The total number of diprotic acids among the following is:

H₃PO₄, H₂SO₄, H₃PO₃, H₂CO₃, H₂S₂O₇, H₃BO₃, H₃PO₂, H₂CrO₄ and H₂SO₃

The compounds $\mathbf{P}, \mathbf{Q}$ and $\mathbf{S}$ were separately subjected to nitration using $\mathrm{HNO}_3 / \mathrm{H}_2 \mathrm{SO}_4$ mixture. The major product formed in each case respectively, is :

The packing efficiency of the twodimensional square unit cell shown below is

The complex showing a spin-only magnetic moment of 2.82 B.M. is :