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Strong and weak acids and bases

Covers AS91392

Strength means how completely it dissociates

A strong acid is completely dissociated in water — essentially every molecule has given up its proton.
A weak acid is only partly dissociated — an equilibrium is set up, and most of the acid remains as undissociated molecules.

Strong acid, written with a single arrow:

$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$

Weak acid, written with equilibrium arrows:

$\text{CH}_3\text{COOH} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{CH}_3\text{COO}^-$

Strong is not the same as concentrated

This distinction is examined every year, and it is worth stating in your own words.

Strong / weak describes how completely the acid dissociates — a property of the substance itself.
Concentrated / dilute describes how much acid is dissolved — a property of the solution you made.
So a concentrated weak acid and a dilute strong acid are entirely different, and either could have the lower pH depending on the numbers.

Comparing them at the same concentration

	Strong acid	Weak acid
$[\text{H}_3\text{O}^+]$	high — equals the acid concentration	low — much less than the acid concentration
pH	low	higher
electrical conductivity	good	poor
rate of reaction with a metal or carbonate	fast	slower
volume of base needed to neutralise	the same	the same

That last row is the one people get wrong. Both contain the same amount of acid, so both need the same volume of base. Strength affects how much has dissociated at any moment, not how much acid is present in total.

Conductivity measures ions

Electrical conductivity depends on the total concentration of ions in the solution.
A strong acid is fully dissociated, so it has a high ion concentration and conducts well.
A weak acid is barely dissociated, so it has a low ion concentration and conducts poorly.
A soluble salt is fully ionic even though it may not be acidic at all, so it also conducts well.
Conductivity is not a measure of acidity. A solution of sodium ethanoate conducts far better than ethanoic acid of the same concentration, yet it is basic.

$K_a$ measures strength numerically

$K_a = \frac{[\text{H}_3\text{O}^+][\text{A}^-]}{[\text{HA}]}$

A larger $K_a$ means the equilibrium lies further right, so the acid is stronger.
$\text{p}K_a = -\log K_a$ , so a smaller $\text{p}K_a$ means a stronger acid.

Because $\text{p}K_a$ is a negative log, the direction reverses: HF with $\text{p}K_a = 3.17$ is a stronger acid than ethanoic acid with $\text{p}K_a = 4.76$ .

The conjugate relationship

The stronger the acid, the weaker its conjugate base, and vice versa.
$\text{HCl}$ is a strong acid, so $\text{Cl}^-$ is such a weak base that it does not react with water at all.
$\text{CH}_3\text{COOH}$ is a weak acid, so $\text{CH}_3\text{COO}^-$ is a reasonable base and does react with water.
This is why salt solutions can be acidic or basic, and it is the link between this page and the last.

Tips and Tricks

Never use "strong" to mean "concentrated". If a question contrasts two solutions, state explicitly which property you are talking about. Many marks turn on this single distinction.

Remember that a strong and a weak acid of the same concentration need the same volume of base to neutralise them. Only the initial pH, the conductivity and the rate differ.

Use the right arrow. Single arrow for a strong acid, equilibrium arrows for a weak one. The arrow is itself a statement about dissociation.

When comparing $\text{p}K_a$ values, remember the scale runs backwards: smaller $\text{p}K_a$ = stronger acid. Sketch $\text{p}K_a = -\log K_a$ in the margin if you need the reminder.

For a conductivity question, talk about the total concentration of ions, not about acidity. A salt solution can out-conduct an acid.

Worked ExampleIdentifying solutions from their properties

Three colourless solutions, all $0.100$ mol L⁻¹, have lost their labels. They are $\text{HCl}$ , $\text{CH}_3\text{COOH}$ and $\text{CH}_3\text{COONa}$ . Their conductivities and pH values are given below.

Solution	Conductivity	pH
A	good	8.9
B	poor	2.9
C	good	1.0

Identify A, B and C, justifying each in terms of the species present.

Step 1 — Work out what each substance should do

HCl is a strong acid: completely dissociated, so $[\text{H}_3\text{O}^+] = 0.100$ mol L⁻¹ and there are many ions.
$\text{CH}_3\text{COOH}$ is a weak acid: only slightly dissociated, so $[\text{H}_3\text{O}^+] \ll 0.100$ mol L⁻¹ and there are few ions.
$\text{CH}_3\text{COONa}$ is a salt: it is fully ionic, so it has many ions, but the ethanoate ion is a weak base so the solution will be basic.

