The volume = 6.71 L
Further explanation
Charles's Law
When the gas pressure is kept constant, the gas volume is proportional to the temperature
[tex]\tt \dfrac{V_1}{T_1}=\dfrac{V_2}{T_2}[/tex]
V₁=7 L
T₁=25 °C(room temperature) + 273 = 298 K
T₂=12.5(halved) + 273 = 285.5 K
[tex]\tt \dfrac{V_1}{T_1}=\dfrac{V_2}{T_2}\\\\\dfrac{7}{298}=\dfrac{V_2}{285.5}\\\\V_2=6.71~L[/tex]
