO).
HNO? + 3HCl → NOCl + Cl? + 2H?O
The nitrosyl chloride can further decompose: 2NOCl → 2NO + Cl?. The gas evolved in the process is Nitric Oxide (NO).