new syllabus chemistry

New Syllabus Chemistry Chapter 7: Acid-Base Chemistry – Lahore Board Guide

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Explore Chapter 7 of the new Lahore Board Chemistry syllabus, dedicated to Acid-Base Chemistry. This comprehensive guide simplifies key concepts such as the Arrhenius, Bronsted–Lowry, and Lewis theories of acids and bases, pH scale, neutralization reactions, titration techniques, and industrial applications. Ideal for students preparing for exams, this resource includes easy explanations, solved examples, MCQs, short questions, and detailed practice problems aligned with the latest syllabus. Boost your understanding of acid-base reactions and excel in your exams with this in-depth content!

Question (i):

Which acid is not used as a food or mixed with food?
Options:
(a) Tartaric acid
(b) Ascorbic acid
(c) Citric acid
(d) Formic acid
Correct Answer: (d) Formic acid
Explanation: Tartaric acid, ascorbic acid, and citric acid are commonly used in food products as preservatives or flavoring agents, whereas formic acid is not used in food due to its toxic and corrosive properties.

Question (ii):

While baking, which gas is responsible for raising the bread and making it soft?
Options:
(a) Oxygen
(b) Carbon dioxide
(c) Nitrogen
(d) Carbon monoxide
Correct Answer: (b) Carbon dioxide
Explanation: Baking powder or yeast releases carbon dioxide gas during baking, which causes the dough to rise and become soft.

Question (iii):

Predict the main characteristics of the reactions of metals with acids.
Options:
(a) Metals are dissolved
(b) Metals are converted into salts
(c) Hydrogen gas is evolved
(d) All the above-mentioned characteristics are true
Correct Answer: (d) All the above-mentioned characteristics are true
Explanation: When metals react with acids, they dissolve to form metal salts, and hydrogen gas is released as a by-product.

Question (iv):

How many hydroxide ions will calcium hydroxide release in water?
Options:
(a) 1
(b) 2
(c) Zero
(d) 3
Correct Answer: (b) 2
Explanation: Calcium hydroxide (Ca(OH)2) dissociates in water to release two hydroxide ions (OH⁻) per formula unit.

Question (v):

In a neutralization reaction between KOH and H3PO4, how many molecules of KOH will react with one molecule of H3PO4?
Options:
(a) 2
(b) 1
(c) 3
(d) 4
Correct Answer: (c) 3
Explanation: H3PO4 is a triprotic acid and can donate three hydrogen ions, requiring three KOH molecules to fully neutralize it.

Question (vi):

Which acid is used in the preparation of soap?
Options:
(a) Tartaric acid
(b) Citric acid
(c) Stearic acid
(d) Oxalic acid
Correct Answer: (c) Stearic acid
Explanation: Stearic acid is used in soap manufacturing because it reacts with a base (e.g., NaOH) to form soap through saponification.

Question (vii):

Which compound is formed when SO2 is dissolved in water?
Options:
(a) SO3
(b) H2SO3
(c) H2SO4
(d) H2S2O7
Correct Answer: (b) H2SO3
Explanation: When SO2 (sulfur dioxide) dissolves in water, it forms sulfurous acid (H2SO3).

Question (viii):

Which of the following contains oxalic acid?
Options:
(a) Tomato
(b) Orange
(c) Tamarind
(d) Sour milk
Correct Answer: (a) Tomato
Explanation: Tomatoes contain oxalic acid naturally, which contributes to their slightly acidic taste.

Question (ix):

Which compound in the following reaction is behaving as a conjugate base?
CH3COOH+H2O⇋CH3COO−+H3O+
Options:
(a) CH3COOH
(b) H2O
(c) CH3COO−
(d) H3O+
Correct Answer: (c) CH3COO−
Explanation: In the reaction, CH3COOH donates a proton to water, forming CH3COO−, which is its conjugate base.

