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Chapter 4 (Stoichiometry), the most important short questions are very helpful for exam preparation.

More notes are available here

Question 1: Define stoichiometry. What is basis of stoichiometry?
Answer: Stoichiometry is the branch of chemistry which deals with the calculations based on balanced chemical equations. OR
The branch of chemistry which deals with the quantitative relationship between reactants and products in a balance chemical equation. It is based on the law of conservation of mass, which states that matter can neither be created nor destroyed.

Question 2: Define the chemical formula.
Answer: The representation of an element or a compound in terms of symbols is called a chemical formula. For example, the chemical formula of water is H₂O and aluminium sulphate is Al₂(SO₄)₃.

Question 3: Define formula unit.
Answer: The simplest whole number ratio between ions in an ionic compound is called a formula unit. For example, sodium chloride is represented by the formula unit NaCl.

Question 4: Name the elements represented by the following symbols: Hg, Au, Fe, Ni, Co, W, Sn, Na, Ba, Br, Bi.
Answer: Hg (Mercury), Au (Gold), Fe (Iron), Ni (Nickel), Co (Cobalt), W (Tungsten), Sn (Tin), Na (Sodium), Ba (Barium), Br (Bromine), Bi (Bismuth).

Question 5: What elements do the following compounds contain? Sugar, common salt, lime water and chalk.
Answer: Sugar (Carbon, Hydrogen, Oxygen), Common Salt (Sodium, Chlorine), Lime water (Calcium, Oxygen, Hydrogen), and Chalk (Calcium, Carbon, Oxygen).

Question 6: Differentiate between empirical formula and formula unit?
Answer: Empirical formula is the simplest whole number ratio of atoms of different elements in a compound (e.g., CH₂O for glucose), and it applies to both covalent and ionic compounds. A formula unit is specifically the simplest whole number ratio of ions in an ionic compound (e.g., NaCl), and applies only to ionic compounds.

Question 7: How can you differentiate between molecular formula and empirical formula?
Answer: The empirical formula shows the simplest whole number ratio of atoms in a compound, while the molecular formula shows the actual number of atoms of each element present in a molecule. Molecular formula is derived as: Molecular formula = n × empirical formula.

Question 8: Identify the following formulae as formula unit or molecular formulae: H₂O₂, CH₄, C₆H₁₂O₆, C₁₂H₂₂O₁₁, BaCO₃, KBr.
Answer: Molecular Formulae: H₂O₂, CH₄, C₆H₁₂O₆, C₁₂H₂₂O₁₁. Formula Units: BaCO₃, KBr.

Question 9: What is the empirical formula of acetic acid (CH₃COOH)? Find out its molecular mass.
Answer: The empirical formula of acetic acid (C₂H₄O₂) is CH₂O. Its molecular mass is 60 amu [12+3(1)+12+16+16+1].

Question 10: How we can determine the molecular formula of compounds?
Answer: The molecular formula is determined by multiplying the empirical formula by ‘n’, where n = Molecular mass / Empirical formula mass.

Question 11: What is the empirical formula of benzene (C₆H₆)?
Answer: The empirical formula of benzene is CH.

Question 12: What is the relationship between empirical formula and molecular formula?
Answer: Molecular formula = n × (empirical formula), where n is 1, 2, 3, and so on.

Question 13: Calculate the formula masses of Na₂SO₄, ZnSO₄ and CuCO₃.
Answer: Na₂SO₄ = 142 amu; ZnSO₄ = 161 amu; CuCO₃ = 123.5 amu.

Question 14: Which term is used to represent the mass of 1 mole of molecules of a substance?
Answer: Gram molecular mass or gram molecule is used to represent the mass of 1 mole of molecules.

Question 15: How many atoms are present in one gram atomic mass of a substance?
Answer: One gram atomic mass of a substance contains Avogadro’s number (6.02 × 10²³) of atoms.

Question 16: Explain the relationship between mass and mole of a substance.
Answer: Number of moles = Mass of substance / Molar mass. Therefore, Mass of substance = Number of moles × Molar mass.

Question 17: Find out the mass of 3 moles of oxygen atoms.
Answer: Mass = 3 mol × 16 g mol^-1 = 48 g.

Question 18: How many molecules of water will be present in half mole of water?
Answer: Number of molecules = 0.5 mol × 6.02 × 10²³ = 3.01 × 10²³ molecules.

Question 19: What are chemical reactions?
Answer: The process in which a chemical change occurs in the nature and composition of substances is called a chemical reaction, such as the rusting of iron.

Question 20: Differentiate between reactants and products.
Answer: Reactants are the substances that combine in a chemical reaction (written on the left side), while products are the new substances formed (written on the right side).

Question 21: How many atoms of sodium are present in 3 moles of sodium and what is its mass?
Answer: Number of atoms = 3 × 6.02 × 10²³ = 1.806 × 10²⁴ atoms. Mass = 3 mol × 23 g mol^-1 = 69 g.

Question 22: How many atoms are in 1 amu and 1 g of hydrogen (H)?
Answer: There are 6.02 × 10²³ atoms in 1 g of hydrogen, and 1 atom in 1 amu of hydrogen.

Question 23: How many atoms are present in 16 g of O and 8 g of S?
Answer: 16 g of Oxygen contains 6.02 × 10²³ atoms. 8 g of Sulphur contains 1.505 × 10²³ atoms.

Question 24: Is the mass of 1 mole of O and 1 mole of S same?
Answer: No, the mass of 1 mole of O is 16 g and the mass of 1 mole of S is 32 g.

Question 25: What do you mean by 1 atom of C and 1 gram atom of C?
Answer: 1 atom of C means a single particle with a mass of 12 amu. 1 gram atom of C means 1 mole (12 g) of carbon containing 6.02 × 10²³ atoms.

Question 26: If 16 g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen in grams.
Answer: Mass of 1 atom = 16 g / (6.02 × 10²³) = 2.65 × 10^-23 g.

Question 27: How many times is 1 mole of oxygen atoms heavier than 1 mole of hydrogen atoms?
Answer: 1 mole of oxygen (16 g) is 16 times heavier than 1 mole of hydrogen (1 g).

Question 28: Why does 10 g nitrogen gas contain the same number of molecules as 10 g of carbon monoxide?
Answer: Because both N₂ and CO have the same molar mass (28 g/mol), meaning 10 g of each represents the same number of moles (0.35 mol) and thus the same number of molecules.

Question 29: Write down the chemical formula of barium nitride.
Answer: The formula is Ba₃N₂.

Question 30: Find out the molecular formula of a compound whose empirical formula is CH₂O and its molar mass is 180.
Answer: n = 180 / 30 = 6. Molecular formula = (CH₂O)₆ = C₆H₁₂O₆.

Question 31: How many molecules are present in 1.5 g H₂O?
Answer: Moles = 1.5 / 18 = 0.0833. Molecules = 0.0833 × 6.02 × 10²³ = 5.01 × 10²² molecules.

Question 32: What is the difference between a mole and Avogadro’s number?
Answer: A mole is the quantity of a substance containing Avogadro’s number of particles. Avogadro’s number is the specific constant value (6.02 × 10²³) representing the number of particles in one mole.

Question 33: Write down the chemical equation for the reaction: Copper + Sulphuric acid → Copper sulphate + Sulphur dioxide + Water.