What is the Arrhenius definition of an acid?

An Arrhenius acid is a substance that produces H + (or H 3 O + ) ions when dissolved in water. This model applies to aqueous solutions only and predates the broader Brønsted–Lowry definition.

pH is defined as the negative base-10 logarithm of the hydronium ion concentration: pH = –log 10 [H 3 O + ] where [H 3 O + ] is measured in M. It is a logarithmic scale — each unit represents a 10-fold change in [H 3 O + ].

True or False? A solution with pH 4 has ten times the [H 3 O + ] of a solution with pH 5.

True. The pH scale is logarithmic (base 10). A decrease of 1 pH unit corresponds to a 10-fold increase in [H 3 O + ], so pH 4 has ten times more H 3 O + than pH 5.

pOH is defined as the negative base-10 logarithm of the hydroxide ion concentration: pOH = –log 10 [OH - ] where [OH - ] is measured in M. The relationship pH + pOH = pK w (= 14 at 25 °C) links pH and pOH.

Why do acidic solutions always have a pH below 7?

Acidic solutions contain more H 3 O + than OH - , so [H 3 O + ] > 10 -7 M. Because pH = –log[H 3 O + ], a concentration greater than 10 -7 M gives a logarithm less negative than –7, so pH < 7.

True or False? At 25 °C, pure water has a pH of exactly 7.

True. At 298 K, [H 3 O + ] = [OH - ] = 10 -7 M in pure water. Therefore pH = –log(10 -7 ) = 7.

At 25 °C, pH + pOH = .......... , and the concentration of H 3 O + in pure water equals .......... M.

At 25 °C, pH + pOH = 14 , and the concentration of H 3 O + in pure water equals 10 -7 M.

Define the autoionization constant of water, K w .

K w is the equilibrium constant for the self-ionization of water: K w = [H 3 O + ][OH - ] At 25 °C, K w = 1.00 × 10 -14 . In pure water, [H 3 O + ] = [OH - ] = √K w = 1.00 × 10 -7 M.

True or False? Increasing temperature causes the pH of pure water to decrease.

True. The ionization of water is endothermic, so by Le Châtelier's principle, raising the temperature shifts the equilibrium right, increasing [H 3 O + ] and therefore increasing K w and decreasing pH. The water remains neutral (equal [H 3 O + ] and [OH - ]) — it is just a lower pH at that temperature.

Why does the concentration of water not appear in the K w expression?

The concentration of liquid water is so large and changes so negligibly during ionization that it is treated as a constant and incorporated into the equilibrium constant itself. This leaves only the ion concentrations in the expression: K w = [H 3 O + ][OH - ].

pK w = –log(K w ). At 25 °C, pK w = 14, which gives the fundamental relationship: pH + pOH = 14 This relationship holds at 25 °C; at other temperatures, pK w differs from 14.

A solution has [OH - ] = 4.60 × 10 -5 M at 25 °C. How would you find its pH?

Step 1: Calculate pOH : pOH = –log(4.60 × 10 -5 ) = 4.34 Step 2: Use pH + pOH = 14: pH = 14 – 4.34 = 9.66 Alternatively, calculate [H 3 O + ] = K w / [OH - ] = (1.00 × 10 -14 ) / (4.60 × 10 -5 ) = 2.17 × 10 -10 M, then pH = –log(2.17 × 10 -10 ) = 9.66.

True or False? At 25 °C, a neutral solution always has pH = 7 regardless of what is dissolved in it.

False. A neutral solution is one where [H 3 O + ] = [OH - ], which gives pH = 7 at 25 °C, but temperature changes K w so the neutral pH departs from 7 at temperatures other than 25 °C — meaning a neutral solution at a different temperature will not have pH 7.

The autoionization constant of water is K w = [H 3 O + ][OH - ] = .......... at 25 °C, so in pure water [H 3 O + ] = .......... M.

The autoionization constant of water is K w = [H 3 O + ][OH - ] = 1.00 × 10 -14 at 25 °C, so in pure water [H 3 O + ] = 1.00 × 10 -7 M.

