Since we want the pressure to remain constant we have to use Charles Law of gases, that relates the volume and the temperature at a constant pressure:
![(V1)/(T1)=(V2)/(T2)](https://img.qammunity.org/2023/formulas/chemistry/college/cmstxvrq2gfhyxp5oorx26lc4wishckekq.png)
In this case, we know the values of V1, T1 and V2, which are 10L, 25°C and 6.0L. Using these values we have to find T2:
![\begin{gathered} T2=(V2\cdot T1)/(V1) \\ T2=(6.0L\cdot25)/(10L) \\ T2=15 \end{gathered}](https://img.qammunity.org/2023/formulas/chemistry/college/jrmoj23hrtnte7swuv7fl91hieku01zz3l.png)
The gas has to be stored at 15°C.