Molecular weight is the sum of all atomic weights in a molecule
To find the molecular weight of any compound, you add up the atomic weights of every atom in its chemical formula. Each element has a standard atomic weight listed on the periodic table. You multiply that weight by how many atoms of that element appear in the formula, then add all the results together.
The process takes three steps: identify each element and count how many atoms you have, look up the atomic weight for each element, and multiply and add. You do not need a calculator beyond basic multiplication and addition, though one makes the work faster and more accurate.
Key Takeaways
- Atomic weights are listed on the periodic table, usually to two decimal places, and represent the average mass of an atom of that element.
- Count the atoms in your formula by reading the subscript numbers after each element symbol; no subscript means there is one atom.
- Multiply each element's atomic weight by its atom count, then add all the products to get the total molecular weight.
- Molecular weight is measured in atomic mass units (amu) or grams per mole (g/mol), which are numerically identical.
Reading the periodic table for atomic weights
The periodic table shows each element's symbol (like H for hydrogen, O for oxygen, C for carbon) and its atomic weight. The atomic weight appears as a decimal number, usually with one or two digits after the decimal point. For example, hydrogen is 1.008, carbon is 12.01, and oxygen is 16.00.
These numbers represent the average mass of one atom of that element in atomic mass units. You use these exact numbers in your calculation, not rounded versions. If you are working from a printed periodic table, write down the atomic weight for each element in your formula before you start multiplying.
Counting atoms in a chemical formula
A chemical formula uses element symbols followed by small numbers called subscripts. The subscript tells you how many atoms of that element are in one molecule. For example, in H₂O, the subscript 2 after H means there are two hydrogen atoms, and the lack of a subscript after O means there is one oxygen atom.
When atoms are grouped in parentheses with a subscript outside, multiply the subscript by the number of each atom inside the parentheses. In Ca(OH)₂, the subscript 2 applies to both O and H, so you have one calcium, two oxygens, and two hydrogens. Write out the count for each element before you begin the math.
The calculation: multiply, then add
Once you have the atomic weight and atom count for each element, multiply them together. For H₂O: hydrogen is 1.008 and you have 2 atoms, so 1.008 × 2 = 2.016. Oxygen is 16.00 and you have 1 atom, so 16.00 × 1 = 16.00. Then add all the products: 2.016 + 16.00 = 18.016.
The result is the molecular weight. For water, it is 18.016 amu (atomic mass units). If you are calculating molar mass — the weight of one mole of the substance — the number is the same but the unit changes to g/mol (grams per mole). One mole of water weighs 18.016 grams.
Working through a longer example: glucose
Glucose has the formula C₆H₁₂O₆. Start by listing what you have: 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. From the periodic table, carbon is 12.01, hydrogen is 1.008, and oxygen is 16.00.
Multiply each: carbon is 12.01 × 6 = 72.06; hydrogen is 1.008 × 12 = 12.096; oxygen is 16.00 × 6 = 96.00. Add them: 72.06 + 12.096 + 96.00 = 180.156. The molecular weight of glucose is 180.156 amu, or 180.156 g/mol.
Common mistakes and how to avoid them
The most frequent error is forgetting to multiply the atomic weight by the atom count. If you see H₂O and use only 1.008 for hydrogen instead of 1.008 × 2, your answer will be wrong. Always multiply first, then add the products.
Another mistake is misreading subscripts, especially in formulas with parentheses. Write out the full atom count on paper before you start calculating. If a formula shows Ca(OH)₂, do not forget that the 2 outside the parentheses applies to both O and H inside it. Checking your atom count twice takes seconds and prevents hours of confusion.
Using atomic weight tables and resources
A standard periodic table is all you need. Most chemistry textbooks include one, and many are available free online. The NIST (National Institute of Standards and Technology) periodic table is a reliable source if you need weights to more decimal places than a basic table shows.
For routine calculations, a basic periodic table with atomic weights to two decimal places is sufficient. If you are doing research or work that requires high precision, use the NIST version or the table in your lab manual. The difference between 16.00 and 15.999 for oxygen matters only in specialized work.
Frequently Asked Questions
What is the difference between atomic weight and atomic mass?
Atomic weight is the average mass of an atom of an element, accounting for the different isotopes that exist in nature. Atomic mass is the mass of a single isotope. For most calculations, you use atomic weight from the periodic table, which is what you need for molecular weight.
Do I round the final answer?
Round to the same number of decimal places as the least precise measurement you used. If your atomic weights go to two decimal places, round your final answer to two decimal places. For water, 18.016 rounds to 18.02. Check your assignment or lab manual for the rounding rule your instructor expects.
Why is molecular weight the same number as molar mass?
Molecular weight in amu and molar mass in g/mol are numerically identical because of how the mole is defined. One mole contains Avogadro's number of particles (about 6.022 × 10²³), and this number was chosen so that the mass in grams equals the molecular weight in amu. The units are different, but the numbers match.
Can I use a calculator for this, or do I have to do it by hand?
A calculator is fine and makes the work faster and more accurate. The important skill is knowing which numbers to multiply and add, not doing arithmetic in your head. Use a calculator for the math once you have identified the atomic weights and atom counts correctly.