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pH and pOH Scales
From the handout on pH scale (given during class) or from the website listed below, pH indicates the relative concentration of the hydronium ion ([H+] or [H3O+]). It is not the concentration of the acid molecule, [HA]. Since strong acids and weak acids ionize to different extent, you need to handle them differently with respect to pH calculations.
pH with strong acids
Since strong acid ionize 100%, you can use stoichiometry to figure out the hydronium ion concentration ([H+] or [H3O+]). Once you find the hydronium ion concentration, you can use the pH equation given in the other handout or website below to determine the pH of the solution.
Example:What is the pH of a 0.025M HCl solution.
1.As with all stoichiometry problem, you must write down chemical reaction,
1 HCl + 1 H2O --> 1 H3O+ + Cl1-
2. So (0.025M HCl)(1mole H3O+ / 1 mole HCl) = 0.025M H3O+
3. Plug into pH = -log[H3O+], pH = -log(0.025M H3O+)
pH= -(-1.602), pH = 1.602 , pH = 1.60
pH with weak acids
As you know, weak acid do not ionize 100% so you have both the acid molecule and its ions. Therefore, this is an equilibrium problem and you must use equilibrium constants to determine the hydronium ion concentration.
For acids, the equilibrium constants are called acid ionization constant, Ka. Several are listed in Fig 19-8 (Pg 553) and Fig 19-19 (Pg 565). The rule is higher acid ionization constant value, the higher strength of the weak acid (more like a strong acid). Using the same logic, strong acids would have very large Ka (we don't usually use them since they are so great a value).
pOH Scale
There is a corresponding scale for OH1- called pOH Scale . Strong base are treated like strong acids except it is hydroxide concentration you look for. Weak base also have equilibrium constant. They are calledbase dissociation constant. Kb. Your book does not provide any. As with pH scale, once you have hydroxide concentration, you use pOH = -log[OH1-] equation to determine pOH of the solution.
Math Equations to go between pH and pOH scales
Finally as the other handout shows, there are two equations that allow you to go between the two scale.(one of the equations is pH + pOH = 14.00.
Sig figs and pH scale
Ask me about sig figs for pH calculations, there are special rules.
pH Calculation Problem Set