Molar MassCommon mistakes in molar mass sums
Most lost marks in mole calculations come from a short list of slips. Here they are, each with an example.
1. Ignoring the number after a bracket
In Mg(OH)2 the 2 applies to both O and H. Wrong: 24.305 + 15.999 + 1.008 = 41.31. Right: 24.305 + 2 × 15.999 + 2 × 1.008 = 58.32.
2. Multiplying only the nearest atom
In Al2(SO4)3 the 3 outside multiplies the 4 inside: 12 oxygen atoms, not 4 and not 7.
3. Forgetting that some elements are two-atom molecules
Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine exist as H2, N2, O2, F2, Cl2, Br2 and I2. The molar mass of oxygen gas is 32.00, not 16.00.
4. Mixing up capital and small letters
CO is carbon monoxide (28.01). Co is cobalt (58.93). NO is nitrogen monoxide; No is nobelium. Every element symbol starts with a capital, and a second letter is always small.
5. Treating the hydrate dot as multiplication
CuSO4·5H2O is the salt plus five waters: 159.60 + 90.08 = 249.68. It is not 159.60 × 5 × 18.02.
6. Using the anhydrous figure for crystals
If the bottle says FeSO4·7H2O, weigh with 278.01, not 151.90.
7. Dividing when you should multiply
Moles = grams ÷ molar mass. Grams = moles × molar mass. If 10 g of something with molar mass 100 comes out as 1000 moles, you multiplied by mistake. Ten grams is a tenth of the molar mass, so it must be 0.1 mol.
8. Leaving the mass in the wrong unit
Change milligrams and kilograms to grams first. 250 mg is 0.250 g.
9. Rounding in the middle
Rounding 0.04996 to 0.05 early is usually harmless, but rounding 0.0164 to 0.02 changes the answer by a fifth. Keep the digits in your calculator and round once, at the end.
10. Counting atoms when the question asks for molecules
One mole of water has 6.022 × 1023 molecules but three times as many atoms. Read which particle the question wants.
11. Confusing molar mass with mass
Molar mass does not change with the size of the sample. 5 g of water and 5 kg of water have the same molar mass, 18.02 g/mol.
12. Comparing grams directly in an equation
The equation 2H2 + O2 → 2H2O does not say 2 g of hydrogen react with 1 g of oxygen. It says 2 moles react with 1 mole: about 4 g with 32 g.
A quick routine that catches most of them
- Rewrite the formula with every bracket opened out: Ca(NO3)2 becomes Ca, 2 N, 6 O.
- Lay the working out as a table: element, atoms, atomic weight, total.
- Ask whether the answer is sensible. A molar mass smaller than the heaviest atom in the formula is impossible.
The calculator lays out exactly this table, so you can compare it with your own line by line. Then try the practice questions.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.