A white crystalline salt X dissolves in hot water to give an alkaline solution, swells and fuses to a glassy mass on strong heating, gives a green-edged flame when heated with ethanol and concentrated H2SO4, and also imparts a persistent golden-yellow colour to the flame. The anion of X is the cyclic [B4O5(OH)4]^2-. The number of sp^3 hybridised boron atoms in this anion is

JEE Advanced levelSingle correct optionDifficulty 3 of 3qa068

A white crystalline salt X dissolves in hot water to give an alkaline solution, swells and fuses to a glassy mass on strong heating, gives a green-edged flame when heated with ethanol and concentrated $H X_{2} SO X_{4}$ , and also imparts a persistent golden-yellow colour to the flame. The anion of X is the cyclic $[B X_{4} O X_{5} (OH) X_{4}] X^{2 -}$ . The number of $s p^{3}$ hybridised boron atoms in this anion is

The observations identify X as borax,

Na X_{2} B X_{4} O X_{7} \cdot 10 H X_{2} O

(green ester flame, alkaline hydrolysis, glassy

NaBO X_{2}

B X_{2} O X_{3}

bead, Na flame). In

[B X_{4} O X_{5} (OH) X_{4}] X^{2 -}

two boron atoms are tetrahedral (four-coordinate,

s p^{3}

, each carrying a formal negative charge) and two are trigonal planar (

s p^{2}

). The number of

s p^{3}

boron atoms is 2.