Question (x):

When a chemical reaction is carried out with a substance Z, a gas is produced which turns red litmus paper blue. What is the reaction?
Options:
(a) Reaction of an acid with a metal carbonate
(b) Reaction of an acid with a base
(c) Reaction of an acid with a metal oxide
(d) Reaction of an acid with ammonium salt
Correct Answer: (b) Reaction of an acid with a base
Explanation: The reaction between an acid and a base produces ammonia gas (NH3 or similar compounds, which turn red litmus blue due to their basic nature.

2. Questions for Short Answers

(i) Choose Arrhenius Acids among the following compounds:

HF, NH3, H2SO4, SO2, H2S, H2O
Answer: HF, H2SO4, H2S, and H2O.
Explanation: According to the Arrhenius concept, acids are substances that increase H+concentration in water. HF, H2SO4, and H2S release H+ ions when dissolved in water, making them Arrhenius acids. Water (H2O) can also act as a weak acid in certain reactions.

(ii) How does calcium metal react with dilute H2SO4?

Answer: Calcium reacts with dilute sulfuric acid to produce hydrogen gas and calcium sulfate:
Ca+H2SO4→CaSO4+H2
Explanation: Calcium is a reactive metal that displaces hydrogen from sulfuric acid, forming an ionic salt (CaSO4) and hydrogen gas.

(iii) Which salt is formed when HCl reacts with BaCO3?

Answer: Barium chloride (BaCl2) is formed.
Reaction:
BaCO3+2HCl→BaCl2+CO2+H2O
Explanation: Hydrochloric acid reacts with barium carbonate to form barium chloride, releasing carbon dioxide gas and water as by-products.

(iv) How will you justify that HSO4−is a Bronsted–Lowry acid?

Answer: HSO4−can donate a proton (H+) to form SO42−:
HSO4−⇋SO42−+H+
Explanation: According to the Bronsted–Lowry concept, acids are proton donors. HSO4– donates a proton in this reaction, making it a Bronsted–Lowry acid.

(v) What chemical name will you give to soap as a compound?

Answer: Soap is chemically called sodium stearate or potassium stearate, depending on the alkali used.
Explanation: Soaps are salts of fatty acids (like stearic acid) formed by the reaction of a base (e.g., NaOH) with a fat or oil in the process of saponification. For example:
C17H35COOH+NaOH→C17H35COONa+H2O
This reaction forms sodium stearate, which is the chemical name for soap.

4. Descriptive Questions

(i) Explain Arrhenius concept of acids and bases.

The Arrhenius concept defines acids and bases based on their behavior in water:

  • Acids: Substances that increase the concentration of hydrogen ions (H+) or protons in an aqueous solution. For example, HCl dissociates in water to produce H+ ions:
    HCl→H++Cl−
  • Bases: Substances that increase the concentration of hydroxide ions (OH−) in an aqueous solution. For example, NaOH dissociates in water to produce OH−:
    NaOH→Na++OH−

The Arrhenius concept is simple but limited to aqueous solutions. It does not explain the behavior of substances like NH3, which acts as a base but does not contain OH− ions.

(ii) Compare Arrhenius and Bronsted–Lowry concepts of acids and bases.

  • Arrhenius Concept:
    • Acids are substances that increase H+ ion concentration in water.
    • Bases are substances that increase OH− ion concentration in water.
    • Limited to aqueous solutions.
    • Example: HCl is an acid, and NaOH is a base.
  • Bronsted–Lowry Concept:
    • Acids are proton (H+) donors.
    • Bases are proton (H+) acceptors.
    • Not restricted to aqueous solutions.
    • Example: NH3 accepts a proton to form NH4+, acting as a base.

The Bronsted–Lowry concept is more general and can describe acid-base behavior in non-aqueous systems.

(iii) How does sulphuric acid react with the following compounds?

