CHEMISTRY THE CENTRAL SCIENCE
16 ACID–BASE EQUILIBRIA
16.5 STRONG ACIDS AND BASES
The chemistry of an aqueous solution often depends critically on pH. It is therefore important to examine how pH relates to acid and base concentrations. The simplest cases are those involving strong acids and strong bases. Strong acids and bases are strong electrolytes, existing in aqueous solution entirely as ions. There are relatively few common strong acids and bases (see Table 4.2).
The seven most common strong acids include six monoprotic acids (HCl, HBr, HI, HNO3, HClO3, and HClO4), and one diprotic acid (H2SO4). Nitric acid (HNO3) exemplifies the behavior of the monoprotic strong acids. For all practical purposes, an aqueous solution of HNO3 consists entirely of H3O+ and NO3– ions:
We have not used equilibrium arrows for this equation because the reaction lies entirely to the right. (Section 4.1) As noted in Section 16.3, we use H3O+(aq) and H+(aq) interchangeably to represent the hydrated proton in water. Thus, we can simplify this acid ionization equation to
In an aqueous solution of a strong acid, the acid is normally the only significant source of H+ ions.* As a result, calculating the pH of a solution of a strong monoprotic acid is straightforward because [H+] equals the original concentration of acid. In a 0.20 M solution of HNO3(aq), for example, [H+] = [NO3–] = 0.20 M. The situation with the diprotic acid H2SO4 is more complex, as we will see in Section 16.6.
SAMPLE EXERCISE 16.8 Calculating the pH of a Strong Acid
What is the pH of a 0.040 M solution of HClO4?
Analyze and Plan Because HClO4 is a strong acid, it is completely ionized, giving [H+] = [ClO4–] = 0.040 M.
Check Because [H+] lies between 1 × 10–2 and 1 × 10–1, the pH will be between 2.0 and 1.0. Our calculated pH falls within the estimated range. Furthermore, because the concentration has two significant figures, the pH has two decimal places.
An aqueous solution of HNO3 has a pH of 2.34. What is the concentration of the acid?
Answer: 0.0046 M
The most common soluble strong bases are the ionic hydroxides of the alkali metals, such as NaOH, KOH, and the ionic hydroxides heavier alkaline earth metals, such as Sr(OH)2. These compounds completely dissociate into ions in aqueous solution. Thus, a solution labeled 0.30 M NaOH consists of 0.30 M Na+(aq) and 0.30 M OH– (aq); there is essentially no undissociated NaOH.
GIVE IT SOME THOUGHT
Which solution has the higher pH, a 0.001 M solution of NaOH or a 0.001 M solution of Ba(OH)2?
SAMPLE EXERCISE 16.9 Calculating the pH of a Strong Base
What is the pH of (a) a 0.028 M solution of NaOH, (b) a 0.0011 M solution of Ca(OH)2?
Analyze We are asked to calculate the pH of two solutions of strong bases.
Plan We can calculate each pH by either of two equivalent methods. First, we could use Equation 16.16 to calculate [H+] and then use Equation 16.17 to calculate the pH. Alternatively, we could use [OH–] to calculate pOH and then use Equation 16.20 to calculate the pH.
(a) NaOH dissociates in water to give one OH– ion per formula unit. Therefore, the OH– concentration for the solution in (a) equals the stated concentration of NaOH, namely 0.028 M.
(b) Ca(OH)2 is a strong base that dissociates in water to give two OH– ions per formula unit. Thus, the concentration of OH–(aq) for the solution in part (b) is 2 × (0.0011 M) = 0.0022 M.
What is the concentration of a solution of (a) KOH for which the pH is 11.89, (b) Ca(OH)2 for which the pH is 11.68?
Answers: (a) 7.8 × 10–3M, (b) 2.4 × 10–3M
Although all of the alkali metal hydroxides are strong electrolytes, LiOH, RbOH, and CsOH are not commonly encountered in the laboratory. The hydroxides of the heavier alkaline earth metals—Ca(OH)2, Sr(OH)2, and Ba(OH)2—are also strong electrolytes. They have limited solubility, however, so they are used only when high solubility is not critical.
Strongly basic solutions are also created by certain substances that react with water to form OH–(aq). The most common of these contain the oxide ion. Ionic metal oxides, especially Na2O and CaO, are often used in industry when a strong base is needed. The O2– reacts with water to form OH–, leaving virtually no O2– in the solution:
Thus, a solution formed by dissolving 0.010 mol of Na2O(s) in enough water to form 1.0 L of solution has [OH–] = 0.020 M and a pH of 12.30.
GIVE IT SOME THOUGHT
The CH3– ion is the conjugate base of CH4, and CH4 shows no evidence of being an acid in water. What happens when CH3– is added to water?