Molar Mass of Sodium Hydroxide

Sodium hydroxide's formula mass rounds to 40.00 g/mol so cleanly that generations of technicians have made 1 M caustic by weighing 40 g into a litre flask without ever looking the number up. As a strong base it dissociates completely, so 0.1 M NaOH has pOH 1 and pH 13, and its equivalent weight equals its formula weight because it supplies one hydroxide per formula unit.

The catch is that solid NaOH is aggressively hygroscopic and absorbs atmospheric CO₂ to form sodium carbonate, so a solution made by weighing is never a primary standard — it must be standardised against potassium hydrogen phthalate before it can be trusted to four figures. Dissolving it is also strongly exothermic, roughly −44 kJ/mol, which is why caustic is always added to water rather than water to caustic, and why bulk 50 % solution is stored heated to keep it from crystallising at 12 °C.