MgBr2(aq) is an ionic compound which will have the releasing of 2 Br⁻ ions ions in water for every molecule of MgBr2 that dissolves.
MgBr2(s) --> Mg+(aq) + 2 Br⁻(aq)
[Br⁻] = 0.065 mol MgBr2/1L × 2 mol Br⁻ / 1 mol MgBr2 = 0.13 M
The answer to this question is [Br⁻] = 0.13 M
