Atomic Mass of Chlorine

A_{\mathrm{r}}(\mathrm{Cl}) = 0.03545\ \text{kg/mol}

Value	0.03545 kg/mol
Status	Measured: ± 0.00001 kg/mol (0.00028 relative)
Source	IUPAC
Categories	Chemistry atomic-mass

A_r(Cl) in every molar mass unit

gram per mole	35.45 g/mol
kilogram per mole	0.03545 kg/mol

Learning zone

Chlorine is the element every teacher reaches for when explaining why atomic weights are not whole numbers: it is roughly 76 % ³⁵Cl and 24 % ³⁷Cl, and the weighted average lands at 35.45 g/mol. No individual chlorine atom weighs anything like that. Mass spectrometry makes the mixture visible in another way — any chlorine-containing fragment shows a companion peak two mass units higher at about a third the intensity, a fingerprint analysts use to spot chlorinated compounds at a glance.

IUPAC quotes chlorine only to four figures, with an interval of 35.446 to 35.457, because the isotope ratio varies measurably between seawater, rock salt and industrial sources. In water treatment the number converts free chlorine residual to moles of oxidant, and it carries into hypochlorite dosing: sodium hypochlorite is sold by "available chlorine", which is expressed in terms of Cl₂ at 70.90 g/mol rather than the sodium salt itself.

Used by these solvers

Moles from Mass (n = m/M)

n = \frac{m}{M}