If the formula of Borax is Na₂B₄Oₓ(OH)_y . zH₂O, then x+y+z= ___________

Solution

Chemistry ma’am · 4 steps

Write borax the way its crystal is built: the [B₄O₅(OH)₄]²⁻ ion plus 8 waters.

1. Real formula. The textbook formula Na₂B₄O₇.10H₂O only gives the composition. The crystal actually contains the anion [B₄O₅(OH)₄]²⁻, so the true formula is Na₂[B₄O₅(OH)₄].8H₂O.
2. Inside the anion. Each of the four boron atoms carries one OH group, so y = 4. Five bridging oxygens hold the rings together, so x = 5. Two borons are tetrahedral (BO₄ units) and two are trigonal (BO₃ units).
3. Check it. B₄O₅(OH)₄ is the same as B₄O₇.2H₂O. Two waters are hidden inside the anion and 8 remain as water of crystallisation, so z = 8. The total is 2 + 8 = 10, which matches the decahydrate.

   x + y + z = 5 + 4 + 8

4. So. x + y + z = 17.

The common slip: 19. Taking z = 10 from Na₂B₄O₇.10H₂O while also counting the four OH groups gives 19. Those OH groups already account for 2 of the 10 waters.

The idea to keep. Borax is Na₂[B₄O₅(OH)₄].8H₂O. Two of the ten waters in Na₂B₄O₇.10H₂O are really the four OH groups on boron, so only 8 are true water of crystallisation.

Answer from the NTA final answer key. Solution by PrepWiser.