Question

Determine the concentrations of MgCl2, Mg2 , and Cl– in a solution prepared by dissolving 2.51 × 10–4 g MgCl2 in 2.00 L of water. Express all three concentrations in molarity. Additionally, express the concentrations of the ionic species in parts per million (ppm).

Answer 1

moles of MgCl2 = 2.51 x 10^-4 / 95.21

                       = 2.64 x 10^-4

volume = 2.00 L

molarity = moles / volume

              = 2.64 x 10^-6 / 2.00

              = 1.32 x 10^-6 M

concentration of MgCl2 = 1.32 x 10^-6 M

[Mg+2] concentration = 1.32 x 10^-6 M

[Cl-] concentration = 2.64 x 10^-6 M

concentration in ppm

Mg+2 = 1.32 x 10^-7 mol / L x 24 g / mol

            = 3.17x 10^-6 g / L

           = 3.17 x 10^-3 mg /L

         = 3.17 x 10^-2 ppm

Cl- concentration = 2.64 x 10^-7 mol / L

                          = 9.37 x 10^-2 ppm