Which must be the same when comparing 1 mol of oxygen gas, cap o sub 2 , with 1 mol of carbon monoxide gas, CO ?

One mole of any substance contains the same number of particles: Avogadro’s number. So 1 mol of O₂ and 1 mol of CO have the same number of molecules. Their masses and numbers of oxygen atoms differ, and their volumes are not guaranteed to match unless temperature and pressure are the same.

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