The HBr is a strong acid, that is, it is fully consumed in the reaction
molar mass HBr = 80.91 g/mol
number of moles = 5.0 x 10⁻⁴ / 80.91 = 6.17 x 10⁻⁶ moles
molarity = 6.17 x 10⁻⁶ / 1.2 = 5.14 x 10⁻⁶ M
HBr ------------------- H₃O⁺ + Br ⁻
↓ ↓ ↓
5.14 x 10⁻⁶ 0 0
- 5.14 x 10⁻⁶ 5.14 x 10⁻⁶ 5.14 x 10⁻⁶
_____________________________________
0 5.14 x 10⁻⁶ M 5.14 x 10⁻⁶ M
PH = - log [ H₃O⁺ ]
PH = - log [ 5.14 x 10⁻⁶ ]
PH = 5.82