#"Charles' Law"# holds that for a given quantity of gas...
#VpropT#, i.e. #V=kT#, where #k# is some constant of proportionality...and given constant pressure, i.e. the gas is enclosed in a piston...we solve for #k# and get...
#V/T=k#...else....#V_1/T_1=V_2/T_2#
#"Volume"# changes proportionally with absolute temperature...