Borax

Borax commonly denotes hydrated sodium tetraborate, conventionally written $\mathrm{N}{\mathrm{a}}_2{\mathrm{B}}_4{\mathrm{O}}_7\cdot 10{\mathrm{H}}_2\mathrm{O}$ for the decahydrate. Anhydrous and other hydrated sodium borates are distinct materials, so a mass-based chemical calculation must state the hydration form.^[ref]

The decahydrate's formal formula counts ten waters per tetraborate formula unit. Those waters contribute to its molar mass and change the amount of borate delivered by a weighed sample. In solution, acid–base equilibria and pH influence the distribution of boron-containing species; the crystalline formula does not require every dissolved boron unit to remain as one unchanged anion.