Molar mass is the total weight of all atoms in one mole of a substance

Molar mass is the mass in grams of exactly one mole of a compound — that is, 6.022 × 10²³ particles of it. To find molar mass, you add up the atomic masses of every atom in the chemical formula, using the periodic table as your reference. The result is always expressed in grams per mole (g/mol).

The process is straightforward: identify each element in the formula, find its atomic mass on the periodic table, multiply by how many atoms of that element appear in the formula, then add all the results together. This works the same way whether you are calculating molar mass for a straightforward compound like water or a complex one like calcium carbonate.

Key Takeaways

  • Find the atomic mass of each element on the periodic table, usually shown as a decimal number below the element symbol.
  • Multiply each atomic mass by the number of atoms of that element in the chemical formula.
  • Add all the products together to get the total molar mass in grams per mole.
  • Round your final answer to two decimal places unless your teacher or assignment specifies otherwise.
  • Check your work by verifying that you counted every atom in the formula and used the correct atomic masses.

Read the periodic table to find atomic masses

The periodic table lists the atomic mass of each element, usually printed as a decimal number directly below the element's symbol. For hydrogen, the atomic mass is approximately 1.008. For carbon, it is 12.01. For oxygen, it is 16.00. These numbers represent the average mass of one atom of that element in atomic mass units (amu), which converts directly to grams per mole.

When you look up an element, use the number shown on the periodic table your teacher provided or the one in your textbook. Different periodic tables may round slightly differently — some show hydrogen as 1.01, others as 1.008 — so consistency matters more than which version you choose. Write down the atomic mass for each element in your formula before you start multiplying.

Count the atoms of each element in the chemical formula

The chemical formula tells you how many atoms of each element are present. In water (H₂O), the subscript 2 after hydrogen means there are two hydrogen atoms and one oxygen atom. In calcium carbonate (CaCO₃), there is one calcium atom, one carbon atom, and three oxygen atoms.

When there is no subscript written, the number is always 1. So in carbon dioxide (CO₂), carbon appears once and oxygen appears twice. If the formula has parentheses, like in calcium hydroxide Ca(OH)₂, multiply the subscript outside the parentheses by each subscript inside. In this case, there is one calcium atom, two oxygen atoms (because the subscript 2 multiplies the one oxygen inside), and two hydrogen atoms (because the subscript 2 multiplies the one hydrogen inside).

Multiply each atomic mass by the number of atoms

Once you have the atomic mass and the atom count for each element, multiply them together. For water (H₂O): hydrogen has an atomic mass of 1.008 and appears twice, so 1.008 × 2 = 2.016. Oxygen has an atomic mass of 16.00 and appears once, so 16.00 × 1 = 16.00.

Write each product on a separate line so you can see your work clearly. This step is where most mistakes happen — either forgetting to multiply by the atom count or misreading the subscript. Double-check that you have multiplied every element and that your subscripts match the formula.

Add all the products to find total molar mass

Add together all the products from the previous step. For water: 2.016 + 16.00 = 18.016 g/mol. Round to two decimal places unless your assignment says otherwise, giving you 18.02 g/mol for water.

The final answer always includes the unit g/mol (grams per mole). This tells you that one mole of water weighs 18.02 grams. If you forget the unit, your answer is incomplete, and your teacher will not know whether you understand what molar mass represents.

Work through a complete example: sodium chloride

Sodium chloride is NaCl. First, find the atomic masses: sodium (Na) is 22.99 and chlorine (Cl) is 35.45. Next, count the atoms: one sodium atom and one chlorine atom. Then multiply: 22.99 × 1 = 22.99 and 35.45 × 1 = 35.45. Finally, add: 22.99 + 35.45 = 58.44 g/mol.

Try a more complex example: calcium carbonate (CaCO₃). Calcium is 40.08, carbon is 12.01, and oxygen is 16.00. There is one calcium, one carbon, and three oxygen atoms. Multiply: 40.08 × 1 = 40.08; 12.01 × 1 = 12.01; 16.00 × 3 = 48.00. Add: 40.08 + 12.01 + 48.00 = 100.09 g/mol. Notice how the three oxygen atoms contribute almost half the total mass.

Common mistakes to avoid

The most frequent error is forgetting to multiply the atomic mass by the number of atoms. If you see H₂O and use only 1.008 for hydrogen instead of 2.016, your answer will be too low. Always multiply first, then add.

Another common mistake is misreading subscripts, especially in formulas with parentheses. In Mg(OH)₂, the subscript 2 applies to both the oxygen and the hydrogen, so you have one magnesium, two oxygens, and two hydrogens — not one of each. Write out the atom count separately before you calculate.

Using the wrong atomic mass is also straightforward to do. Nitrogen (N) is 14.01, but nitrogen is not the same as sodium (Na), which is 22.99. Read the element symbol carefully and match it exactly to the periodic table. If you are unsure, say the element name aloud to yourself.

Frequently Asked Questions

What if the periodic table shows different atomic masses than my textbook?

Atomic masses are based on the weighted average of all naturally occurring isotopes of an element, and these averages are updated periodically. Small differences between sources are normal. Use whichever periodic table your teacher gave you or specified, and stay consistent throughout your assignment. The difference between 1.008 and 1.01 for hydrogen will not change your final answer significantly.

Do I need to round during the calculation or only at the end?

Round only at the end. Keep all decimal places while you multiply and add, then round your final molar mass to two decimal places. Rounding at each step introduces small errors that add up. Most calculators will hold enough decimal places automatically, so just enter the numbers as shown on the periodic table.

How do I handle formulas with parentheses?

Multiply the subscript outside the parentheses by each subscript inside. In Al₂(SO₄)₃, the subscript 3 outside means you have three sulfate groups, so three sulfur atoms and twelve oxygen atoms (3 × 4). Aluminum appears twice. Count carefully and write out the total atom count before you start multiplying atomic masses.

What is the difference between atomic mass and molar mass?

Atomic mass is the mass of a single atom, measured in atomic mass units (amu). Molar mass is the mass of one mole of atoms or molecules, measured in grams per mole (g/mol). The numerical value is the same — hydrogen has an atomic mass of 1.008 amu and a molar mass of 1.008 g/mol — but the units and scale are different.

Can I use molar mass to find how many grams are in a sample?

Yes. If you know the number of moles in a sample, multiply by the molar mass to find the mass in grams. For example, 2 moles of water (H₂O) weighs 2 × 18.02 = 36.04 grams. This is one of the main reasons chemists calculate molar mass — to convert between moles and grams in the lab.