Define a strong acid .

A strong acid is one that completely ionizes in aqueous solution. Because ionization is essentially 100%, [H + ] equals the molar concentration of the acid: [H + ] = M(strong acid) Examples include HCl and HNO 3 .

True or False? For a strong acid, [H 3 O + ] equals the molar concentration of the acid.

True. Strong acids ionize completely, so every formula unit contributes exactly one H + ion. For a monoprotic strong acid at concentration c: [H 3 O + ] = c M.

How do you calculate the pH of a solution containing 0.00052 M Ba(OH) 2 ?

Ba(OH) 2 is a strong base that releases 2 OH - per formula unit: [OH - ] = 0.00052 × 2 = 0.00104 M pOH = –log(0.00104) = 2.98 pH = 14 – 2.98 = 11.02

True or False? A Group 1 metal hydroxide produces more than one OH - ion per formula unit.

False. Group 1 hydroxides (e.g. NaOH, KOH) each produce exactly one OH - ion per formula unit. It is Group 2 hydroxides such as Ca(OH) 2 and Ba(OH) 2 that produce two OH - ions per formula unit.

Why does a strong base produce a high pH rather than a low one?

Strong bases completely dissociate, generating a high [OH - ] . This suppresses [H 3 O + ] because K w = [H 3 O + ][OH - ] must remain constant. A lower [H 3 O + ] means a higher pH (pH = –log[H 3 O + ]).

To find the pH of a strong acid with [H + ] = 1.6 × 10 -4 M, calculate pH = –log( .......... ) = .......... (to 2 d.p.).

To find the pH of a strong acid with [H + ] = 1.6 × 10 -4 M, calculate pH = –log( 1.6 × 10 -4 ) = 3.80 (to 2 d.p.).

AP®ChemistryCollege BoardFlashcardsUnit 8: Acids & BasespH & pOH

pH & pOH (College Board AP® Chemistry): Flashcards

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What is the Arrhenius definition of an acid?

