The volume of a gas with a pressure of 1.2 atm increases from 1.0 l to 4.0 l. what is the final pressure of the gas, assuming no change in moles or temperature?

Boyle's law gives the relationship between pressure and volume of gas.
for a fixed amount of gas, at constant temperature the pressure of gas is inversely proportional to its volume.
PV = k
where P - pressure, V - volume and k - constant
P1V1 = P2V2
where parameters for the first instance are on the left side and parameters for the second instance are on the right side.
substituting the values in the equation
1.2 atm x 1.0 L = P x 4.0 L 
P = 0.3 atm
new pressure is 0.3 atm

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