Which must be the same when comparing 1 mol of oxygen gas, O2, with 1 mol of carbon monoxide gas, CO?

One mole of any substance contains the same number of particles: Avogadro’s number, about 6.02 × 10²³. So 1 mol of O₂ and 1 mol of CO contain the same number of molecules. Their masses differ because their molar masses differ, and they contain different numbers of oxygen atoms per molecule.

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