An aqueous solution containing Pb^ 2+ ions reacts with K_2CrO_4 to...

When potassium chromate is added to a solution containing $Pb^{2+}$ ions, a yellow precipitate of lead chromate (A) is formed.

$Pb^{2+} + CrO_4^{2-} \rightarrow PbCrO_4 \downarrow$

Lead is an amphoteric metal, and its salts like lead chromate dissolve in excess of strong alkalis such as sodium hydroxide. The dissolution occurs due to the formation of a soluble tetrahydroxoplumbate(II) complex (B).

$PbCrO_4 + 4NaOH \rightarrow Na_2[Pb(OH)_4] + Na_2CrO_4$

Thus, the soluble complex (B) is $Na_2[Pb(OH)_4]$.

Answer: $Na_2[Pb(OH)_4]$