  1. NH4Cl:
    H2SO4 reacts with NH4Cl to release hydrogen chloride gas:
    NH4Cl+H2SO4→NH4HSO4+HCl
    This is an acid-base reaction where H2SO4 acts as a stronger acid.
  2. NH3:
    H2SO4 reacts with NH3 to form ammonium sulfate:
    2NH3+H2SO4→(NH4)2SO4
    The reaction neutralizes H2SO4, producing a salt.
  3. MgO:
    H2SO4 reacts with MgO to form magnesium sulfate and water:
    MgO+H2SO4→MgSO4+H2O
    This reaction shows that magnesium oxide acts as a basic oxide.
  4. MgCO3:
    H2SO4 reacts with MgCO3 to produce magnesium sulfate, carbon dioxide, and water:
    MgCO3+H2SO4→MgSO4+CO2+H2O
    This is a typical reaction of acids with carbonates.

(iv) What happens when a base reacts with a non-metallic oxide? What do you infer about the nature of non-metallic oxide?

When a base reacts with a non-metallic oxide, a salt and water are formed. For example:
2NaOH+CO2→Na2CO3+H2O
This reaction indicates that non-metallic oxides are acidic in nature, as they react with bases to form salts and water.

(v) State the reason for showing acidic character by both dry HCl gas and HCl solution in water.

  • Dry HCl Gas: Dry HCl does not ionize because there is no water to dissociate it into H+ and Cl−. It does not show acidic properties in the absence of water.
  • HCl in Water: In water, HCl ionizes completely to form H+ ions, which are responsible for its acidic character:
    HCl→H++Cl−

Thus, the presence of water is essential for HCl to exhibit acidic properties.

(vi) Differentiate between an acid and its conjugate base.

  • Acid: A substance that donates a proton (H+) in a chemical reaction.
  • Conjugate Base: The species that remains after the acid has donated its proton.

Example:
In the reaction:
CH3COOH⇋CH3COO−+H+

  • CH3COOH is the acid.
  • CH3COO− is its conjugate base.

Acids and their conjugate bases differ by one proton.

5. Investigative Questions

(i) Why is significant CO2 passing through limewater on this statement?

When CO2 gas is passed through limewater (Ca(OH)2, it initially reacts to form insoluble calcium carbonate (CaCO3), which turns the solution milky:
CO2+Ca(OH)2→CaCO3+H2O
If more CO2CO_2 is passed, the calcium carbonate reacts further to form soluble calcium bicarbonate, causing the milkiness to disappear:
CaCO3+CO2+H2O→Ca(HCO3)2
This demonstrates that the reaction depends on the concentration of CO2 and the solubility of the products.

(ii) What is observed when CO2 is passed through limewater for a short duration (a) and for a long duration (b)?

  • (a) Short Duration: Limewater turns milky due to the formation of insoluble calcium carbonate:
    CO2+Ca(OH)2→CaCO3+H2O
  • (b) Long Duration: The milkiness disappears because calcium carbonate reacts with excess CO2 and water to form soluble calcium bicarbonate:
    CaCO3+CO2+H2O→Ca(HCO3)2

This reaction is used as a test for the presence of CO2 gas.

Chapter 4 Stoichiometry SLO based solved Exercise 9th class new syllabus

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Get comprehensive solutions for Chapter 4 Stoichiometry from the 9th Class New Syllabus. This SLO-based solved exercise is designed to help students master the concepts of stoichiometry effectively.

(i) How many atoms are present in one gram of H2O?

1 mole of H2O = 18 g and contains 6.022×1023 molecules.
1 molecule of H2O = 3 atoms (2 H + 1 O).

Thus,
Atoms in 1 gram of H2O: 6.022×1023/18×3≈10.02×1022

Correct answer: (a) 10.02×1022 atoms

(ii) Which is the correct formula of calcium phosphide?

The formula of calcium phosphide is based on the charges of Ca2+ and P3−. To balance charges, the formula is Ca3P2

Correct answer: (c) Ca3P2

(iii) How many atomic mass units (amu) are there in one gram?

1 amu = 1.66×10−24g.

