General

Chemistry Help please

A sample of has is enclosed in a Syringe with it's tip sealed. If the plunger is pushed in, how would you expect the number of particles per millimeter to change (increase, decree or stay the same)
If the syringe and the gas it contained were then heated, what would happen to the number of particles per millimeter if the volume was kept constant?
Why would it be necesssary to apply a greater pressure to the syringe plunger to keep the volume constant as the gas was heated?

Thanks