Prepare for PTB exams with solved exercises on Acids, Bases, and Salts. Understand key concepts like pH, acid-base reactions, and salt formation through detailed solutions and explanations.
1. Name three common household substances having:
a. pH value greater than 7:
- Baking soda (sodium bicarbonate)
- Soap
- Bleach b. pH value less than 7:
- Vinegar (acetic acid)
- Lemon juice
- Milk c. pH value equal to 7:
- Distilled water
2. Define a base and explain that all alkalis are bases, but all bases are not alkalis.
A base is a substance that can accept protons (H⁺ ions) or donate electron pairs. Alkalis are a subset of bases that are soluble in water and release hydroxide ions (OH⁻) in solution. Thus, while all alkalis are bases, not all bases dissolve in water to form an alkaline solution (e.g., ammonia is a base but not an alkali because it is weak and does not fully dissolve in water).
3. Define Bronsted-Lowry base and explain with an example that water is a Bronsted-Lowry base.
A Bronsted-Lowry base is a substance that can accept protons (H⁺ ions) during a chemical reaction. Water can act as a Bronsted-Lowry base because it accepts a proton from an acid.
For example, in the reaction between hydrochloric acid (HCl) and water:
HCl + H2O → H3O^+ + Cl^-
Water accepts a proton from HCl and forms the hydronium ion (H₃O⁺).
4. How can you justify that Bronsted-Lowry concept of acid and base is applicable to non-aqueous solutions?
The Bronsted-Lowry concept is based on proton transfer and does not require water as a solvent. For instance, in liquid ammonia (a non-aqueous solvent), ammonium ions (NH₄⁺) act as acids, and ammonia (NH₃) acts as a base according to the Bronsted-Lowry theory:
5. Which kind of bond is formed between Lewis acid and a base?
A coordinate covalent bond (or dative bond) is formed when a Lewis base donates a pair of electrons to a Lewis acid.
6. Why H⁺ ion acts as a Lewis acid?
The H⁺ ion acts as a Lewis acid because it lacks electrons and can accept a pair of electrons from a Lewis base to form a bond.
7. Name two acids used in the manufacture of fertilizers.
- Sulfuric acid (H₂SO₄)
- Nitric acid (HNO₃)
8. Define pH. What is the pH of pure water?
pH is the negative logarithm of the hydrogen ion concentration ([H^+]) in a solution, representing its acidity or alkalinity. The pH of pure water is 7, which is neutral.
9. How many times a solution of pH 1 will be stronger than that of a solution having pH 2?
A solution with a pH of 1 is 10 times more acidic than a solution with a pH of 2 because each pH unit represents a tenfold difference in hydrogen ion concentration.
10. Define the followings:
i. Normal salt: A salt formed by the complete neutralization of an acid by a base, e.g., sodium chloride (NaCl).
ii. Basic salt: A salt that contains hydroxide ions (OH⁻) or can produce them in water, e.g., sodium bicarbonate (NaHCO₃).
11. Na₂SO₄ is a neutral salt while NaHSO₄ is an acid salt. Justify.
Sodium sulfate (Na₂SO₄) is a neutral salt because it is formed from a strong acid (H₂SO₄) and a strong base (NaOH), which neutralize each other completely. Sodium bisulfate (NaHSO₄), on the other hand, is an acidic salt because it still contains an acidic hydrogen (HSO₄⁻) that can donate protons in solution.
12. Give a few characteristic properties of salts.
- Salts are ionic compounds.
- They are usually solid and crystalline at room temperature.
- Many salts are soluble in water, producing ions.
- Salts conduct electricity when dissolved in water or molten.
- They have high melting and boiling points.
13. How are the soluble salts recovered from water?
Soluble salts can be recovered from water through processes such as evaporation or crystallization, where water is evaporated, leaving the dissolved salt behind.
14. How are the insoluble salts prepared?
Insoluble salts can be prepared by a precipitation reaction, where two aqueous solutions containing ions react to form a solid salt.
For example:
AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
15. Why is a salt neutral? Explain with an example.
A salt is neutral when it is formed from the reaction between a strong acid and a strong base, resulting in a neutral pH solution.
Example: Sodium chloride (NaCl) is neutral because it is formed from hydrochloric acid (HCl) and sodium hydroxide (NaOH), both strong and completely dissociating in water.
16. Name an acid used in the preservation of food.
Acetic acid (CH₃COOH), found in vinegar, is commonly used in the preservation of food.
17. Name the acids present in:
i. Vinegar: Acetic acid (CH₃COOH)
ii. Ant sting: Formic acid (HCOOH)
iii. Citrus fruit: Citric acid (C₆H₈O₇)
iv. Sour milk: Lactic acid (C₃H₆O₃)
18. How can you justify that Pb(OH)NO₃ is a basic salt?
Lead hydroxynitrate (Pb(OH)NO₃) is considered a basic salt because it contains hydroxide ions (OH⁻) in its structure, which makes it capable of neutralizing acids.
19. You are in need of an acidic salt. How can you prepare it?
An acidic salt can be prepared by the partial neutralization of a polybasic acid with a base. For example, by reacting sulfuric acid (H₂SO₄) with sodium hydroxide (NaOH) in a 1:1 molar ratio, you get sodium bisulfate (NaHSO₄), an acidic salt.
20. Which salt is used to prepare plaster of Paris?
Plaster of Paris is prepared from calcium sulfate hemihydrate (CaSO₄·0.5H₂O).
Extensive Questions
1. Define an acid and a base according to Bronsted-Lowry concept and justify with examples that water is an amphoteric compound.
