Q2. What is the dimensional formula of $a b ^ { - 1 }$ in the equation $\left( \mathrm { P } + \frac { \mathrm { a } } { \mathrm { V } ^ { 2 } } \right) ( \mathrm { V } - \mathrm { b } ) = \mathrm { RT }$, where letters have their usual meaning.\\
(1) $\left[ \mathrm { M } ^ { - 1 } \mathrm {~L} ^ { 5 } \mathrm {~T} ^ { 3 } \right]$\\
(2) $\left[ \mathrm { M } ^ { 6 } \mathrm {~L} ^ { 7 } \mathrm {~T} ^ { 4 } \right]$\\
(3) $\left[ \mathrm { ML } ^ { 2 } \mathrm {~T} ^ { - 2 } \right]$\\
(4) $\left[ \mathrm { M } ^ { 0 } \mathrm {~L} ^ { 3 } \mathrm {~T} ^ { - 2 } \right]$