Answer
(a) $2HF(aq) + Ba(OH)_2(aq) -- \gt BrF_2(aq) + 2H_2O(l)$
(b) $HClO_4(aq) + NaOH(aq) -- \gt NaClO_4(aq) + H_2O(l)$
(c) $2HBr(aq) + Ca(OH)_2(aq) -- \gt CaBr_2(aq) + 2 H_2O(l)$
(d) $HCl(aq) + KOH(aq) -- \gt KCl(aq) + H_2O(l)$
Work Step by Step
1. Write the double replacement equation.
The cation in the acid will always be $H^+$, and, in this case, the anions of the bases are $OH^-$.
2. Balance the equation.
Remember the reaction between an acid and a base with hydroxide ions follows the pattern:
$Acid + Base --\gt Salt + Water$
Where the salt is the ionic compound created by the mixture of the 2 conjugate pairs of the acid and the base.