Which among the following is NOT a feature of $$\mathrm{S}_{\mathrm{N}} 2$$ mechanism?

What is the pH of 0.005 M NaOH solution?

What is the oxidation number of sulfur in $$\mathrm{H}_2 \mathrm{SO}_5$$ ?

If $$\mathrm{N}_2$$ gas is compressed at 2 atmosphere from 9.0 L to $$3.0 \mathrm{~L}$$ at $$300 \mathrm{~K}$$, find the final pressure at same temperature.