Number of amu in 1 g=1/1.66×10−24≈6.022×1023

Correct answer: (b) 6.022×1023

(iv) Structural formula of 2-hexene is CH3−CH=CH−(CH2)2−CH3. What is its empirical formula?

The molecular formula of 2-hexene is C6H12
Empirical formula is obtained by dividing subscripts by their greatest common divisor (6). Empirical formula=CH2

Correct answer: (b) CH2

(v) How many moles are there in 25 g of H2SO4?

Molar mass of H2SO4 = 2+32+(4×16)=98 g/mol

Moles=Mass/Molar mass=25/98≈0.255 moles.

Correct answer: (c) 0.255 moles

(vi) A necklace has 6 g of diamonds in it. What are the number of carbon atoms in it?

Diamonds are made of carbon.
Molar mass of carbon = 12 g/mol.
Number of moles of carbon: Moles=6/12=0.5 moles

Number of carbon atoms: 0.5×6.022×1023≈3.01×1023

Correct answer: (d) 3.01×1023

(vii) What is the mass of Al in 204 g of aluminum oxide (Al2O3)?

Molar mass of Al2O3 = 2(27)+3(16)=102 g/mol
Mass fraction of Al: 2(27)/102=54/102=0.5294

Mass of Al in 204 g of Al2O3: 0.5294×204≈108 g

Correct answer: (d) 108 g

(viii) Which one of the following compounds will have the highest percentage by mass of nitrogen?

  • For CO(NH2)2(urea):
    Molar mass = 12+16+2(14)+4(1)=60
    Nitrogen mass = 28
    Percentage of N = 28/60×100=46.67%
  • For NH4NO3:
    Molar mass = 14+4+14+3(16)=80
    Nitrogen mass = 28.
    Percentage of N = 28/80×100=35%

Correct answer: (a) CO(NH2)2

(ix) When one mole of each of the following compounds is reacted with oxygen, which will produce the maximum amount of CO2?

The number of moles of CO2 depends on the number of carbon atoms in the compound. Among the given options:

  • Methane (CH4): Produces 1 mole of CO2.
  • Ethane (C2H6): Produces 2 moles of CO2.
  • Diamond (C): Produces 1 mole of CO2

Correct answer: (b) Ethane

(x) What mass of 95% Ca(OH)2 will be required to neutralize 50 cm3 of 0.5 M H2SO4?

  • Reaction: Ca(OH)2+H2SO4→CaSO4+2H2O
    1 mole of H2SO4 reacts with 1 mole of Ca(OH)2.
    Moles of H2SO4: 0.5×0.05=0.025 moles
    Mass of pure Ca(OH)2: 0.025×(40+2(16+1))=0.025×74=1.85 g.
  • Mass of 95% pure Ca(OH)2: 1.850.95≈1.95 g

Correct answer: (d) 1.95 g

2. Questions for Short Answers

i. Write down the chemical formula of barium nitride.
The chemical formula of barium nitride is Ba₃N₂.
Explanation: Barium (Ba) is an alkaline earth metal with a +2 oxidation state, and nitride (N) is a nonmetal with a -3 charge. To balance the charges, three barium ions combine with two nitride ions, forming Ba₃N₂.

ii. Find out the molecular formula of a compound whose empirical formula is CH₂O and its molar mass is 180.
Step 1: Calculate the molar mass of the empirical formula (CH₂O).
Molar mass of CH₂O = 12 (C) + 2(1) (H) + 16 (O) = 30 g/mol.

Step 2: Determine the ratio of the molecular formula to the empirical formula.
Ratio=Molar mass of compound / Molar mass of empirical formula=180/30=6.

Step 3: Multiply the empirical formula by the ratio.
Molecular formula = (CH2O)6=C6H12O6 = C₆H₁₂O₆.
The molecular formula of the compound is C₆H₁₂O₆ (glucose).

iii. How many molecules are present in 1.5 g H₂O?
Step 1: Calculate the molar mass of water (H₂O).
Molar mass = 2(1) + 16 = 18 g/mol.

