Question

A 1.87 g sample of Mg reacts with 80.0 mL of a hydrochloric acid solution (pH = -0.544). What is the pH of the solution after all the Mg has reacted? Assume constant volume.

Answer 1

The pH of the acid is given as -0.544 which means -0.544 = - log [H+]

Thus [H+] = 3.499M

    The reaction is   Mg     + 2 HCl -------> MgCl₂ + H₂(g)

                                 1.87/24 3.499x80x10^-3    0 0 initial concentrations

   =0.0799 = 0.2799   

   0.0 0.2    0.0799 0.0799 after reaction

Thus [H+] after reaction = 0.2x 1000/80 = 2.5M

Therefore pH of solution after reaction = - log [2.5] = -0.3979