The number of three-centre two-electron $(3c-2e)$ bonds in a molecule of diborane $(B_2H_6)$ is

Which of the following statements is incorrect?

$A$ certain salt $X$ gives the following results:
$(i)$ Its aqueous solution is alkaline to litmus.
$(ii)$ It swells up to a glassy material $Y$ on strong heating.
$(iii)$ When conc. $H_2SO_4$ is added to a hot solution of $X$,white crystals of an acid $Z$ separate out.
Identify $X, Y$ and $Z$ and write the equations for the reactions.

$A$ nonmetallic element of group $13$,used in making bulletproof vests,is an extremely hard solid of black colour. It can exist in many allotropic forms and has an unusually high melting point. Its trifluoride acts as a Lewis acid towards ammonia. The element exhibits a maximum covalence of four. Identify the element and write the reaction of its trifluoride with ammonia. Explain why the trifluoride acts as a Lewis acid.

Dilute aqueous solution of $.......$ acid is used as an antibiotic.

In which of the following reactions is hydrogen one of the products?
$i$. $2NaBH_4 + I_2 \longrightarrow$
$ii$. $2BF_3 + 6NaH \xrightarrow{450 \ K}$
$iii$. $4BF_3 + 3LiAlH_4 \longrightarrow$
$iv$. $3B_2H_6 + 6NH_3 \xrightarrow{\text{heat}} 2B_3N_3H_6 + 12H_2$