- According to the Bronsted-Lowry concept, an acid is a substance that donates protons (H⁺ ions), while a base is a substance that accepts protons.
- Water (H₂O) is an amphoteric compound, meaning it can act both as an acid and as a base depending on the situation. For example:
- As a base: When reacting with hydrochloric acid (HCl):
HCl + H_2O → H3O^+ + Cl^- - As an acid: When reacting with ammonia (NH₃):
NH3 + H_2O → NH4^+ + OH^-
In both cases, water accepts or donates a proton, demonstrating its amphoteric nature.
2. Explain the Lewis concept of acids and bases.
- According to the Lewis concept, an acid is a substance that accepts an electron pair, while a base is a substance that donates an electron pair.
- Lewis acid: Example – Aluminum chloride (AlCl₃) accepts an electron pair from chloride ions.
- Lewis base: Example – Ammonia (NH₃) donates an electron pair to form a bond with hydrogen ions (H⁺).
3. What is auto-ionization of water? How is it used to establish the pH of water?
- Auto-ionization of water refers to the process where water molecules react with each other to produce hydronium ions (H₃O⁺) and hydroxide ions (OH⁻):
2H2O(l) ⇌ H3O^+(aq) + OH^-(aq) - The equilibrium constant for this reaction is called the ionization constant of water (Kw), and at 25°C, Kw = (1.0 \times 10^{-14}). The pH of water is calculated from the concentration of ( H3O^+ ) ions, which is 1.0 × 10⁻⁷ M, giving water a pH of 7 (neutral).
4. Define a salt and give the characteristic properties of salts.
- A salt is an ionic compound formed when an acid reacts with a base, usually through neutralization. It is made up of positive ions (cations) from the base and negative ions (anions) from the acid.
- Properties:
- They are typically solid and crystalline.
- Many salts are soluble in water.
- Salts have high melting and boiling points.
- In solution, salts can conduct electricity due to ion dissociation.
5. Explain with examples how are soluble salts prepared?
- Soluble salts are typically prepared by reacting an acid with a base or by combining a metal with an acid.
- Neutralization reaction: Hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to form sodium chloride (NaCl), a soluble salt.
HCl + NaOH → NaCl + H_2O - Reaction of metal with acid: Zinc reacts with sulfuric acid to form zinc sulfate (ZnSO₄), which is soluble in water.
Zn + H2SO4 → ZnSO4 + H2
6. Give the characteristics of an acidic salt.
- Acidic salts are formed by the partial neutralization of a polybasic acid with a base. These salts contain replaceable hydrogen ions (H⁺).
- They react with bases to form normal salts.
- They can release protons (H⁺) in solution, making the solution acidic.
- Example: Sodium bisulfate (NaHSO₄) is an acidic salt because it can release H⁺ ions in solution.
7. Give four uses of calcium oxide.
- Calcium oxide (quicklime) is used in various industries:
- Construction: It is used to make cement and mortar.
- Water treatment: It is used to remove impurities from water.
- Steel manufacturing: Used in the purification of steel.
- Soil treatment: It is used to neutralize acidic soils.
8. You are having a strong acid (HNO₃) and strong base (NaOH) on mixing:
i. What type of salt will you have?
- The reaction between nitric acid (HNO₃) and sodium hydroxide (NaOH) will produce sodium nitrate (NaNO₃), which is a neutral salt. ii. What type of reaction will it be?
- This is a neutralization reaction, where an acid and a base react to form water and a salt. iii. Will it be soluble or insoluble salt?
- Sodium nitrate (NaNO₃) is a soluble salt. iv. If it is soluble, how will it be recovered?
- Sodium nitrate can be recovered by evaporation of the water from the solution, leaving behind solid sodium nitrate crystals.
9. Explain why:
i. HCl forms only one series of salts.
- Hydrochloric acid (HCl) is a monoprotic acid, meaning it has only one replaceable hydrogen ion, so it can only form one series of salts, like sodium chloride (NaCl). ii. H₂SO₄ forms two series of salts.
- Sulfuric acid (H₂SO₄) is a diprotic acid with two replaceable hydrogen ions. It forms two types of salts:
- Normal salts, e.g., sodium sulfate (Na₂SO₄), where both hydrogen ions are replaced.
- Acidic salts, e.g., sodium bisulfate (NaHSO₄), where only one hydrogen ion is replaced.
- Phosphoric acid (H₃PO₄) is a triprotic acid, with three replaceable hydrogen ions, so it forms three types of salts:
- Primary salts: NaH₂PO₄
- Secondary salts: Na₂HPO₄
- Tertiary salts: Na₃PO₄
10. Classify the following salts as soluble or insoluble salts:
i. Sodium chloride: Soluble
ii. Lead chloride: Insoluble
iii. Barium sulphate: Insoluble
iv. Sodium carbonate: Soluble
v. Silver nitrate: Soluble
vi. Copper sulphate: Soluble
vii. Ammonium chloride: Soluble
viii. Calcium carbonate: Insoluble
ix. Calcium chloride: Soluble
x. Magnesium sulphate: Soluble
11. Complete and balance the following equations:
i. Aluminium + Hydrochloric acid
2Al + 6HCl → 2AlCl3 + 3H2
ii. Copper oxide + Sulphuric acid
CuO + H_2SO_4 → CuSO_4 + H_2O
iii. Iron sulphide + Sulphuric acid
FeS + H2SO4 → FeSO4 + H2S
iv. Ammonium chloride + Sodium hydroxide
NH4Cl + NaOH → NH3 + NaCl + H_2O
v. Ferric chloride + Sodium hydroxide
FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl
