15.5
Strong Acid and Base Solutions
A strong acid is a compound that dissociates completely in an aqueous solution and produces a concentration of hydronium ions equal to the initial con…
Strong acids dissociate completely in water. For example, nitric acid dissociates completely into hydronium ions and nitrate ions. As the hydronium ions generated from the autoionization of water are negligible, the concentration of the hydronium ions in water is equal to the concentration of the strong acid.
The pH of these solutions can be determined using the initial concentration of the strong acid.
For example, in a 0.10 molar HCl solution, HCl will dissociate completely into the hydronium ions and chloride ions, and therefore the hydronium ion concentration of the solution will also be 0.10 molar. By taking the negative logarithm of this concentration, the pH of the solution is equal to one.
Conversely, the pH of a solution can be used to determine the hydronium ion concentration of a solution. For example, for a solution of pH 3.60, its hydronium ion concentration can be determined by solving the equation 3.60 is equal to the negative log of the hydronium ion concentration.
To solve for the concentration, multiply both sides by negative one, and then take the antilog of both sides. The hydronium ion concentration equals 2.5 times ten to the negative four molar.
Strong bases that are group one metal hydroxides, like sodium hydroxide and potassium hydroxide, dissociate completely into solution. For example, 0.20 molar sodium hydroxide will dissociate completely in water and produce 0.20 molar sodium ions and 0.20 molar hydroxide ions.
However, group two metal hydroxides, like barium hydroxide and calcium hydroxide, produce two moles of hydroxide ions for each mole of base. For example, 0.020 molar calcium hydroxide will dissociate completely in water and will produce 0.020 molar calcium ions and 0.040 molar hydroxide ions.
Ionic metal oxides, like sodium oxide and calcium oxide, are also strong bases. Their oxide ion reacts with water and produces hydroxide ions.
The concentration of hydroxide ions can be used to calculate a pOH and pH of the solution. For example, a five times ten to the negative five molar potassium hydroxide solution has an equal amount of hydroxide ions as strong base and therefore has a pOH of 4.30.
Like pH, a pOH of the solution can also be used to determine hydroxide ion concentration by solving the equation: pOH equals the negative log of the hydroxide ion concentration.
Since pH plus pOH is equal to 14 and the pOH is 4.3, the pH of the solution is 9.7.
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Q1: Why does a strong acid produce hydronium ions equal to its initial concentration?
Strong acids dissociate completely in water, meaning every acid molecule breaks apart into hydronium ions and anions. Since the hydronium ions from water's autoionization are negligible, the hydronium ion concentration equals the acid's starting concentration. For example, a 0.10 molar HCl solution produces 0.10 molar hydronium ions.
Q2: How do you calculate pH from the hydronium ion concentration of a strong acid?
pH is calculated using the negative logarithm of the hydronium ion concentration: pH = -log[H+]. For a 0.10 molar HCl solution with 0.10 molar hydronium ions, pH = -log(0.10) = 1. Conversely, if pH is known, you can find hydronium concentration by taking the antilog of the negative pH value.
Q3: What is the difference between Group 1 and Group 2 metal hydroxides as strong bases?
Group 1 metal hydroxides like sodium hydroxide produce one mole of hydroxide ions per mole of base. Group 2 metal hydroxides like calcium hydroxide produce two moles of hydroxide ions per mole of base. For example, 0.020 molar calcium hydroxide generates 0.040 molar hydroxide ions, making it more basic than Group 1 hydroxides at the same concentration.
Q4: How do ionic metal oxides function as strong bases in aqueous solution?
Ionic metal oxides like sodium oxide and calcium oxide are strong bases because their oxide ions react with water to produce hydroxide ions. This reaction generates OH- ions that increase the solution's basicity. The resulting hydroxide ion concentration can then be used to calculate pOH and pH values.
Q5: How can you determine hydroxide ion concentration from pOH?
Hydroxide ion concentration is found using the equation pOH = -log[OH-]. If pOH is known, rearrange to solve for [OH-] by multiplying both sides by negative one and taking the antilog. For example, a solution with pOH 4.30 has a hydroxide ion concentration of 5 × 10⁻⁵ molar.
Q6: What is the relationship between pH and pOH in aqueous solutions?
pH and pOH are related by the equation pH + pOH = 14 at 25°C. This relationship allows you to calculate one value if you know the other. For instance, if a strong base solution has pOH 4.30, its pH is 9.7, indicating a basic solution with low hydronium ion concentration.
Q7: How do molar ratios help determine ion concentrations in strong acid and base solutions?
Molar ratios from the dissociation equation show how many ions form per molecule of acid or base. Since strong acids and bases dissociate completely, these ratios directly give ion concentrations. For example, 0.030 molar HCl produces 0.030 molar hydronium ions using a 1:1 ratio, while 0.015 molar Ba(OH)2 produces 0.030 molar hydroxide ions using a 1:2 ratio.