Gracie squeezed some grapes and kept the juice in a vessel for 5 days at room temperature. on the fifth day she found that the juice tasted sour and appeared frothy.
what caused this change?

4.08 atm was the original pressure

Explanation:

When the temperature and the number of moles keeps on constant in 2 situations with a gas, we can apply this relation:

P₁ . V₁ = P₂ . V₂

It is an undirectly proportion, When the volume decrease, the pressure increase so if the pressue decrease, the volume will be increased.

P₁ . 50L = 3 atm . 68L

As the volume was increased, the initial pressure must be higher than the final pressure.

P₁  = (3 atm . 68L) / 50L = 4.08 atm