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What is the Arrhenius definition of an acid?
An Arrhenius acid is a substance that produces H⁺ (or H₃O⁺) ions when dissolved in water. This model applies to aqueous solutions only and predates the broader Brønsted–Lowry definition.
Define pH.
pH is defined as the negative base-10 logarithm of the hydronium ion concentration:
pH = –log₁₀[H₃O⁺]
where [H₃O⁺] is measured in M. It is a logarithmic scale — each unit represents a 10-fold change in [H₃O⁺].
True or False?
A solution with pH 4 has ten times the [H₃O⁺] of a solution with pH 5.
True.
The pH scale is logarithmic (base 10). A decrease of 1 pH unit corresponds to a 10-fold increase in [H₃O⁺], so pH 4 has ten times more H₃O⁺ than pH 5.
Define pOH.
pOH is defined as the negative base-10 logarithm of the hydroxide ion concentration:
pOH = –log₁₀[OH^-]
where [OH^-] is measured in M. The relationship pH + pOH = pK_w (= 14 at 25 °C) links pH and pOH.
Why do acidic solutions always have a pH below 7?
Acidic solutions contain more H₃O⁺ than OH^-, so [H₃O⁺] > 10^-7 M. Because pH = –log[H₃O⁺], a concentration greater than 10^-7 M gives a logarithm less negative than –7, so pH < 7.
True or False?
At 25 °C, pure water has a pH of exactly 7.
True.
At 298 K, [H₃O⁺] = [OH^-] = 10^-7 M in pure water. Therefore pH = –log(10^-7) = 7.
At 25 °C, pH + pOH = .........., and the concentration of H₃O⁺ in pure water equals .......... M.
At 25 °C, pH + pOH = 14, and the concentration of H₃O⁺ in pure water equals 10^-7 M.
Define the autoionization constant of water, K_w.
K_w is the equilibrium constant for the self-ionization of water:
K_w = [H₃O⁺][OH^-]
At 25 °C, K_w = 1.00 × 10^-14. In pure water, [H₃O⁺] = [OH^-] = √K_w = 1.00 × 10^-7 M.
True or False?
Increasing temperature causes the pH of pure water to decrease.
True.
The ionization of water is endothermic, so by Le Châtelier's principle, raising the temperature shifts the equilibrium right, increasing [H₃O⁺] and therefore increasing K_w and decreasing pH. The water remains neutral (equal [H₃O⁺] and [OH^-]) — it is just a lower pH at that temperature.
Why does the concentration of water not appear in the K_w expression?
The concentration of liquid water is so large and changes so negligibly during ionization that it is treated as a constant and incorporated into the equilibrium constant itself. This leaves only the ion concentrations in the expression: K_w = [H₃O⁺][OH^-].
Define pK_w.
pK_w = –log(K_w). At 25 °C, pK_w = 14, which gives the fundamental relationship:
pH + pOH = 14
This relationship holds at 25 °C; at other temperatures, pK_w differs from 14.
A solution has [OH^-] = 4.60 × 10^-5 M at 25 °C. How would you find its pH?
Step 1: Calculate pOH: pOH = –log(4.60 × 10^-5) = 4.34
Step 2: Use pH + pOH = 14: pH = 14 – 4.34 = 9.66
Alternatively, calculate [H₃O⁺] = K_w / [OH^-] = (1.00 × 10^-14) / (4.60 × 10^-5) = 2.17 × 10^-10 M, then pH = –log(2.17 × 10^-10) = 9.66.
True or False?
At 25 °C, a neutral solution always has pH = 7 regardless of what is dissolved in it.
False.
A neutral solution is one where [H₃O⁺] = [OH^-], which gives pH = 7 at 25 °C, but temperature changes K_w so the neutral pH departs from 7 at temperatures other than 25 °C — meaning a neutral solution at a different temperature will not have pH 7.
The autoionization constant of water is K_w = [H₃O⁺][OH^-] = .......... at 25 °C, so in pure water [H₃O⁺] = .......... M.
The autoionization constant of water is K_w = [H₃O⁺][OH^-] = 1.00 × 10^-14 at 25 °C, so in pure water [H₃O⁺] = 1.00 × 10^-7 M.
Define a strong acid.
A strong acid is one that completely ionizes in aqueous solution. Because ionization is essentially 100%, [H⁺] equals the molar concentration of the acid:
[H⁺] = M(strong acid)
Examples include HCl and HNO₃.
True or False?
For a strong acid, [H₃O⁺] equals the molar concentration of the acid.
True.
Strong acids ionize completely, so every formula unit contributes exactly one H⁺ ion. For a monoprotic strong acid at concentration c: [H₃O⁺] = c M.
How do you calculate the pH of a solution containing 0.00052 M Ba(OH)₂?
Ba(OH)₂ is a strong base that releases 2 OH^- per formula unit:
[OH^-] = 0.00052 × 2 = 0.00104 M
pOH = –log(0.00104) = 2.98
pH = 14 – 2.98 = 11.02
True or False?
A Group 1 metal hydroxide produces more than one OH^- ion per formula unit.
False.
Group 1 hydroxides (e.g. NaOH, KOH) each produce exactly one OH^- ion per formula unit. It is Group 2 hydroxides such as Ca(OH)₂ and Ba(OH)₂ that produce two OH^- ions per formula unit.
Why does a strong base produce a high pH rather than a low one?
Strong bases completely dissociate, generating a high [OH^-]. This suppresses [H₃O⁺] because K_w = [H₃O⁺][OH^-] must remain constant. A lower [H₃O⁺] means a higher pH (pH = –log[H₃O⁺]).
To find the pH of a strong acid with [H⁺] = 1.6 × 10^-4 M, calculate pH = –log(.......... ) = .......... (to 2 d.p.).
To find the pH of a strong acid with [H⁺] = 1.6 × 10^-4 M, calculate pH = –log(1.6 × 10^-4) = 3.80 (to 2 d.p.).

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