The sample is acidic.

Acids cause methyl orange to turn red (colour change 3.1-4.4), and cause phenolphthalein to turn colourless (colour change 8.3-10.0).

There is a table of indicator colours in the data book.

The relationship between [OH⁻] and [H⁺] is given by the ionic product of water (which should be learned):

K_w = [OH⁻] × [H⁺] = 1 × 10⁻¹⁴ mol² dm⁻⁶

Therefore [H⁺] = K_w / [OH⁻] = 1 × 10⁻¹⁴ / 1.5 × 10⁻⁸ = 6.7 × 10⁻⁷

6.7 × 10⁻⁷ is therefore the correct answer.

Incorrect answers

1.5 × 10⁶ is the answer if the fraction is inverted ([OH⁻] / K_w)

−1.5 × 10⁻⁸ is the answer if an incorrect addition expression is used for K_w.

0.67 is the answer if the powers of 10 are omitted.

pH can be found by pH = −log₁₀[H⁺]

pH = −log₁₀ 1.7 × 10⁻³ = 2.854 = 2.9 (2sf)

2.9 is therefore the correct answer.

Incorrect answers

2.8 is a rounding error (fives round up by convention)

−2.9 is missing out the minus sign in the equation

−2.8 is missing out the minus sign in the equation and a rounding error (fives round up by convention)

The relationship between [OH⁻] and [H⁺] is given by the ionic product of water (which should be learned):

K_w = [OH⁻] × [H⁺] = 1 × 10⁻¹⁴ mol² dm⁻⁶

Therefore [H⁺] = K_w / [OH⁻] = 1 × 10⁻¹⁴ / 2.5 × 10⁻² = 4.00 × 10⁻¹³

pH can be found by pH = −log₁₀[H⁺]

pH = −log₁₀ 4.00 × 10⁻¹³ = 12.397 = 12.4 (3sf)

12.4 is therefore the correct answer.

(This can also be solved by using pH + pOH = 14, finding pOH first (1.60) and taking away from 14 to give 12.4)

Incorrect answers

1.60 is pOH −log₁₀[OH⁻]

Strong acids completely dissociate: HCl_(aq) → H⁺_(aq) + Cl⁻_(aq)

One mole of HCl gives one mole of H⁺ ions, so [HCl] = [H⁺]

pH can be found by pH = −log₁₀[H⁺]

pH = −log₁₀ 0.45 = 0.346 = 0.35 (2sf)

0.35 is therefore the correct answer.

Incorrect answers

0.34 is a rounding error

−0.35 is missing out the minus sign in the equation

−0.34 is missing out the minus sign in the equation and a rounding error

Strong alkalis completely dissociate: NaOH_(aq) → Na⁺_(aq) + OH⁻_(aq)

One mole of NaOH gives one mole of OH⁻ ions, so [NaOH] = [OH⁻]

The relationship between [OH⁻] and [H⁺] is given by the ionic product of water (which should be learned):

K_w = [OH⁻] × [H⁺] = 1 × 10⁻¹⁴ mol² dm⁻⁶

Therefore [H⁺] = K_w / [OH⁻] = 1 × 10⁻¹⁴ / 0.0800 = 1.25 × 10⁻¹³

pH can be found by pH = −log₁₀[H⁺]

pH = −log₁₀ 1.25 × 10⁻¹³ = 12.903... = 12.9 (3sf)

12.9 is therefore the correct answer.

(This can also be solved by using pH + pOH = 14, finding pOH first (1.10) and taking away from 14 to give 12.9)

Incorrect answers

1.10 is pOH −log₁₀[OH⁻]

Strong acids completely dissociate: H₂SO_4(aq) → 2H⁺_(aq) + SO₄⁻_(aq)

One mole of H₂SO₄ gives two moles of H⁺ ions, so 2×[H₂SO₄] = [H⁺] = 0.060

pH can be found by pH = −log₁₀[H⁺]

pH = −log₁₀ 0.060 = 1.2218... = 1.2 (2sf)

1.2 is therefore the correct answer.

Incorrect answers

1.5 is taking [H⁺] = 0.060

1.8 is taking [H⁺] = 0.015 (dividing conc. of acid by two instead of multiplying by two)

0.030 is the concentration of the acid

Strong acids and alkalis fully dissociate into ions in aqueous solution, weak acids and alkalis only partially dissociate. Strong acids and alkalis will react faster than weak acids and alkalis, and will have greater electrical conductivity.

Hydrochloric acid is a strong acid, so it does fully dissociate into ions and is a good conductor.

It is a strong acid of 1 mol dm⁻³ concentration so will have a pH of 0 (−log₁₀1 = 0) not a pH greater than 7.

Therefore 1 and 3 only is the correct answer.

Strong acids and alkalis fully dissociate into ions in aqueous solution, weak acids and alkalis only partially dissociate. Strong acids and alkalis will react faster than weak acids and alkalis, and will have greater electrical conductivity.

Ammonia solution is a weak alkali so it does not fully dissociate into ions.

It will have a pH greater than 7 as it is alkaline.

Dissociation should not be represented using → (as this suggests complete dissociation) but should be represented using ⇌ (an equilibrium sign). The equation is overwise correct however.

Therefore 2 only is the correct answer.

Strong acids and alkalis fully dissociate into ions in aqueous solution, weak acids and alkalis only partially dissociate. Strong acids and alkalis will react faster than weak acids and alkalis, and will have greater electrical conductivity.

Hydrochloric acid is a strong acid and ethanoic acid is a weak acid.

The pH of hydrochloric acid will be lower than that of ethanoic acid as at any moment in time there will be a higher concentration of H⁺ ions (as HCl fully dissociates).

The acids have equal concentration of the acid (but not of H+ ions in solution) and equal volumes, so the number of moles of the acids will be the same. (Moles = Concentration × Volume)

Therefore 1, 2 and 3 is the correct answer.

Rain water is naturally acidic due to dissolved carbon dioxide, but this only causes mild acidity (pH ≈ 5.6).

Acid deposition occurs when oxides of nitrogen and sulfur dissolve in rain water.

Therefore 1 and 2 only is the correct answer.

Acid deposition occurs when oxides of nitrogen and sulfur dissolve in rain water.

Acid deposition can be prevented by spraying waste gases with a limestone slurry (wet scrubbing in factory chimneys for example) or by passing waste gases through a catalytic converter (in cars for example).

Filtering will not remove gases.

Therefore 2 and 3 only is the correct answer.