You collect 6.36 moles of hydrogen gas from your lab experiment involving the decomposition of water by electrolysis. The pressure inside your...

we can solve this  problem with ideal gas equation, which is pv=nRT

where, p=pressure of the container

          v=volume of the container

          n=number of moles of the gas

          R= Universal gas constant

          T= temperature in kelvin

If we substitute the values in the first instant in to the equation;(that is before adding the second volume of gas)

1x v =6.36x Rx T----(1)

If we substitute the second  set of values( that is after the addition of second gas sample)

p2x v = (6.36+1.28) x Rx T----(2)

note:  * since the question doesn't say about a change of temperature

           we can assume that  the temperature is constant

          * Container is said to be  rigid so we can assume  that the   volume is             constant.

Divide equation number (1) by (2)

  (1x v =6.36x Rx T)  ----(1)

____________________

(p2x v = 7.64x Rx T)----(2)

By simplifying these equations  you would get

6.36p2 =7.64

p2       = 7.64/6.36

          =1.20 atm//