Answer
$m=20.39kg$
Work Step by Step
First we calculate the piston pression:
$P=P_{abs}-P_{atm}=180kPa-100kPa=80kPa$
Area of piston will be:
$25cm^2*\frac{1m^2}{10000cm^2}=0.0025m^2$
The force make by the piston is:
$F=PA=80000Pa*0.0025m^2=200N$
And the mass of the piston is:
$m=\frac{F}{g}=\frac{200N}{9.81\frac{m}{s^2}}=20.39kg$
