In systems with multiple equilibria, the equilibrium calculations use a systematic approach involving a series of steps to make sure there are as many equations as chemical species. Firstly, identify all the chemical reactions in the system. Then, formulate the equilibrium constant expression for each reaction. Next, create mass-balance equations that reveal two sources of hydroxide ions. The hydroxide ion concentration from the dissociation of magnesium hydroxide is twice the magnesium ion concentration, and the hydroxide ion concentration from the dissociation of water is equal to the hydronium ion concentration. Then, make an expression for the charge balance of ionic species that follows the principle of electroneutrality, meaning the positive and negative charge concentrations in the solution must be equal. Finally, determine the number of independent equations. If the number is equal to or exceeds the number of chemical species involved or unknowns, the equations are solvable. Assumptions about the relative concentrations of species are often made to simplify the calculations. The validity of the assumptions must be checked in the final answer.