Notes:
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Strong Acids and Strong Bases
- The strength of the acid is determined by how far the equilibrium lies to the right. Qualitatively, this may be judged by the Ka of the acid. A large Ka indicates a strong acid; a small Ka indicates a weak acid. Strong acids, such as HCl, have Ka values in the vicinity of infinity. This implies that the dissociation of HCl is virtually complete, and the equilibrium lies completely to the right, therefore, the concentration of the acid equals the concentration of hydronium ions produced. For instance, a 0.01 M HCl solution will completely dissociate into 0.01 M H3O+ and 0.01 M Cl-. The effective concentration of HCl after "equilibrium" has been reached will be zero! Analogously, strong bases, such as NaOH, will dissociate completely. The concentration of OH- in solution will be equal to the concentration of the strong base.
- A typical strong acid problem might be: What is the pH of a 0.010 M HCl solution? Since HCl is a strong acid, the hydronium ion concentration will be equal to the HCl concentration:
[H3O+] = 0.010 M
- The pH can be found by taking the negative log of the hydronium ion concentration:
pH = -log[H3O+] = -log(0.010) = 2.00
- A typical strong base problem might be: What is the pH of a 0.010 M NaOH solution? Since NaOH is a strong base, the hydroxide ion concentration will be equal to the NaOH concentration:
[OH-] = 0.010 M
- The pH can be found by first finding the pOH by taking the negative log of the hydroxide ion concentration, and then converting the pH to pOH. To find the pOH:
pOH = -log[OH-] = -log(0.010) = 2.00
- The pH can then be calculated from the equation pH + pOH = 14:
pH = 14.00 - 2.00 = 12.00