A. The balanced equation for the reaction between sodium and oxygen is:
4 Na + O2 -> 2 Na2O
B. Dimensional analysis:
From the balanced equation, we can see that 4 moles of sodium react with 1 mole of oxygen to produce 2 moles of sodium oxide.
Given that 7.5 moles of sodium are used, we can set up the following proportion:
4 moles Na / 1 mole O2 = 7.5 moles Na / x moles O2
Solving for x, we get:
x = (7.5 moles Na) / (4 moles Na/ 1 mole O2) = 7.5 moles Na * (1 mole O2/4 moles Na) = 1.875 moles O2
Therefore, the number of moles of oxygen used in the reaction is 1.875 moles.