Hydrogen chloride, $$$\mathrm{HCl} $$$, is an acidic gas. Hydrogen chloride dissolves in water to form a solution of hydrochloric acid. A student titrated aqueous ammonia with hydrochloric acid using the apparatus shown below. Describe how the $$$\mathrm{pH}$$$ value of the solution in B changes as hydrochloric acid is added until the acid is in excess.

Chemistry

IGCSE&ALevel

CAIE

Exam No：0620_w12_qp_21 Year：2012 Question No：2(b)(ii)

Answer:

pH starts above 7 / stated value above 7;
allow: high $\mathrm{pH}$

decreases (on addition of acid);

$(\mathrm{pH})$ ends at below $7 /$ stated value below 7 ;
allow: low $\mathrm{pH}$
note: $\mathrm{pH}$ decreases to $\mathrm{pH} 7=2$ marks
note: $\mathrm{pH}$ goes from alkali to acid $=1$ mark

Knowledge points:

7.1.1.1. metals

7.1.1.2. bases

7.1.1.3. carbonates

7.1.10. Define a strong acid as an acid that is completely dissociated in aqueous solution and a weak acid as an acid that is partially dissociated in aqueous solution

7.1.11. State that hydrochloric acid is a strong acid, as shown by the symbol equation,

7.1.12. State that ethanoic acid is a weak acid, as shown by the symbol equation

7.1.2.1. litmus

7.1.2.2. thymolphthalein

7.1.2.3. methyl orange

7.1.3. State that bases are oxides or hydroxides of metals and that alkalis are soluble bases

7.1.4.1. acids

7.1.4.2. ammonium salts

7.1.5.1. litmus

7.1.5.2. thymolphthalein

7.1.5.3. methyl orange

7.1.6. State that aqueous solutions of acids contain

7.1.7. Describe how to compare hydrogen ion concentration, neutrality, relative acidity and relative alkalinity in terms of colour and pH using universal indicator paper

7.1.8. Describe the neutralisation reaction between an acid and an alkali to produce water

7.1.9. Define acids as proton donors and bases as proton acceptors

7.3.1.1. an alkali by titration

7.3.1.2. excess metal

7.3.1.3. excess insoluble base

7.3.1.4. excess insoluble carbonate

7.3.2.1. sodium, potassium and ammonium salts are soluble

7.3.2.2. nitrates are soluble

7.3.2.3. chlorides are soluble, except lead and silver

7.3.2.4. sulfates are soluble, except barium, calcium and lead

7.3.2.5. carbonates are insoluble, except sodium, potassium and ammonium

7.3.2.6. hydroxides are insoluble, except sodium, potassium, ammonium and calcium (partially)

7.3.3. Define a hydrated substance as a substance that is chemically combined with water and an anhydrous substance as a substance containing no water

7.3.4. Describe the preparation of insoluble salts by precipitation

7.3.5. Define the term water of crystallisation as the water molecules present in hydrated crystals

Answer:

Knowledge points:

Solution:

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