2. Acidity and basicity of solutions of common substances at 25 ºC: a. Calculate the pH of a sample of lemon juice with [H+ ] = 3.8 x 10–4 M. b. Calculate the pH of a commonly available window-cleaning solution with [OH– ] = 1.9 x 10–6 M. c. A sample of freshly pressed apple juice has a pH of 3.76. What is the [H+ ]? d. A solution formed by dissolving an antacid tablet has a pH of 9.18. What is the [OH– ]?

a. The pH of a sample of lemon juice with [H⁺] = 3.8*10⁻⁴ M is 3.4, indicating acidity.

b. The pH of a commonly available window-cleaning solution with [OH⁻] = 1.9*10⁻⁶ M is 8.28,  indicating basicity.

c. The [H⁺] is 1.74*10⁻⁴ M.

d. The [OH⁻] is 1.51*10⁻⁵ M.

Explanation:

pH is a measure of acidity or alkalinity that indicates the amount of hydrogen [H⁺] or hydronium [H₃O⁺] ions present in a solution or substance. Mathematically the pH is the negative logarithm of the molar concentration of the hydrogen or proton ions (H⁺) or hydronium ions (H₃O⁺):

pH= -log₁₀ [H⁺]= -log₁₀ [H₃O⁺] or pH= -log [H⁺]= -log [H₃O⁺]

The pH scale ranges from 0 to 14. Values less than 7 indicate the acidity range and those greater than 7 indicate alkalinity or basicity. Value 7 is considered neutral.

Case a

You know [H⁺]= 3.8*10⁻⁴ M. Then:

pH= -log (3.8*10⁻⁴)

pH=3.42

The pH of a sample of lemon juice with [H⁺] = 3.8*10⁻⁴ M is 3.4, indicating acidity.

Case b

Like pH, pOH (or OH potential) is a measure of the basicity or alkalinity of a solution.

The pOH indicates the concentration of hydroxyl ions [OH-] present in a solution:

pOH= -log₁₀ [OH⁻]= -log [OH⁻]

The pOH is related to the pH by the formula:

pH + pOH= 14

In this case you know [OH⁻]=1.9*10⁻⁶ M. Then:

pOH= -log(1.9*10⁻⁶)= 5.72

So pH + 5.72= 14

pH= 14 - 5.72

pH= 8.28

The pH of a commonly available window-cleaning solution with [OH⁻] = 1.9*10⁻⁶ M is 8.28,  indicating basicity.

Case c

You know pH=3.76. Then:

3.76= -log [H⁺]

-3.76= log [H⁺]

[H⁺]=

[H⁺]= 1.74*10⁻⁴ M

The [H⁺] is 1.74*10⁻⁴ M.

Case d

You know that pH + pOH=14 and pH is 9.18. Then

9.18 + pOH=14

pOH=14 - 9.18

pOH= 4.82

You know pOH= - log [OH⁻]. Then:

4.82= -log [OH⁻]

-4.82= log [OH⁻]

[OH⁻]=

[OH⁻]= 1.51*10⁻⁵ M

The [OH⁻] is 1.51*10⁻⁵ M.

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