Molar MassMolar mass of bases and alkalis
Bases neutralise acids. Those that dissolve in water are called alkalis. Their molar masses are needed for standard solutions and for neutralisation sums.
Sodium and potassium hydroxide take up water and carbon dioxide from the air, so a solution made by weighing them is only roughly the intended strength and is checked against a standard acid. Bases with two hydroxide groups, such as calcium hydroxide, neutralise two moles of hydrochloric acid for each mole.
This page lists 8 bases and alkalis with their formulas and molar masses, lightest first. The lightest here is ammonia at 17.03 g/mol and the heaviest is barium hydroxide at 171.34 g/mol. Tap a name for the full working, the percentage of each element and worked examples.
| Compound | Formula | Molar mass, g/mol |
|---|---|---|
| Ammonia | NH3 | 17.03 |
| Ammonium hydroxide ammonia solution | NH4OH | 35.05 |
| Sodium hydroxide caustic soda | NaOH | 40.00 |
| Potassium hydroxide caustic potash | KOH | 56.10 |
| Magnesium hydroxide milk of magnesia | Mg(OH)2 | 58.32 |
| Calcium hydroxide slaked lime | Ca(OH)2 | 74.09 |
| Aluminium hydroxide | Al(OH)3 | 78.00 |
| Barium hydroxide baryta | Ba(OH)2 | 171.34 |
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.
Not in the list?
Type any formula into the calculator. It handles brackets and water of crystallisation and shows the same working as these pages. The method by hand is on the step by step page.
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