Step 2: Find the number of moles in 1.5 g of H₂O.
Moles = Mass/Molar mass=1.5/18=0.0833 mol

Step 3: Calculate the number of molecules using Avogadro’s number (6.022×1023 molecules/mol)
Number of molecules = 0.0833×6.022×1023=5.02×1022
Thus, there are approximately 5.02×1022

iv. What is the difference between a mole and Avogadro’s number?

  • Mole: A mole is a unit in chemistry that represents a quantity of substance. One mole of any substance contains the same number of particles (atoms, molecules, or ions) as there are in 12 g of carbon-12.
  • Avogadro’s Number: Avogadro’s number is a constant that defines the number of particles in one mole of a substance. It is equal to 6.022×1023 particles/mol

Difference: The mole is a quantity (like a dozen), while Avogadro’s number provides the specific count of particles in one mole.

v. Write down the chemical equation for the given reaction.
Copper + Sulphuric acid → Copper sulphate + Sulphur dioxide + Water
Balanced chemical equation:
Cu+2H₂SO₄→CuSO₄+SO₂+2H₂O

3. Constructed Response Questions

i. Different compounds will never have the same molecular formula but can have the same empirical formula. Explain.

  • The empirical formula represents the simplest whole-number ratio of elements in a compound, whereas the molecular formula shows the exact number of atoms of each element in a molecule.
  • Example: Acetic acid (C₂H₄O₂) and glucose (C₆H₁₂O₆) have the same empirical formula (CH₂O), but their molecular formulas are different.
  • Explanation: Compounds with different molecular structures can share the same empirical formula because the molecular formula is a multiple of the empirical formula.

ii. Write down the chemical formulas of the following compounds.

  1. Calcium phosphate: Ca₃(PO₄)₂
  2. Aluminium nitride: AlN
  3. Sodium acetate: CH₃COONa
  4. Ammonium carbonate: (NH₄)₂CO₃
  5. Bismuth sulphate: Bi₂(SO₄)₃

iii. Why does Avogadro’s number have immense importance in chemistry?
Avogadro’s number (6.022×1023 is essential in chemistry because:

  • It establishes a connection between the microscopic (atoms/molecules) and macroscopic (grams/moles) scales.
  • It allows chemists to calculate the number of particles in a given mass of substance.
  • It is crucial for determining stoichiometric relationships in chemical reactions, enabling precise calculations for reactants and products.

iv. When 8.657 g of a compound were converted into elements, it gave 5.217 g of carbon, 0.962 g of hydrogen, and 2.478 g of oxygen. Calculate the percentage of each element present in this compound.
Step 1: Calculate the total mass of the compound.
Total mass = 5.217+0.962+2.478=8.657 g

Step 2: Calculate the percentage of each element.

  • Carbon: 5.2178.657×100=60.3%
  • Hydrogen: 0.9628.657×100=11.1%
  • Oxygen: 2.4788.657×100=28.6%

Thus, the percentages are:

  • Carbon: 60.3%
  • Hydrogen: 11.1%
  • Oxygen: 28.6%.

4. Descriptive Questions

i. Which conditions must be fulfilled before writing a chemical equation for a reaction?
To write a chemical equation, the following conditions must be fulfilled:

  1. Correct identification of reactants and products: Ensure the substances involved in the reaction are accurately identified.
  2. Balancing the equation: The equation must follow the law of conservation of mass, where the number of atoms for each element is the same on both sides.
  3. Physical states of substances: Indicate the physical states of reactants and products (solid (s), liquid (l), gas (g), or aqueous (aq)).
  4. Reaction conditions: Specify the conditions such as temperature, pressure, or the use of catalysts if required for the reaction.

ii. Explain the concepts of Avogadro’s number and mole.

