Cl2 Molar Mass: Chlorine Gas, Atoms, Isotopes and Worked Examples

The Cl₂ molar mass is about 70.90 g/mol. Chlorine gas is made of two-atom molecules, so you double chlorine's standard atomic weight of 35.45: 2 × 35.45 g/mol = 70.90 g/mol. A single chlorine atom (Cl) or a chloride ion (Cl⁻) has a molar mass of 35.45 g/mol, which is why both numbers show up in homework answers.

This guide explains where 35.45 comes from, why IUPAC now gives chlorine's atomic weight as a range, and how the two stable isotopes average out. It also covers the mass of one chlorine atom in grams, worked conversions between grams, moles, molecules and liters, a table of common chlorine compounds and the difference between molar mass and molecular weight.

Key Takeaways

• The Cl₂ molar mass, the value for chlorine gas, is 70.90 g/mol (2 × 35.45).
• The molar mass of atomic chlorine and of the chloride ion is 35.45 g/mol. The one extra electron in Cl⁻ adds too little mass to change the value.
• IUPAC's Commission on Isotopic Abundances and Atomic Weights (CIAAW) lists chlorine's standard atomic weight as the interval [35.446, 35.457], with 35.45 ± 0.01 as the value for everyday calculations.
• Natural chlorine is roughly 76% chlorine-35 and 24% chlorine-37, which is why its average mass is not a whole number.
• One chlorine atom has a mass of about 5.887 × 10⁻²³ g, and one Cl₂ molecule about 1.177 × 10⁻²² g.

What Is the Molar Mass of Chlorine? Atom vs Molecule vs Ion

"Chlorine" can mean three different things in a chemistry problem, and each one has its own formula. Read the question carefully before you pick a number.

Why does the chloride ion have the same molar mass as the atom? It carries one extra electron, and an electron has a mass of only about 0.000549 u, according to NIST's CODATA value. Adding that to 35.45 gives 35.4505, which still rounds to 35.45. So "chloride molar mass" and "molar mass of chloride" questions use 35.45 g/mol.

Chlorine is one of seven elements that exist as diatomic molecules in their elemental form (H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂). Any problem that says "chlorine gas," "Cl₂(g)" or simply uses chlorine as a reactant element needs the molar mass of chlorine gas, 70.90 g/mol.

Chlorine's Standard Atomic Weight Is an Interval

For decades chlorine had a single standard atomic weight. The CIAAW chlorine page shows the 1961 value as 35.453(1). Since 2009, CIAAW has given the standard atomic weight of chlorine as an interval:

A_r(Cl) = [35.446, 35.457]

The interval exists because the ratio of chlorine-35 to chlorine-37 varies slightly from one natural source to another. CIAAW says the atomic weights of normal materials are expected to fall inside this range. A sample's true atomic weight could be anywhere in that window, so a single number with tiny uncertainty would overstate how well we know it.

For calculations you still need one number. CIAAW's abridged standard atomic weights list chlorine as 35.45 ± 0.01, and the IUPAC atomic weights table gives 35.45 as chlorine's single value. This is the value textbooks use, and it falls inside the interval.

Applied to chlorine gas, the interval gives a Cl₂ molar mass between 2 × 35.446 = 70.892 g/mol and 2 × 35.457 = 70.914 g/mol. You may see 70.906 g/mol in older references, which is double the old 35.453 value. For homework, 70.90 g/mol is the correct figure to four significant figures. Some intro courses round to 71.0 g/mol, which is fine if your instructor allows three significant figures.

Chlorine Isotopes and Why the Average Is 35.45

Chlorine has two stable isotopes. Each atom of chlorine-35 has 17 protons and 18 neutrons, and each atom of chlorine-37 has 17 protons and 20 neutrons. The values below come from the NIST atomic weights and isotopic compositions database.

The average atomic mass is a weighted average of the two isotopes:

1. Chlorine-35 contribution: 0.7576 × 34.968853 = 26.4924
2. Chlorine-37 contribution: 0.2424 × 36.965903 = 8.9605
3. Sum: 26.4924 + 8.9605 = 35.4529, which rounds to 35.45

This explains a common point of confusion. No individual chlorine atom has a mass of 35.45 u. Every atom weighs about 34.97 u or about 36.97 u, and 35.45 is the average of a very large number of them. CIAAW gives the natural abundance ranges as 0.755 to 0.761 for chlorine-35 and 0.239 to 0.245 for chlorine-37. Plugging the ends of those ranges into the same weighted average gives values close to the ends of the [35.446, 35.457] interval.

The isotopes also mean that Cl₂ molecules come in three masses. Using the NIST abundances, the expected fractions are:

That roughly 10 : 6 : 1 pattern is what you see in the mass spectrum of chlorine gas. The weighted average of the three is 70.906 u, exactly twice the isotope-weighted atomic mass of 35.453. That agrees with 2 × 35.45 = 70.90 within the ±0.01 uncertainty CIAAW attaches to chlorine's abridged value.

How to Calculate the Cl2 Molar Mass Step by Step

1. Write the formula. Chlorine gas is Cl₂, two chlorine atoms per molecule.
2. Look up the atomic weight. Chlorine is 35.45.
3. Multiply by the subscript. 2 × 35.45 = 70.90.
4. Add units. Molar mass is expressed in grams per mole, so the answer is 70.90 g/mol.

OpenStax Chemistry 2e defines molar mass as the mass in grams of one mole of a substance and notes that it is numerically equal to the formula mass in atomic mass units. For more practice with multi-element formulas, see our walkthrough of calculating the molecular mass of C₆H₆.

Mass of a Chlorine Atom in Grams

To find the mass of one atom, divide the molar mass by the Avogadro constant, which NIST lists as exactly 6.022 140 76 × 10²³ mol⁻¹.

• One average chlorine atom: 35.45 g/mol ÷ 6.02214076 × 10²³ mol⁻¹ = 5.887 × 10⁻²³ g
• One average Cl₂ molecule: 70.90 g/mol ÷ 6.02214076 × 10²³ mol⁻¹ = 1.177 × 10⁻²² g

In atomic mass units, the average chlorine atom is 35.45 u and the average Cl₂ molecule is 70.90 u. One dalton (Da) is the same as one u.

Worked Calculations with Chlorine Gas

Grams to Moles

Problem: How many moles are in 25.0 g of Cl₂?

1. Divide mass by molar mass: n = m ÷ M.
2. n = 25.0 g ÷ 70.90 g/mol = 0.3526 mol.
3. Round to three significant figures: 0.353 mol Cl₂.

Moles to Grams

Problem: What is the mass of 0.750 mol of Cl₂?

1. Multiply moles by molar mass: m = n × M.
2. m = 0.750 mol × 70.90 g/mol = 53.175 g.
3. Answer: 53.2 g Cl₂.

Grams to Molecules and Atoms

Problem: How many Cl₂ molecules and how many chlorine atoms are in 25.0 g of chlorine gas?

1. Convert to moles (from above): 0.3526 mol.
2. Multiply by the Avogadro constant: 0.3526 mol × 6.022 × 10²³ mol⁻¹ = 2.12 × 10²³ molecules.
3. Each molecule has two atoms: 2 × 2.12 × 10²³ = 4.25 × 10²³ chlorine atoms.

Density of Chlorine Gas at STP

The ideal gas law gives gas density as d = PM ÷ RT, as shown in OpenStax Chemistry 2e, section 9.3. Using STP as 0 °C (273.15 K) and 1 atm:

1. d = (1.00 atm × 70.90 g/mol) ÷ (0.08206 L·atm·mol⁻¹·K⁻¹ × 273.15 K)
2. Denominator: 0.08206 × 273.15 = 22.41 L·atm/mol
3. d = 70.90 ÷ 22.41 = 3.16 g/L

Two cautions apply. First, this is an ideal-gas estimate. Chlorine molecules attract each other more than small molecules such as N₂ do, so real chlorine gas deviates from ideal behavior and its measured density is slightly higher. Second, some courses define standard pressure as 1 bar (100 kPa) instead of 1 atm. At 1 bar the same calculation gives about 3.12 g/L. Use whichever definition your textbook uses.

The same numbers convert moles to volume. At 0 °C and 1 atm, one mole of an ideal gas occupies about 22.41 L, so 25.0 g of Cl₂ (0.3526 mol) occupies about 0.3526 × 22.41 = 7.90 L.

Stoichiometry: Sodium and Chlorine

Problem: How many grams of sodium chloride form when 10.0 g of Cl₂ reacts completely with sodium? The balanced equation is 2Na(s) + Cl₂(g) → 2NaCl(s).

1. Moles of Cl₂: 10.0 g ÷ 70.90 g/mol = 0.1410 mol.
2. Mole ratio: 1 mol Cl₂ gives 2 mol NaCl, so 0.1410 × 2 = 0.2821 mol NaCl.
3. Mass of NaCl: 0.2821 mol × 58.44 g/mol = 16.5 g NaCl.

The most common error here is using 35.45 g/mol for Cl₂, which doubles the moles and the final answer.

