Balanced chemical equation:
FeBr2(aq) + 2AgNO3(aq) --→ Fe(NO3)2(aq) + 2AgBr(s)
Moles of FeBr2:
moles FeBr2 = mass / molar mass
moles FeBr2 = 12.7 g / 215.86 g/mol
moles FeBr2 = 0.0588 mol
Moles of Fe2+ ions:
moles Fe2+ = moles FeBr2 = 0.0588 mol
Total volume of the solution:
Total volume = volume of FeBr2 solution + volume of AgNO3 solution
Total volume = 250. mL + 0 mL = 250. mL = 0.250 L
Final molarity of Fe2+ ions:
Molarity Fe2+ = moles Fe2+ / total volume
Molarity Fe2+ = 0.0588 mol / 0.250 L
Molarity Fe2+ = 0.235 M
Rounded to 3 significant digits:
Molarity Fe2+ = 0.24 M