Calculating pH from Strong Acids and Weak Acids

What is equilibrium? Equilibrium is basically the state in which the concentrations of both the reactants and products are no longer changing. Reactants still convert to products and vice versa in an equilibrium state but they convert in a similar ratio so that the overall concentrations do not change.

Equilibrium is different for each compound. For strong acids, the equilibrium lies really far to the right. This means that there is more product than reactant. The acid is almost fully dissociated.

Weak acids at equilibrium lie very far to the left, meaning that most of the acid is still fully associated with its hydrogen.

This is important when we start solving for pH using strong acids vs. weak acids. To solve for pH using a strong acid, most of the acid is dissociated, so the pH is based on the actual concentration of the acid present in the solution. For example, if we have 1.0M HCl, most of it dissociates to H^+  ion, so in effect we have 1.0M H^+ and we can easily solve for pH.

The tricky part comes in when we are solving for weak acids. Since weak acids do not fully dissociate, and in fact most exist in its hydrogen form, 1.0M HF does NOT mean we have 1.0M H^+ in the solution. We could have something like 98% in the HF form and only 2% in the H^+. Therefore to calculate pH we have to find out how much of HF is dissociated. This is where the ice boxes come into play.

Say we have 1.0M HF solution and we would like to calculate the pH of this solution. We have to set up a table to figure out how much of the HF dissociates into its H^+ form.

X is the amount of HF that does decide to dissociate. This will yield our H^+ concentration from which we can calculate the pH.

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