The group 7A elements are called Halogens.
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It depends on the process.
Like for example if the process is isothermal(temperature is constant), you can use,
PV = constant or P1V1 = P2V2 where P1V1 are initial conditions and P2V2 are final.
For adiabatic process,
PV^gamma = constant or P1V1 ^gamma = P2V2 ^gamma.
where gamma = Cp
------
Cv
Cp = specific heat at constant pressure and Cv = specific at constant volume.
Value of Gamma will be given in question.
Hope this helps!
<span>The answer is "D" where the number of collisions per unit area is reduced by one-half. Drawing back on the piston means the volume is increased. The pressure is reduced. There are fewer collisions when the pressure is reduced.</span>