How to Calculate Molecular Weight

M_molar = Σ (count × atomic mass)

You read the counts from the chemical formula. You read the atomic masses from a periodic table. You do not add atomic numbers. Carbon is 12.011, not 6.

The molecular weight calculator stores a table of average atomic masses and adds them. No account. It will not expand a formula you typed as text if you enter the wrong counts, and it will not balance an equation.

Subscripts multiply the element, not the neighbor

H2O is 2 H and 1 O: 2(1.008) + 15.999 = 18.015 g/mol.

H2O2 is 2 H and 2 O: 34.014 g/mol. That is not “water plus 2.”

CO2 is 1 C and 2 O: 44.009 g/mol. People who add C + O + 2 get 30 and then wonder why a dry-ice problem will not close.

Coefficients in front of a formula in a reaction (2 H2O) mean two moles of water, each 18.015 g/mol. The molar mass of water is still 18.015 g/mol. You multiply by 2 when you want the mass of two moles, not when you compute the formula mass.

Glucose by hand: C6H12O6

Using the same averages as a typical table:

Element	Count	Atomic mass (g/mol)	Subtotal
C	6	12.011	72.066
H	12	1.008	12.096
O	6	15.999	95.994
Total			180.156

Report 180.16 g/mol if you are matching a calculator that rounds to two decimals, or 180.2 g/mol if the table only had 12.01, 1.008, 16.00.

Check: a mole of glucose is about 180 g. A 5.00 g sample is 5.00 / 180.16 = 0.0278 mol. If you used 18.02 (water) by reading “glucose” and thinking “sugar water,” every later molarity is off by 10.

Parentheses, hydrates, and ions

Ca(NO3)2. Inside the parentheses: N + 3 O. Times 2: 2 N + 6 O. Plus Ca.

Ca 40.078
N 2 × 14.007 = 28.014
O 6 × 15.999 = 95.994
164.086 g/mol

If you treat (NO3)2 as “NO32,” you invent an element. If you add only one nitrate, you get ~102 g/mol and a titration will not land.

CuSO4·5H2O. Formula mass = CuSO4 + 5(H2O).

CuSO4: 63.546 + 32.065 + 4(15.999) = 159.607
5 H2O: 5 × 18.015 = 90.075
249.682 g/mol

Moles of hydrate = grams / 249.682. Moles of Cu²⁺ equal moles of hydrate if that is the formula you weighed. Using 159.6 after you weighed the blue crystals is the hydrate trap.

NaCl is a formula mass, 58.44 g/mol. There is no NaCl molecule in the crystal. You still add Na + Cl the same way.

Average mass versus an isotope

The periodic table’s carbon is 12.011 because of ¹³C (and a little ¹⁴C in living samples). A problem about a ¹⁴C-labeled compound that wants the mass of that exact molecule should use 14.003, not 12.011, for that carbon.

Atomic mass units versus g/mol: one formula unit in u has the same number as one mole in g/mol. 180.156 u per molecule ↔ 180.156 g/mol.

Where the number goes next

n = mass / M_molar, then M = n/V for molarity.

A reaction that says 2 mol A → 1 mol B needs the stoichiometry calculator after both molar masses are known. The molecular-weight page will not apply the 2:1 ratio for you.

pH problems do not use molar mass unless you were given grams of a solid acid and need [H+] first.

Common addition errors

Using atomic number. Forgetting oxygen’s subscript in sulfate (4, not 1). Double-counting hydrogen in a hydrate (the 5 H2O already includes 10 H). Rounding each element to an integer and then comparing to a four-decimal key. Adding the coefficient 2 from 2 CO2 into the molar mass of CO2.

Another: computing mass percent with a wrong total. Percent C in glucose is 72.066 / 180.156 × 100% = 40.00%. If the total was 162 (forgot hydrogens), the percents will not add to 100%.

Add it locally, then use it

Enter each element and its count in the molecular weight calculator and compare to your hand sum. Then take that g/mol into moles, molarity, or the stoichiometry calculator. For a side-by-side of concentration units that use molar mass, see molarity vs molality.