How do I find the # of each element in counting atoms

Iknow the answer is a but i don't know why

How would the number of moles (n) of o2 change if the atmospheric pressure doubled but all other variables stayed the same

The diagram below shows a cell placed in a solution.a cell is shown placed inside a beaker. it is labeled cell. the solution inside the beaker is labeled 40% salt solution and the solution inside the cell is labeled 20% salt solution.only water is allowed to move in and out of the cell. what will most likely happen to the cell? it will expand as water moves out of it. it will shrink as water moves out of it.it will expand as water moves into it. it will shrink as water moves into it.