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

The number of moles of a gas is directly proportional to the pressure of the gas at constant temperature and volume.

This comes from the ideal gas law:

PV=nRT or PV/nT = R, a constant

We can assume constant volume because the container is rigid, and there's no indication that the temperature changed. 

When V and T are unchanged they can be left out of the equation so P/n equals a constant. When pressure and...

The number of moles of a gas is directly proportional to the pressure of the gas at constant temperature and volume.

This comes from the ideal gas law:

PV=nRT or PV/nT = R, a constant

We can assume constant volume because the container is rigid, and there's no indication that the temperature changed. 

When V and T are unchanged they can be left out of the equation so P/n equals a constant. When pressure and number of moles change, P1/n1 = P2/n2.

We can rearrange this to solve for the new pressure: P2 = (P1)(n2)/n1

P1 = 1.00 atm

n1 = 6.36 moles

n2 = 6.36 moles + 1.28 moles = 7.64 moles

P2 = (1.00 atm)(7.64 mol)/(6.36 mol) = 1.20 atm

This answer is consistent with what we expect, a higher pressure because the number of moles of gas increased.