Answer: The new pressure is 3 psi.
Explanation:
Given:
= 12 psi, 
= ?, 
Formula used is as follows.

Thus, we can conclude that the new pressure is 3 psi.
Answer:
Explanation:
You need to remember that the oxidation number of H is +1, except when it is in a metal hydrites like NaH, where its oxidation number is -1. Then, the oxidation number of O is -2, but in peroxides is -1. So with these rules you just have to multiply the ox. number with the name of atoms and all the elements in the reaction must sum 0.