There are only 6 * 10 ^ 23 molecules in 1 mole of a substance. This is Avogadro’s number NA and in what follows we will operate with it.
a) 5 mol of O2 contains 5 * 2 * NA = 10 * NA = atoms.
b) 3 mol of AlCl3, this is 3 * 4 * NA = 12 * NA atoms.
c) 3 mol of NaOH, this is 3 * 3 * NA = 9 * NA atoms.
d) 3 mol of H2SO4 is 3 * 7 * NA = 21 * NA atoms. So we got the desired maximum result.
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