Meaning of the label “sodium tetraborate decahydrate” on a borax product
A brand of borax that is sodium tetraborate decahydrate is identified by the ingredient name itself: the words sodium tetraborate decahydrate indicate the compound commonly called borax decahydrate, a hydrated ionic solid containing a borate anion and ten waters of crystallization.
Label check (practical chemistry): The term decahydrate means “10 water molecules per formula unit.” If the label instead says pentahydrate or omits the hydrate (e.g., “sodium tetraborate” without “decahydrate”), it is not the same hydrate and will have a different molar mass and water content.
Chemical formula of sodium tetraborate decahydrate (borax decahydrate)
The standard formula is:
\[ \mathrm{Na_2B_4O_7 \cdot 10H_2O} \]
The dot “\(\cdot\)” indicates waters of crystallization in the crystal lattice, not covalent bonding to boron in a single molecule.
Atom counts per formula unit
| Component | How counted | Atoms contributed |
|---|---|---|
| \(\mathrm{Na_2B_4O_7}\) | Subscripts in the anhydrous salt | \(\mathrm{Na: 2,\ B: 4,\ O: 7}\) |
| \(\mathrm{10H_2O}\) | \(10 \times\) one water molecule | \(\mathrm{H: 10 \times 2 = 20,\ O: 10 \times 1 = 10}\) |
| Total | Add the contributions | \(\mathrm{Na: 2,\ B: 4,\ O: 7 + 10 = 17,\ H: 20}\) |
Molar mass (formula mass) of sodium tetraborate decahydrate
Using standard atomic masses (in \(\mathrm{g\ mol^{-1}}\)) \( \mathrm{Na}=22.99\), \( \mathrm{B}=10.81\), \( \mathrm{O}=16.00\), \( \mathrm{H}=1.008\):
1) Molar mass of the anhydrous part \(\mathrm{Na_2B_4O_7}\):
\[ M(\mathrm{Na_2B_4O_7}) = 2(22.99) + 4(10.81) + 7(16.00) = 45.98 + 43.24 + 112.00 = 201.22\ \mathrm{g\ mol^{-1}} \]
2) Molar mass of the water part \(\mathrm{10H_2O}\):
\[ M(\mathrm{H_2O}) = 2(1.008) + 16.00 = 18.016\ \mathrm{g\ mol^{-1}} \] \[ M(10\mathrm{H_2O}) = 10 \times 18.016 = 180.16\ \mathrm{g\ mol^{-1}} \]
3) Total molar mass:
\[ M(\mathrm{Na_2B_4O_7 \cdot 10H_2O}) = 201.22 + 180.16 = 381.38\ \mathrm{g\ mol^{-1}} \]
Helpful visualization: “anhydrous salt + 10 waters of crystallization”
Optional extension: mass percent of water in borax decahydrate
Water mass in one mole is \(180.16\ \mathrm{g}\), and total molar mass is \(381.38\ \mathrm{g}\), so:
\[ \%\ \mathrm{H_2O} = \frac{180.16}{381.38} \times 100 = 47.23\% \]
This explains why different hydrates (different “-hydrate” names) change measured mass relationships in stoichiometry and solution preparation.