Step 2 — Use the pH to sort acidic from basic

Solution A has pH 8.9, which is above 7 — it is the only basic solution.

Only one of the three is basic: sodium ethanoate, because $\text{CH}_3\text{COO}^-$ accepts a proton from water:

$\text{CH}_3\text{COO}^- + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COOH} + \text{OH}^-$

Step 3 — Separate the two acids by conductivity and pH

B and C are both acidic, so they are the two acids. Both differ sharply in both properties.

Solution C: pH 1.0 and good conductivity. A pH of 1.0 means $[\text{H}_3\text{O}^+] = 1 \times 10^{-1} = 0.100$ mol L⁻¹ — exactly equal to the concentration of the acid.

That can only happen if the acid is completely dissociated, and the high ion concentration explains the good conductivity.

Solution B: pH 2.9 and poor conductivity. A pH of 2.9 means $[\text{H}_3\text{O}^+] \approx 1.3 \times 10^{-3}$ mol L⁻¹, which is far less than 0.100 — only about 1.3 % of the acid has dissociated.

The solution therefore contains mostly undissociated $\text{CH}_3\text{COOH}$ molecules and few ions, which is why it conducts poorly.

Step 4 — Check the reasoning holds together

	Identity	Why the conductivity	Why the pH
A	$\text{CH}_3\text{COONa}$	fully ionic salt → many ions	$\text{CH}_3\text{COO}^-$ is a weak base → basic
B	$\text{CH}_3\text{COOH}$	barely dissociated → few ions	weak acid → only mildly acidic
C	HCl	fully dissociated → many ions	strong acid → $[\text{H}_3\text{O}^+] = c$

Notice that A and C conduct equally well for completely different reasons — one is an ionic salt and the other a fully dissociated acid. Conductivity alone could not have separated them; it was the pH that did. Neither measurement was sufficient on its own.

Strong and weak acids and bases

Covers AS91392

Strength means how completely it dissociates

A strong acid is completely dissociated in water — essentially every molecule has given up its proton.
A weak acid is only partly dissociated — an equilibrium is set up, and most of the acid remains as undissociated molecules.

Strong acid, written with a single arrow:

$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$

Weak acid, written with equilibrium arrows:

$\text{CH}_3\text{COOH} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{CH}_3\text{COO}^-$

Strong is not the same as concentrated

This distinction is examined every year, and it is worth stating in your own words.

Strong / weak describes how completely the acid dissociates — a property of the substance itself.
Concentrated / dilute describes how much acid is dissolved — a property of the solution you made.
So a concentrated weak acid and a dilute strong acid are entirely different, and either could have the lower pH depending on the numbers.

Comparing them at the same concentration

	Strong acid	Weak acid
$[\text{H}_3\text{O}^+]$	high — equals the acid concentration	low — much less than the acid concentration
pH	low	higher
electrical conductivity	good	poor
rate of reaction with a metal or carbonate	fast	slower
volume of base needed to neutralise	the same	the same

That last row is the one people get wrong. Both contain the same amount of acid, so both need the same volume of base. Strength affects how much has dissociated at any moment, not how much acid is present in total.

Conductivity measures ions

Electrical conductivity depends on the total concentration of ions in the solution.
A strong acid is fully dissociated, so it has a high ion concentration and conducts well.
A weak acid is barely dissociated, so it has a low ion concentration and conducts poorly.
A soluble salt is fully ionic even though it may not be acidic at all, so it also conducts well.
Conductivity is not a measure of acidity. A solution of sodium ethanoate conducts far better than ethanoic acid of the same concentration, yet it is basic.

$K_a$ measures strength numerically

$K_a = \frac{[\text{H}_3\text{O}^+][\text{A}^-]}{[\text{HA}]}$

A larger $K_a$ means the equilibrium lies further right, so the acid is stronger.
$\text{p}K_a = -\log K_a$ , so a smaller $\text{p}K_a$ means a stronger acid.

Because $\text{p}K_a$ is a negative log, the direction reverses: HF with $\text{p}K_a = 3.17$ is a stronger acid than ethanoic acid with $\text{p}K_a = 4.76$ .

