What assumption is commonly used to simplify the process of finding the pH for a solution of a weak acid?
1) The quadratic equation is the fastest way to find the pH of a weak acid, and the assumption is that the quadratic equation will work.
2) For many weak acid solutions, it is assumed that [H3O+] is small relative to the initial concentration of acid, so the "x is small" approximation is used.
3) The concentration of H3O+ in solution is assumed to be equal to the concentration of the weak acid.
4) The concentration of H3O+ is assumed to be directly found from the K a expression, so all that is needed is to plug the initial concentrations of each species into the K a expression.