A sample of sodium peroxide, Na2O2, was reacted with an excess of water.

2 Na2O2 (s) + 2 H2O (l) —> 4 NaOH (aq) + O2 (g)

All of the sodium peroxide reacted, and the oxygen was collected over water at 21 degrees Celsius. The barometric pressure was 771 mm Hg. However, the level of water inside the tube was 25.0 cm above the level of water outside the tube. If the volume of gas in the tube is 31.0 mL, how many grams of sodium peroxide were in the sample?

According to the solutions in the back of my Chemistry book, the answer is 0.194 g Na2O2. I can’t seem to figure out how to get this answer, though. Any help would be very much appreciated!