15.6:

15.6: Strong Acid and Base Solutions

Strong Acid and Base Solutions

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Chemistry

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JoVE Core Chemistry

Strong Acid and Base Solutions

15.5: Relative Strengths of Conjugate Acid-Base Pairs

15.7: Weak Acid Solutions

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03:22 min

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September 24, 2020

Overview

A strong acid is a compound that dissociates completely in an aqueous solution and produces a concentration of hydronium ions equal to the initial concentration of acid. For example, 0.20 M hydrobromic acid will dissociate completely in water and produces 0.20 M of hydronium ions and 0.20 M of bromide ions.

On the other hand, a strong base is a compound that dissociates completely in an aqueous solution and produces hydroxide ions. For example, 0.015 M KOH, a group 1 metal hydroxide, will dissociate completely and produce 0.015 M of OH^– and 0.015 M of K⁺.

Group 2 metal hydroxides, like barium hydroxide [Ba(OH)₂] and strontium hydroxide [Sr(OH)₂], are also strong bases and possess two hydroxide ions. This causes them to produce a more basic solution compared to NaOH or KOH at the same concentration. For example, 0.015 M Ba(OH)₂produces 0.015 M Ba⁺and 0.030 M hydroxide.

As strong acids and bases dissociate completely, molar ratios can be used to determine their hydronium and hydroxide concentrations, which in turn can be used to calculate the pH or pOH of a solution. For example, a 0.030 M HCl solution will produce 0.03 M hydronium ions. Therefore the pH of this solution will be

The pOH of the same solution can be determined using the formula

As the pH of the solution is 1.52, its pOH can be calculated as

Similarly, the concentration of hydroxide ions produced by strong bases can be used to determine the pOH of a solution using the equation

The above equation can also be used to determine the hydroxide ion concentration when pOH is known. For example, if the pOH of a solution is 3.00,

Multiplication of both sides by −1 gives

Now, take the antilog of both sides

Thus, the hydronium ion concentration of the solution with pOH 3 is 1.0 × 10⁻³ M. A similar method can be used to determine the hydronium ion concentration of a solution if its pH is known.

Transcript

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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Strong Acid Solutions Strong Base Solutions Dissociation Hydronium Ions Nitric Acid Nitrate Ions Concentration PH HCl Chloride Ions Logarithm Solution Concentration Hydronium Ion Concentration Antilog Group One Metal Hydroxides Sodium Hydroxide Potassium Hydroxide