The conjugate relationship

The stronger the acid, the weaker its conjugate base, and vice versa.
$\text{HCl}$ is a strong acid, so $\text{Cl}^-$ is such a weak base that it does not react with water at all.
$\text{CH}_3\text{COOH}$ is a weak acid, so $\text{CH}_3\text{COO}^-$ is a reasonable base and does react with water.
This is why salt solutions can be acidic or basic, and it is the link between this page and the last.

Tips and Tricks

Never use "strong" to mean "concentrated". If a question contrasts two solutions, state explicitly which property you are talking about. Many marks turn on this single distinction.

Remember that a strong and a weak acid of the same concentration need the same volume of base to neutralise them. Only the initial pH, the conductivity and the rate differ.

Use the right arrow. Single arrow for a strong acid, equilibrium arrows for a weak one. The arrow is itself a statement about dissociation.

When comparing $\text{p}K_a$ values, remember the scale runs backwards: smaller $\text{p}K_a$ = stronger acid. Sketch $\text{p}K_a = -\log K_a$ in the margin if you need the reminder.

For a conductivity question, talk about the total concentration of ions, not about acidity. A salt solution can out-conduct an acid.

Worked ExampleIdentifying solutions from their properties

Solution	Conductivity	pH
A	good	8.9
B	poor	2.9
C	good	1.0

Identify A, B and C, justifying each in terms of the species present.

Step 1 — Work out what each substance should do

HCl is a strong acid: completely dissociated, so $[\text{H}_3\text{O}^+] = 0.100$ mol L⁻¹ and there are many ions.
$\text{CH}_3\text{COOH}$ is a weak acid: only slightly dissociated, so $[\text{H}_3\text{O}^+] \ll 0.100$ mol L⁻¹ and there are few ions.
$\text{CH}_3\text{COONa}$ is a salt: it is fully ionic, so it has many ions, but the ethanoate ion is a weak base so the solution will be basic.

Step 2 — Use the pH to sort acidic from basic

Solution A has pH 8.9, which is above 7 — it is the only basic solution.

Only one of the three is basic: sodium ethanoate, because $\text{CH}_3\text{COO}^-$ accepts a proton from water:

$\text{CH}_3\text{COO}^- + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COOH} + \text{OH}^-$

Step 3 — Separate the two acids by conductivity and pH

B and C are both acidic, so they are the two acids. Both differ sharply in both properties.

Solution C: pH 1.0 and good conductivity. A pH of 1.0 means $[\text{H}_3\text{O}^+] = 1 \times 10^{-1} = 0.100$ mol L⁻¹ — exactly equal to the concentration of the acid.

That can only happen if the acid is completely dissociated, and the high ion concentration explains the good conductivity.

The solution therefore contains mostly undissociated $\text{CH}_3\text{COOH}$ molecules and few ions, which is why it conducts poorly.

Step 4 — Check the reasoning holds together

	Identity	Why the conductivity	Why the pH
A	$\text{CH}_3\text{COONa}$	fully ionic salt → many ions	$\text{CH}_3\text{COO}^-$ is a weak base → basic
B	$\text{CH}_3\text{COOH}$	barely dissociated → few ions	weak acid → only mildly acidic
C	HCl	fully dissociated → many ions	strong acid → $[\text{H}_3\text{O}^+] = c$

Strong and weak acids and bases

Strength means how completely it dissociates

Strong is not the same as concentrated

Comparing them at the same concentration

Conductivity measures ions

KaK_aKa​ measures strength numerically

The conjugate relationship

Step 1 — Work out what each substance should do

Step 2 — Use the pH to sort acidic from basic

Step 3 — Separate the two acids by conductivity and pH

Step 4 — Check the reasoning holds together

Strong and weak acids and bases

Strength means how completely it dissociates

Strong is not the same as concentrated

Comparing them at the same concentration

Conductivity measures ions

KaK_aKa​ measures strength numerically

The conjugate relationship

Step 1 — Work out what each substance should do

Step 2 — Use the pH to sort acidic from basic

Step 3 — Separate the two acids by conductivity and pH

Step 4 — Check the reasoning holds together

$K_a$ measures strength numerically

$K_a$ measures strength numerically