What is the concentration of chloride ions in a solution formed by mixing 150. ml of a 1.50 m nacl solution with 250. ml of a 0.750 m mgcl2 solution?

Here, we apply a mass balance:
Moles of chloride ions in final solution = sum of moles of chloride ions in added solutions

We must also not that each mole of sodium chloride will release one mole of chloride ions, while each mole of magnesium chloride will release two moles of chloride ions.
Moles = concentration * volume
Moles in final solution = moles in NaCl solution + moles in MgCl₂ solution
C * (150 + 250) = 1.5 * 150 + 2 * 0.75 * 250
C = 1.5 M

The final concentration is 1.5 M