Chemistry Self-Ionisation and pH Scale

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Questions and Answers

What occurs during the self-ionisation of water?

Only OH- ions are formed from water molecules.

Water molecules combine to form only H+ ions.

H2O molecules dissociate into H+ and OH- ions. (correct)

Water remains unchanged with no ions produced.

What is the value of K_w at 25°C?

1 × 10^-7

1 × 10^-1

1 × 10^-14 (correct)

1 × 10^0

How is the pH of a solution related to its hydrogen ion concentration?

pH = -log10[H+] (correct)

pH = log10[H+]

pH = 1/[H+]

pH = -2 log10[H+]

What does a pH value of 6 generally indicate about a solution?

The solution is acidic. Signup and view all the answers

If the hydrogen ion concentration is 7.1 x 10^-7 mol/L, what is the pH of the solution?

6.80 Signup and view all the answers

What is the hydrogen ion concentration of a solution with a pH of 3.7?

1.995 x 10^-4^ mol/L Signup and view all the answers

Which of the following statements is true regarding strong acids?

Strong acids dissociate completely in a solution. Signup and view all the answers

How is the strength of a weak acid represented?

By a small value of Ka. Signup and view all the answers

Which of the following describes a weak base?

Poor proton acceptor. Signup and view all the answers

What characterizes the conjugate base of a strong acid?

It is a weak proton donor. Signup and view all the answers

What happens when OH^- is added to a system containing the weak base indicator XOH?

The reverse reaction is favored, causing color 1 to be seen. Signup and view all the answers

Which indicator changes from red to yellow in the pH range of 3 to 5?

Methyl Orange Signup and view all the answers

During a pH titration, what role does the magnetic stir bar serve?

To ensure the acid is well mixed with the base. Signup and view all the answers

What is the primary purpose of the pH sensor in a pH titration experiment?

To record and measure pH changes throughout the titration. Signup and view all the answers

What is the definition of the range of an indicator?

The pH interval where the indicator displays a distinct color change. Signup and view all the answers

What is the pH of a 0.04 M H2SO4 solution?

0.097 Signup and view all the answers

What is the pOH of a 0.15 M NaOH solution?

0.82 Signup and view all the answers

What assumption is made when calculating the pH of a weak acid at low concentrations?

Since K is very low, 0.1 - x is approximately 0.1. Signup and view all the answers

How is the pH of a weak acid calculated using its dissociation constant?

pH = -log10[sqrt(Ka x Macid)] Signup and view all the answers

What does an acid-base indicator do according to a solution's pH?

It changes color in response to the pH level. Signup and view all the answers

What is the calculated pH of a 0.1 M solution of ethanoic acid given a K_a of 1.8 x 10^-5?

2.9 Signup and view all the answers

Which of the following is NOT a limitation of the pH scale?

It works well at very low concentrations. Signup and view all the answers

What is the pH of a 0.1 M solution of methanoic acid with K_a = 2.1 x 10^-4?

2.34 Signup and view all the answers

What occurs at the endpoint of a titration of a strong acid against a strong base?

There is a large jump in pH. Signup and view all the answers

Which indicator is suitable for titrating a weak acid against a strong base?

Phenolphthalein Signup and view all the answers

In water, how do strong acids behave compared to weak acids according to the Arrhenius theory?

Strong acids fully dissociate, weak acids partially dissociate. Signup and view all the answers

What happens when a weak acid is titrated against a weak base?

There is a gradual rise in pH without a sudden change. Signup and view all the answers

What color is observed when a few drops of indicator solution are added to a 0.5 M NaOH solution?

Purple Signup and view all the answers

Which of the following statements about indicators used in titrations is correct?

Indicators change color at specific pH ranges. Signup and view all the answers

What is a conjugate pair according to Brønsted-Lowry theory?

An acid and a base that differ by one proton. Signup and view all the answers

Study Notes

Self-Ionisation of Water

Water conducts electricity when ions are present
Even pure water has a small current due to self-ionisation
Self-ionisation equation: H₂O ⇌ H⁺ + OH⁻
Concentration of H⁺ and OH⁻ in pure water is very small
Equilibrium strongly favours the left side of the equation
Concentration of H₂O is constant, so can be factored out of the equilibrium constant
Equilibrium Constant: K = [H⁺][OH⁻]/[H₂O]
Kw (Ionic Product of Water): K [H₂O] = [H⁺][OH⁻]
At 25°C, Kw = 1 x 10⁻¹⁴
[H⁺]= [OH⁻] = 1 x 10⁻⁷

Measurement of Acidity - The pH Scale

pH = -log₁₀[H⁺]
Square brackets indicate concentration in moles per litre
pH of a solution is the negative base-10 logarithm of the hydrogen ion concentration.
pH scale ranges from 0 to 14
0-6 acidic
7 neutral
8-14 alkaline/basic

Strengths of Acids and Bases

Strong Acid: Good proton donor (e.g., HCl, H₂SO₄)
Weak Acid: Poor proton donor (e.g., Ethanoic acid)
Strong Base: Good proton acceptor (e.g., NaOH, KOH)
Weak Base: Poor proton acceptor (e.g., NH₃)
Ka: Acid dissociation constant
Kb: Base dissociation constant

Calculating the pH of Strong Acids and Strong Bases

Strong acids and bases are assumed to be fully dissociated in water
Calculate pH using [H⁺] or [OH⁻] and the known concentration
Example: For a 0.04 M H₂SO₄ solution, calculate pH from [H⁺]

Calculating the pH of Weak Acids and Weak Bases

For weak acids or bases, dissociation constants (Ka or Kb) must be known
Example: To calculate the pH of a 0.1 M ethanoic acid solution, using Ka.

Acid-Base Indicators

Definition: A substance that changes colour according to the pH of the solution
Indicator as a weak acid: Hln ⇌ H⁺ + In⁻ (Colour 1 ⇌ Colour 2) -Adding H⁺(acid), shifts reaction to the left (Colour 1 will be seen) -Adding OH⁻(base), removes H⁺, shifting reaction to the right (Colour 2 will be seen)
Indicator as a weak base: Xon ⇌ X⁺ + on⁻
Definition: Range of an indicator is the pH interval with a clear change in colour.
Examples: Methyl Orange (3-5), Litmus (5-8), Phenolphthalein (8-10)

pH Titration

Experiment to track pH changes during titration
Base is placed in a burette and acid in a beaker.
A pH sensor measures changes in pH
Observations are recorded in a graph, a pH titration curve
Example: Strong acid (e.g., HCl) against a strong base (e.g., NaOH) solution
Example: Strong acid against a weak base solution
Example: Weak acid against a strong base solution
Example: Weak acid against a weak base solution

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Description

Explore the concepts of self-ionisation of water and the pH scale in this quiz. Understand how water conducts electricity, the equilibrium constant, and the strengths of acids and bases. Test your knowledge on the principles that govern acidity and basicity.