Solutions of a Strong Acid or Base

Consider an aqueous solution of a strong acid or base. Suppose you dissolve 0.10 mol HCl in 1.0 L of aqueous solution, giving O.IOM HCl. You would like to know the concentration of H30 ion in this solution. In addition to the self-ionization of water, you have the reaction of HCl with water, which also produces H30 ion. A strong acid such as hydrochloric acid, HCl(ag), essentially reacts completely with water. 

Because you started with 0.10 mol HCl in 1.0 L of solution, the reaction will produce 0.10mol H30, so the concentration of H30 ion from HCl is O.IOM. 

Now consider the concentration of H30 ion produced by the self-ionization of water. In pure water, the concentration of H30 produced is 1.0 X 10 M in an acid solution, the contribution of HsO from water will be even smaller. You can see this by applying Le Chatelier s principle to the self-ionization reaction. When you increase the concentration of H30 in water by adding an acid, the self-ionization of water reverses until a new equilibrium is obtained. 

Consequently, the concentration of produced by the self-ionization of water 

In a solution of a strong acid, you can normally ignore the self-ionization of water as a source of HsO. The concentration is usually determined by the strong 

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Calculate the concentrations of ions in a solution of a strong acid or base (Example 10.2). 

When you are asked to solve problems involving acids and bases, you need to pay attention to their strengths, even if the word strength isn t mentioned in the problem. Strong acids and bases are treated as if they ionize completely, so the concentration of the ions in an aqueous solution of a strong acid or base is directly related to the molarity of the acid or base. If a question asks for the concentration of H30+ ions in a 0.001 molar solution of HC1, the answer would be 0.001 M, because each molecule of HC1, a strong acid, is thought to provide one hydronium ion. 

Calculating Concentrations of HbO and OH in Solutions of a Strong Acid or Base... 

Calculating concentrations of HjO" and OH in solutions of a strong acid or base Given the concentration of a strong acid or base, calculate the hydronium-ion and hydroxide-ion concentrations. (EXAMPLE 16.4)... 

The difficulties appear whenever two or more different reactions contribute comparable concentrations of the same species. We have already seen an example in a very dilute solution of a strong acid or base (page 268). H comes not only from the reaction... 

In a 1 M solution of a strong acid or base at 298 K, more than 95% of the molecules react with water. In a 1 M solution of a weak acid or base, only a small fraction of molecules react with water. 

We have seen how to estimate the pH of a solution of a weak acid or base (Chapter 10), but suppose that a salt of the acid or base is also present. How does that salt affect the pH of the solution Suppose we have a dilute hydrochloric acid solution and add to it appreciable concentrations of the conjugate base, the Cl- ion, as sodium chloride. Because the acid is strong, its conjugate base is extremely weak and so has no measurable effect on pH. The pH of 0.10 M HCl(aq) is about 1.0, even after 0.10 mol NaCl has been added to a liter of the solution. Now suppose instead that the solution contains acetic acid to which sodium acetate has been added (the acetate ion, CH jC()2, is the conjugate base of CH COOH). Because the conjugate base of a weak acid is a base, we can predict that adding acetate ions (as sodium acetate) to a solution of acetic acid will increase the pH of the solution. Similarly, suppose we have a solution of ammonia and add ammonium chloride to it. The... 

Buffering refers to the ability of a solution to resist change in pH after the addition of a strong acid or base. The body s principal buffer system is the carbonic acid/bicarbonate (H2C03/HC03 ) system. 

A buffer comprises (1) a weak acid and a salt of that acid, (2) a weak base and a salt of that base, or (3) it may contain an acid salt. We define an acid-base buffer as a solution whose pH does not change after adding (small amounts of) a strong acid or base . Sodium ascorbate is a favourite buffer in the food industry. 

Just because an acid or base is strong doesn t mean a solution of that acid or base is corrosive. The corrosive action of an acidic solution is caused by the hydronium ions rather than by the acid that generated those hydronium ions. Similarly, the corrosive action of a basic solution results from the hydroxide ions it contains, regardless of the base that generated those hydroxide ions. A very dilute solution of a strong acid or a strong base may have litde corrosive action because in such solutions there are only a few hydronium or hydroxide ions. (Almost all the molecules of the strong acid or base break up into ions,... 

The pH of a solution of a strong acid or a strong base can be calculated conversely, [H+] and [OH ] can be determined from the measured pH of the solution. This is shown in the following examples. 

A buffer solution (or a buffered solution) is a solution that is resistant to change when small amounts of acid or bases are added. Such a solution contains relatively large amounts of both a weak acid (or weak base) and its strong salt (one that ionizes very well). If a small amount of a strong acid (or base) is added to a buffer, most of the added H+ (or OH-) will combine with an equivalent amount of the weak base (or acid) of the buffer to form the conjugate acid (or base) of that weak base (or acid). The result is that the hydrogen ion and hydroxide concentrations in the solution display very little change. 

Very dilute solution -> Concentration of a strong acid or base is less than 10 m. 

CAUTION Dilution of a concentrated solution, especially of a strong acid or base, frequently liberates a great deal of heat. This can vaporize drops of water as they hit the concentrated solution and can cause dangerous spattering. 

CAUTION Dilution of a concentrated solution, especially of a strong acid or base, frequently liberates a great deal of heat. This can vaporize drops of water as they hit the concentrated solution and can cause dangerous spattering. As a safety precaution, concentrated solutions of acids or bases are always poured slowly into water, allowing the heat to be absorbed by the larger quantity of water. Calculations are usually simpler to visualize, however, by assuming that the water is added to the concentrated solution. 

Buffer - A solution designed to maintain a constant pH when small amounts of a strong acid or base are added. Buffers usually consist of a fairly weak acid and its salt with a strong base. Suitable concentrations are chosen so that the pH of the solution remains close to the pK of the weak acid. 

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