http://www.ck12.org Chapter 21. Acids and Bases
Ka= 4. 5 × 10 −^4 =(x)(x)
2. 00 −x=
27.24 X
2. 00 −xThe quadratic equation is required to exactly solve this equation for x. However, a simplification can be made
because of the fact that relatively little of the weak acid will be ionized at equilibrium. Since the reactants are
heavily favored, the value of x will be much less than 2.00, so we can say that 2.00 - x is approximately equal to
2.00.
4. 5 × 10 −^4 =
x^2
2. 00 −x≈
x^2
2. 00
x=√
4. 5 × 10 −^4 ( 2. 00 ) = 2. 9 × 10 −^2 M= [H+]
Since the variable x represents the hydrogen-ion concentration, the pH of the solution can now be calculated.
pH = -log[H+] = -log[2.9× 10 −^2 ] = 1.54Step 3: Think about your result.
The pH of a 2.00 M solution of a strong acid would be equal to –log(2.00) =−0.30. The higher pH of the 2.00 M
nitrous acid is consistent with it being a weaker acid.
Practice Problem- Calculate the pH of a 0.20 M solution of hydrocyanic acid.
The procedure for calculating the pH of a solution of a weak base is similar to that of the weak acid in Sample
Problem 21.5. However, the variable x will represent the concentration of the hydroxide ion. The pH is found by
taking the negative logarithm to get the pOH, followed by subtracting from 14 to get the pH.
Lesson Summary
- Strong acids and bases are those that ionize completely in solution. Weak acids and bases ionize only to a very
small extent in solution. A stronger acid has a correspondingly weaker conjugate base, and vice-versa. - The acid ionization constant (Ka) and base ionization constant (Kb) are numerical measures of the strength of
acids and bases. The larger the ionization constant, the stronger the acid or base. - Solutions of known concentration and known pH can be used to find an unknown Kaor Kb.
- For acids or bases with known Kaor Kb, the pH of the solution can be determined as long as the concentration
of the solution is also known.
Lesson Review Questions
Reviewing Concepts
- Explain why a strong acid is not the same thing as a concentrated acid.
- Acid HX is a strong acid, while HY is a weak acid.