Molar Masses of Common Chlorine Compounds

The values below use CIAAW abridged atomic weights: H 1.0080, C 12.011, O 15.999, Na 22.990, Mg 24.305, Al 26.982, Cl 35.45, K 39.098, Ca 40.078, Fe 55.845, Cu 63.546, Zn 65.38, Ag 107.87 and Ba 137.33. Results are rounded to two decimal places because chlorine's value limits the precision.

The chloroform value matches the 119.37 that OpenStax gives for CHCl₃. If your class uses a periodic table with fewer decimal places, your answers may differ in the last digit, which is normal.

Molar Mass vs Molecular Weight

Students often see "molar mass," "molecular weight," "molecular mass" and "MW" used as if they were the same. They give the same number for chlorine, since the molecular mass of chlorine gas is 70.90 u and its molar mass is 70.90 g/mol, but they are not quite the same quantity.

Because one mole contains Avogadro's number of particles and 1 u × 6.022 × 10²³ is about 1 g, the three values line up numerically. In practice, "chlorine molecular weight" and "chlorine MW" questions almost always want 70.90 for Cl₂. If a question says "atomic mass of Cl" or "atomic weight of chlorine," it wants 35.45.

Common Mistakes with Chlorine Molar Mass

• Using 35.45 g/mol for chlorine gas. Elemental chlorine is Cl₂, so use 70.90 g/mol.
• Using 70.90 g/mol for chloride. In NaCl or CaCl₂, chlorine is present as Cl⁻ ions at 35.45 g/mol each.
• Forgetting the subscript. CaCl₂ contains two chloride ions, so it needs 2 × 35.45.
• Using the chlorine-35 isotope mass. 34.97 is one isotope, not the average atomic weight.
• Rounding too early. Carry 70.90 through the problem and round only the final answer.

If you are preparing for a placement or diagnostic test, these conversions appear constantly. Our ALEKS initial knowledge check chemistry guide covers the topics that tend to come up. When you report results in a lab, the guide to writing a chemistry lab report shows how to present calculations clearly, and our chemistry lab help page covers pre-lab and calculation support.

Getting Help with Chemistry Calculations

Mole conversions get easier once you see the same pattern worked several times on your own problems. If you are stuck on a set of stoichiometry or gas-law questions, a tutor can walk through each step and explain where the units cancel. Our chemistry homework help page explains how that support works.

Frequently Asked Questions

What Is the Molar Mass of Cl2?

The Cl₂ molar mass is 70.90 g/mol. It is twice the standard atomic weight of chlorine, 35.45. Older references may show 70.906 g/mol, based on the earlier single value of 35.453.

What Is the Molar Mass of Chlorine?

It depends on the form. Atomic chlorine, Cl, is 35.45 g/mol, and chlorine gas, Cl₂, is 70.90 g/mol. If the problem mentions chlorine gas or uses chlorine as an element in a reaction, use 70.90 g/mol.

What Is the Atomic Mass of Cl?

The standard atomic weight of chlorine is the interval [35.446, 35.457], and the single value used in calculations is 35.45. This is a weighted average of chlorine-35 (about 34.97 u) and chlorine-37 (about 36.97 u).

What Is the Molar Mass of Chloride (Cl−)?

The chloride ion has a molar mass of 35.45 g/mol. The extra electron adds only about 0.00055 g/mol, which does not change the value at the precision used in chemistry courses.

What Is the Mass of a Chlorine Atom?

An average chlorine atom has a mass of about 5.887 × 10⁻²³ g, or 35.45 u. You get this by dividing 35.45 g/mol by the Avogadro constant, 6.022 × 10²³ mol⁻¹. A single chlorine-35 atom is lighter (34.97 u) and a chlorine-37 atom is heavier (36.97 u).

Is the Molecular Weight of Chlorine 35.45 or 70.90?

The molecular weight of chlorine, meaning the Cl₂ molecule, is 70.90. The value 35.45 is the atomic weight of a single chlorine atom. Chlorine does not exist as single atoms under normal conditions, so "molecular weight" refers to Cl₂.

Why Isn't Chlorine's Atomic Mass a Whole Number?

Natural chlorine is a mixture of two isotopes, about 75.8% chlorine-35 and 24.2% chlorine-37. The atomic weight is the abundance-weighted average of their masses, which lands at 35.45 rather than at either isotope's mass.

Atomic weights and isotope data come from CIAAW's 2024 standard and abridged atomic weights, IUPAC and NIST, with methods from OpenStax Chemistry 2e, as of Nov. 2026.