How to Calculate Molecular Weight

Co-authored by Bess Ruff, MA

Last Updated: December 6, 2022 References

The molecular mass, often called the molecular weight (MW), is the weight of all atoms in a given molecular formula. Molecular weight is measured in Atomic Mass Units, usually expressed as u or amu.^{[1]
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Research source} In order to calculate the molecular weight of a formula, you'll need to add up the atomic masses of each element present.

Method 1

Method 1 of 2:

Calculating Molecular Weight

1
Count how many atoms of each element exist in the molecule. First, list each element present in the molecule. You may use the chemical symbol or write out the name of the element. Then, count the atoms according to the molecular formula and write them next to the element’s name or symbol.^{[2]
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- For example, carbon dioxide or CO₂, list that there is 1 carbon (C) and 2 oxygens (O) present in the molecule.
- For methane, CH₄, list carbon (C) and hydrogen (H). Methane comprises 1 atom of carbon and 4 atoms of hydrogen.
- For glucose, C₆H₁₂O₆, list carbon (C), hydrogen (H), and oxygen (O). Glucose comprises 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.
2
Find the relative atomic mass of each element in the molecule. Use a copy of the Periodic Table of Elements. The Periodic Table lists the atomic mass of each element below the chemical symbol. For example, oxygen has a relative atomic mass of 15.9994 amu.^{[3]
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- For carbon dioxide (CO₂), the relative atomic mass is 12.011 amu for carbon and 15.999 for oxygen.
- The atomic mass of an element is roughly equal to the summed mass of the protons and neutrons that it contains. Note that the relative atomic mass you find in the periodic table is scaled: it accounts for all of the isotopes of the element in the proportions that they naturally occur.^{[4]
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3
Calculate the total mass for each element in the molecule. Multiply the atomic mass of each element by the number of atoms of that element: (Atomic Mass of Element) x (# of atoms of that element). Do this for each element in the molecule.^{[5]
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- In our carbon dioxide example, the mass of the single carbon atom is 12.011 amu. Since there are 2 oxygen atoms, you would write $15.999*2=31.998amu$ .
4
Add up the mass of all the atoms to find the molecular weight. Molecular weight = ∑((Atomic Mass of Element)_n x (# of atoms of that element)_n)^{[6]
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- Round the answer as necessary, using significant digits.
- Remember to use the proper units. amu is the old abbreviation for atomic mass units, but the "most correct" modern unit is a lower-case u.^{[7]
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- For carbon dioxide, the molecular weight can be found like so: $12.011+31.998=44.009u$ .

Method 2

Method 2 of 2:

Practicing with Examples

1
Find the molecular weight of methane (CH₄). The atomic mass of hydrogen is 1.008 u. The atomic mass of carbon is 12.011 u.^{[8]
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Research source} There are 4 hydrogen atoms in methane. Add together the weight of all the atoms in methane to find the molecular weight.^{[9]
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- $1.008*4=4.032u$ .
- $12.011*1=12.011u$
- $12.011+4.032=16.043u$
2
Calculate the molecular weight of glucose (C₆H₁₂O₆). The atomic mass of carbon is 12.011 u. The atomic mass of hydrogen is 1.008 u. The atomic mass of oxygen is 15.999^{[10]
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Research source} There are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Add up the mass of all of the atoms in glucose to calculate the molecular weight.
- $12.011*6=72.066u$
- $1,008*12=12.096u$
- $15.999*6=95.994u$
- $72.066+12.096+95.994=180.156u$

Community Q&A

Question

What is the molecular mass of phosphorus?

Community Answer

The molecular mass of Phosphorus is 30.974 to 3 decimal places, commonly written as 31 on the periodic table.
Question

Why are the hydrogen bonds weak?

Community Answer

Hydrogen has a low electronegativity, and thus is easily taken out of bonds.
Question

What is the molecular weight of CO2?

Williamsm917

Community Answer

44.01 g/mol. C has a molecular mass of 12.01. O has a molecular mass of 16. Therefore, the formula would be 12.01 + 2(16), which gives you the molecular mass 44.01 g/mol.