> A 90-96% yield of etBr from 28.5 mole etOH is impossible
> based on the amount of KBr (10mole) used... Right? OR--
> is the 90-96% yield based on the amount of KBr used?
Yields are always based on the limiting reagent, in this case KBr.
The boiling point of EtBr is very low! If I were you I'd consider ethyl iodide instead. It's more costly to prepare though.
A mixture of 500 g (3.94 mol) of iodine, 800 mL of 84% (w/v) ethanol, and 60 g (2.224 mol) of aluminum foil in small pieces is warmed gently. Once started, the reaction increases progressively in vigor, but can be controlled by cooling; it subsides in about 10 min, and is complete in an hour. The product is distilled until deep red fumes appear. A cold mixture of 700 mL of 84% ethanol and 400 mL of 85% sulfuric acid is added to the cooled residue. After 15 min, distillation is begun, and continued until no more oily drops form in the water in the receiver. The yield of crude ethyl iodide is 260 mL (504 g; approx. 80% yield).
Taken from
https://www.thevespiary.org/rhodium/Rhodium/chemistry/nitroalkane.html