Molar MassMolar mass of potassium aluminium sulphate dodecahydrate (KAl(SO4)2·12H2O)
The molar mass of potassium aluminium sulphate dodecahydrate, KAl(SO4)2·12H2O, is 474.37 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 474.37 grams.
474.37 g/mol
39.098 + 26.982 + 2 × 32.06 + 20 × 15.999 + 24 × 1.0080 = 474.372 · 48 atoms in the formula · also called potash alum, fitkari
About potassium aluminium sulphate dodecahydrate
Potash alum is the fitkari used to clear muddy water and stop bleeding from small cuts. It is a double salt with twelve molecules of water.
Other names: potash alum, fitkari.
The working
The formula KAl(SO4)2·12H2O has 1 potassium atom, 1 aluminium atom, 2 sulphur atoms, 20 oxygen atoms and 24 hydrogen atoms, 48 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Potassium (K) | 1 | 39.098 | 39.098 | 8.24% |
| Aluminium (Al) | 1 | 26.982 | 26.982 | 5.69% |
| Sulphur (S) | 2 | 32.06 | 64.120 | 13.52% |
| Oxygen (O) | 20 | 15.999 | 319.980 | 67.45% |
| Hydrogen (H) | 24 | 1.0080 | 24.192 | 5.10% |
39.098 + 26.982 + 2 × 32.06 + 20 × 15.999 + 24 × 1.0080 = 474.372 g/mol
With the rounded atomic masses used in many school books (K = 39, Al = 27, S = 32, O = 16, H = 1) the answer is 474 g/mol. The difference from 474.37 is too small to matter in most school sums.
The water in the crystal
The part after the dot is water of crystallisation: 12 molecules of water for each KAl(SO4)2. That water has a mass of 12 × 18.015 = 216.18 g/mol, which is 45.6% of the crystal. Without it, the anhydrous part KAl(SO4)2 has a molar mass of 258.19 g/mol. Heating drives the water off, so a sample loses up to 45.6% of its mass. How this is used to find the number of waters is shown on the hydrates page.
Percentage composition
By mass, KAl(SO4)2·12H2O is 8.24% potassium, 5.69% aluminium, 13.52% sulphur, 67.45% oxygen and 5.10% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 8.2 g of potassium, 5.7 g of aluminium, 13.5 g of sulphur, 67.5 g of oxygen and 5.1 g of hydrogen.
Notice that hydrogen has more atoms in the formula (24) than oxygen (20), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 100 g of KAl(SO4)2·12H2O? Divide the mass by the molar mass: 100 ÷ 474.37 = 0.2108 mol.
Moles to grams. What is the mass of 2 mol of KAl(SO4)2·12H2O? Multiply: 2 × 474.37 = 948.7 g.
Counting particles. 100 g contains 0.2108 mol × 6.022 × 1023 = 1.269 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 474.37 = 47.44 g of KAl(SO4)2·12H2O in each litre. For 250 mL, weigh 11.86 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 47.44 g |
| 0.5 mol | 237.2 g |
| 1 mol | 474.4 g |
| 2 mol | 948.7 g |
| 5 mol | 2372 g |
| Mass | Moles |
|---|---|
| 1 g | 0.002108 mol |
| 10 g | 0.02108 mol |
| 100 g | 0.2108 mol |
| 1 kg | 2.108 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of KAl(SO4)2.12H2O?
The molar mass of potassium aluminium sulphate dodecahydrate (KAl(SO4)2.12H2O) is 474.37 g/mol, or 474.372 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of KAl(SO4)2.12H2O?
100 ÷ 474.37 = 0.2108 mol.
What is the mass of one formula unit of KAl(SO4)2.12H2O?
Divide the molar mass by the Avogadro constant: 474.37 ÷ 6.022 × 1023 = 7.877 × 10-22 g.
What is the percentage of oxygen in KAl(SO4)2.12H2O?
Oxygen makes up 67.45% of the mass of KAl(SO4)2.12H2O: 20 × 15.999 ÷ 474.372 × 100.
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.