Using Boyle's Law : the pressure of a gas is inversely proportional to its volume when temperature is constant.
[tex] P_1 V_1 = P_2 V_2 [/tex]
Where, [tex] P_1 [/tex] & [tex]V_1[/tex] are inital pressure and initial volume
[tex]P_2[/tex] & [tex]V_2[/tex] are final pressure and final volume
Given, a balloon with a final volume of 12.0 L reduced it's pressure to 0.5 atmosphere without a change in temperature.
Here,
We need to find the balloon's initial pressure if it has 2.0l of gas before :
Plugging in the given values :
[tex]P_1 \times 2.0 L = 0.5 \: atm \times \: 12.0 \: L[/tex]
[tex]P_1 = 0.5 \: atm \times 12.0 \: L / 2.0 \: L[/tex]
[tex]P_1 = 3.0 \: atm[/tex]
Therefore, the initial pressure of the balloon is 3 atm