  • Avogadro’s Number: Avogadro’s number (6.022×1023 is the number of particles (atoms, molecules, or ions) in one mole of a substance. It bridges the gap between the microscopic scale and the macroscopic scale.
  • Mole: A mole is a unit of measurement used in chemistry to represent 6.022×1023 particles of a substance. It is the amount of a substance that contains as many entities as there are atoms in 12 grams of carbon-12.

iii. How many grams of CO₂ will be produced when we react 10 g of CH₄ with excess of O₂ according to the following equation?
CH4+2O2→CO2+2H2OCH₄ + 2O₂ → CO₂ + 2H₂O

  1. Molar masses:
    • CH₄ = 16 g/mol
    • CO₂ = 44 g/mol
  2. Moles of CH₄:
    Moles = Mass/Molar mass=10/16=0.625 mol
  3. Moles of CO₂:
    From the equation, 1 mole of CH₄ produces 1 mole of CO₂.
    Moles of CO₂ = 0.625 mol.
  4. Mass of CO₂:
    Mass = Moles × Molar mass = 0.625×44=27.5 g

Answer: 27.5 g of CO₂ will be produced.

iv. How many moles of coal are needed to produce 10 moles of CO according to the following equation?
3C+O2+H2O→H2+3CO3C + O₂ + H₂O → H₂ + 3CO

From the equation, 3 moles of carbon (C) produce 3 moles of CO.
Thus, to produce 10 moles of CO: Moles of coal=103=3.33 moles.\text{Moles of coal} = \frac{10}{3} = 3.33 \, \text{moles}.

Answer: 3.33 moles of coal are needed.

v. How much SO₂ is needed in grams to produce 10 moles of sulphur?
2H2S+SO2→2H2O+3S

From the equation, 1 mole of SO₂ produces 1.5 moles of sulphur.
To produce 10 moles of sulphur: Moles of SO₂=10/1.5=6.67 moles

Molar mass of SO₂ = 32 (S) + 2(16) = 64 g/mol.
Mass = Moles × Molar mass = 6.67×64=426.88 g

Answer: 426.88 g of SO₂ are needed.

vi. How much ammonia is needed in grams to produce 1 kg of urea fertilizer?
2NH3+CO2→CO(NH2)2+H2O

  1. Molar masses:
    • NH₃ = 17 g/mol
    • CO(NH₂)₂ (urea) = 60 g/mol
  2. Moles of urea:
    Moles = Mass/Molar mass=1000/60=16.67 mol
  3. Moles of NH₃:
    From the equation, 2 moles of NH₃ produce 1 mole of urea.
    Moles of NH₃ = 16.67×2=33.34 mol.
  4. Mass of NH₃:
    Mass = Moles × Molar mass = 33.34×17=566.78 g

Answer: 566.78 g of ammonia is needed.

vii. Calculate the number of atoms in the following:

(a) 3 g of H₂:
Moles of H₂ = Mass/Molar mass=3/2=1.5 mol
Number of molecules = 1.5×6.022×1023=9.033×1023
Each molecule of H₂ contains 2 atoms, so:
Number of atoms = 9.033×1023×2=1.8066×1024
Number of molecules = 3.4×6.022×1023=2.0475×1024
Each molecule of N₂ contains 2 atoms, so:
Number of atoms = 2.0475×1024×2=4.095×1024
Molar mass of C₆H₁₂O₆ = 6(12)+12(1)+6(16)=180 g/mol
Number of molecules = 0.0556×6.022×1023=3.348×1022
Each molecule contains 6+12+6=246 + 12 + 6 = 24 atoms.
Number of atoms = 3.348×1022×24=8.035×1023

5. Investigative Questions

i. How many moles of water are needed for a single adult?
Volume of water per glass = 400 cm3
Density of water = 1 g/cm3
Mass of water = 400×8=3200 g

Molar mass of water = 18 g/mol.
Moles of water = Mass/Molar mass=3200/18=177.78 mol

Answer: 177.78 moles of water are needed.

ii. How many moles of SiO₂ are present in the glass?
Assuming the mass of glass is provided, moles = Mass of glass/Molar mass of SiO₂

where Molar mass of SiO₂=28+2(16)=60 g/mol

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