How to calculate it
Multiply each element's atomic mass by how many atoms there are, then add: 3 × 1 + 31 + 4 × 16 = 98 g/mol.
| Element | Atoms | Mass (u) | Total | % by mass |
|---|---|---|---|---|
| H | 3 | 1 | 3 | 3.09% |
| P | 1 | 31 | 31 | 31.61% |
| O | 4 | 16 | 64 | 65.31% |
Useful conversions
- 1 mol of H3PO4 weighs 98 g and contains 6.022 × 10²³ formula units.
- 1 g of H3PO4 is 0.0102 mol, or 6.145 × 1021 formula units.
- One formula unit weighs 1.627 × 10−22 g.
Equivalent weight
Equivalent weight = molar mass ÷ n-factor. For phosphoric acid, the n-factor is 3: three replaceable H⁺. So the equivalent weight is 98 ÷ 3 = 32.67 g/eq. Use it for normality: N = M × 3.
Molar masses of common compounds
| Formula | Name | g/mol (exam) |
|---|---|---|
| H2O | water | 18 |
| NaOH | sodium hydroxide | 40 |
| NaCl | sodium chloride | 58.5 |
| H2SO4 | sulphuric acid | 98 |
| HCl | hydrochloric acid | 36.5 |
| HNO3 | nitric acid | 63 |
| KMnO4 | potassium permanganate | 158 |
| K2Cr2O7 | potassium dichromate | 294 |
| CaCO3 | calcium carbonate | 100 |
| NaHCO3 | sodium bicarbonate (baking soda) | 84 |
| Na2CO3 | sodium carbonate | 106 |
| C6H12O6 | glucose | 180 |
| C12H22O11 | sucrose | 342 |
| CO2 | carbon dioxide | 44 |
| NH3 | ammonia | 17 |
| CH4 | methane | 16 |
| C2H5OH | ethanol | 46 |
| CH3COOH | acetic acid | 60 |
| KOH | potassium hydroxide | 56 |
| Ca(OH)2 | calcium hydroxide | 74 |
| Mg(OH)2 | magnesium hydroxide | 58 |
| CuSO4 | copper sulphate (anhydrous) | 159.5 |
| CuSO4·5H2O | copper sulphate pentahydrate (blue vitriol) | 249.5 |
| FeSO4·7H2O | ferrous sulphate heptahydrate (green vitriol) | 278 |
| FeSO4·(NH4)2SO4·6H2O | Mohr's salt | 392 |
| H2C2O4·2H2O | oxalic acid dihydrate | 126 |
| Na2S2O3·5H2O | sodium thiosulphate (hypo) | 248 |
| AgNO3 | silver nitrate | 170 |
| BaCl2 | barium chloride | 208 |
| KCl | potassium chloride | 74.5 |
| Al2O3 | aluminium oxide | 102 |
| Fe2O3 | iron(III) oxide | 160 |
| H3PO4 | phosphoric acid | 98 |
| NH4Cl | ammonium chloride | 53.5 |
| (NH4)2SO4 | ammonium sulphate | 132 |
| KI | potassium iodide | 166 |
| Na2SO4 | sodium sulphate | 142 |
| MgSO4·7H2O | magnesium sulphate heptahydrate (Epsom salt) | 246 |
| C6H6 | benzene | 78 |
| O2 | oxygen gas | 32 |
| N2 | nitrogen gas | 28 |
| Cl2 | chlorine gas | 71 |
| H2O2 | hydrogen peroxide | 34 |
| CO(NH2)2 | urea | 60 |
| Al2(SO4)3 | aluminium sulphate | 342 |
| CaCl2 | calcium chloride | 111 |
| NaNO3 | sodium nitrate | 85 |
| KClO3 | potassium chlorate | 122.5 |
Questions students ask
What is the molar mass of H₃PO₄?
98 g/mol using exam atomic masses (3 × 1 + 31 + 4 × 16), or 97.99 g/mol with exact IUPAC values.
How many moles are in 1 g of H₃PO₄?
1 ÷ 98 = 0.0102 mol.
What is the mass of one molecule of H₃PO₄?
97.99 ÷ 6.022 × 10²³ = 1.627 × 10⁻²² g.
What is the percentage of O in H₃PO₄?
65.31% by mass.
What is the equivalent weight of H₃PO₄?
32.67 g/eq: molar mass 98 ÷ n-factor 3. Basis for the n-factor: three replaceable H⁺.