How would the number of moles (n) of O2 change if the atmospheric pressure doubled but all variables stayed the same?

The no. of moles will be doubled as the pressure is doubled.

Explanation:

We can use the general law of ideal gas: PV = nRT.

where, P is the pressure of the gas in atm.

V is the volume of the gas in L.

n is the no. of moles of the gas in mol.

R is the general gas constant,

T is the temperature of the gas in K.

If V and T are constant, and have different values of P and n:

(P₁n₂) = (P₂n₁).

P₂ = 2P₁,

∴ n₂ = (P₂n₁) / (P₁) = (2P₁n₁) / (P₁) = 2n₁.

So, the no. of moles will be doubled as the pressure is doubled.