Answer
There are $~~1.5~~$ moles of gas.
Work Step by Step
We can use the data at point $a$ to find the number of moles $n$:
$PV = nRT$
$n = \frac{PV}{RT}$
$n = \frac{(2500~Pa)(1.0~m^3)}{(8.31~J/mol\cdot K)(200~K)}$
$n = 1.5~mol$
There are $~~1.5~~$ moles of gas.
