Honors Chemistry
Name: ___________________________________________
Date: _________
Calculating pH, pOH, [H+] and [OH-] for Strong Acids and Bases
Strong Acids:
Strong acids completely dissociate into ions in aqueous solution. Therefore, calculating the pH
of a solution of a strong monoprotic acid is straightforward because [H+] equals the original
concentration of the acid.
1) What is the pH of a 0.20 M solution of HNO3?
2) An aqueous solution of HCl has a pH of 1.12. What is the concentration of this acid?
Strong Bases:
Strong bases also completely dissociate into ions in aqueous solution. Thus, calculating the pH
of their solutions is also straightforward.
1) What is the pH of a 0.028 M NaOH solution?
2) What is the pH of a 0.0011 M Ca(OH)2 solution?
3) What is the concentration of a solution of KOH for which the pH is 11.89?
4) What is the concentration of a solution of Ca(OH)2 for which the pH is 11.68?
Molarity (M):
The concentration of acids and bases is measured using molarity (M). Give the formula for
molarity in the space below:
1) Calculate the pH of a solution containing 1.52 g of HNO3 in 575 mL of solution.
2) Calculate the pH when 2.250 g of LiOH are dissolved in 250.0 mL of solution.
Diluting Strong Acids and Bases:
In the laboratory, dilute solutions are often made from concentrated solutions of both strong
acids and strong bases. List the formulas for the dilution of a concentrated acid and the dilution
of a concentrated base in the space below:
3) Calculate the pH if 5.00 mL of 0.250 M HClO4 is diluted to 50.0 mL.
4) Calculate the pH when 1.00 mL of 0.175 M NaOH is diluted to 2.00 L.
Strong Acid and Strong Base Practice Problems
1) What is the pH of a 0.040 M solution of HClO4?
2) An aqueous solution of HNO3 has a pH of 2.34. What is the concentration of this acid?
3) What is the pH of a 0.063 M LiOH solution?
4) What is the pH of a 0.045 M Sr(OH)2 solution?
5) Calculate the concentration of an aqueous solution of NaOH that has a pH of 11.50.
6) Calculate the concentration of an aqueous solution of Ba(OH)2 that has a pH of 12.05.
7) Calculate the pH of a solution containing 0.525 g of HClO4 in 2000 mL of solution.
8) Calculate the pH if 5.00 mL of 1.00 M HCl is diluted to 0.50 L.
9) Calculate the pH when 1.565 g of KOH is dissolved in 500.0 mL of solution.
10) Calculate the pH when 10.0 mL of 0.015 M Ca(OH)2 is diluted to 500.0 mL.
Calculating pH, Ka and Percentage of Ionization for Weak Acids
Weak Acids:
Most acidic substances are weak acids. Unlike strong acids, weak acids are only partially
ionized in aqueous solution. We can use the equilibrium constant for the ionization reaction to
express the extent to which a weak acid ionizes. If we represent a general weak acid as HA, we
can write the equation for its ionization in the space below.
We can then use this equation to write an equilibrium-constant expression for this reaction:
*Ka is called the acid-dissociation constant.
The magnitude of Ka indicates the tendency of the acid to ionize in water. If the value of Ka is
greater than one, the equilibrium lies to the right, and we are dealing with a ____________ acid.
If the value of Ka is less than one, the equilibrium lies to the left, and we are dealing with a
______________ acid. Therefore, the higher the value of Ka, the ________________ the acid.
Because weak acids only partially ionize in aqueous solution, we must use our skills for solving
equilibrium problems (ICE tables) from Chapter 15 in order to calculate either the Ka value for a
weak acid or the pH of its solutions.
Calculating Ka and Percent Ionization from Measured pH:
1. A student prepared a 0.10 M solution of formic acid (HCHO2) and measured its pH using
a pH meter. The pH at 25°C was found to be 2.38. (a) Calculate Ka for formic acid at
this temperature. (b) What percentage of the acid is ionized in this 0.10 M solution?
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Using Ka to Calculate pH:
2. Calculate the pH of a 0.30 M solution of acetic acid (HC2H3O2), the weak acid
responsible for the characteristic odor and acidity of vinegar, at 25°C. What percentage
of the acid is ionized in this 0.30 M solution? The Ka for acetic acid is 1.8 x 10-5.
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Polyprotic Acids:
Many acids have more than one ionizable H atom. These acids are known as polyprotic acids.
For example, each of the H atoms in sulfurous acid can ionize in successive steps. Write these
steps below.
The acid-dissociation constants for these equilibria are labeled Ka1 and Ka2. The numbers on
the constants refer to the particular proton of the acid that is ionizing. Thus, Ka2 always refers to
the equilibrium involving removal of the second proton of a polyprotic acid.
By viewing the Ka values listed above for the successive ionizations of the H atoms in sulfurous
acid, it is obvious that it is always _____________ to remove the first proton from a polyprotic
acid than to remove the second. For an acid with three ionizable protons, it is easier to remove
the second proton than the third. Thus, Ka values become successively ______________ as
successive protons are removed.
More ICE Table Practice Problems!
1) Lactic acid (HC3H5O3) has one acidic hydrogen. A 0.10 M solution of lactic acid has a
pH of 2.44. Calculate Ka and the percentage of the acid that is ionized.
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2) Phenylacetic acid (HC8H7O2) is one of the substances that accumulates in blood of
people with phenylketonuria, an inherited disorder that can cause mental retardation or
even death. A 0.085 M solution of HC8H7O2 is found to have a pH of 2.68. Calculate Ka
and the percentage of the acid that is ionized.
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3) Calculate the pH of a 0.20 M solution of HCN. What percentage of the acid is ionized in
this solution? The Ka for hydrocyanic acid is 4.9 x 10-10.
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4) Calculate the pH of a 0.50 M solution of benzoic acid (HC7H5O2). What percentage of
the acid is ionized in this solution? The Ka for benzoic acid is 6.3 x 10-5.
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