Question 1

Q1: A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage comp...

Accepted Answer

Given: Total mass of the compound = 0.24 g Mass of boron (B) in the compound = 0.096 g Mass of oxygen (O) in the compound = 0.144 g To Find: The percentage composition of boron and oxygen in the compound by weight. Formula: $$	ext{Percentage of an element} = \frac{	ext{Mass of the element}}{	ext{...

Question 2

Q2: When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbo...

Accepted Answer

Given: - Experiment 1: 3.0 g of carbon reacts with 8.00 g of oxygen to produce 11.00 g of carbon dioxide. - Experiment 2: 3.00 g of carbon is burnt in 50.00 g of oxygen. To Find: - Mass of carbon dioxide formed in the second experiment. - The law of chemical combination that governs the answer. Solu...

Question 3

Q3: What are polyatomic ions? Give examples.

Accepted Answer

Definition: Polyatomic ions are charged species composed of a group of two or more atoms that are covalently bonded together and act as a single unit with a net positive or negative charge. The entire group of atoms carries the charge, not just a single atom within the group. Examples: 1.  Ammonium...

Question 4

Q4: Write the chemical formulae of the following. (a) Magnesium chloride (b) Calcium oxide (c) Copper nitrate (d) Aluminium chloride (e) Calcium carbonate...

Accepted Answer

Solution: We determine the chemical formulae by using the symbols and valencies (charges) of the constituent ions. (a) Magnesium chloride - Magnesium ion: $Mg^{2+}$ (Valency 2) - Chloride ion: $Cl^{-}$ (Valency 1) - Criss-crossing the valencies gives $Mg_1Cl_2$. - Formula: $MgCl_2$ (b) Calcium oxide...

Question 5

Q5: Give the names of the elements present in the following compounds. (a) Quick lime (b) Hydrogen bromide (c) Baking powder (d) Potassium sulphate.

Accepted Answer

Solution: First, we need to know the chemical formula or composition of each compound. (a) Quick lime - Chemical name: Calcium oxide - Formula: CaO - Elements present: Calcium (Ca) and Oxygen (O). (b) Hydrogen bromide - Formula: HBr - Elements present: Hydrogen (H) and Bromine (Br). (c) Baking powde...

Question 6

Q6: Calculate the molar mass of the following substances. (a) Ethyne, $C_2H_2$ (b) Sulphur molecule, $S_8$ (c) Phosphorus molecule, $P_4$ (Atomic mass of...

Accepted Answer

Given: Atomic masses: C = 12 u, H = 1 u, S = 32 u, P = 31 u, Cl = 35.5 u, N = 14 u, O = 16 u. To Find: The molar mass of the given substances. The molar mass is numerically equal to the molecular mass, but the unit is grams per mole (g/mol). Solution: (a) Ethyne, $C_2H_2$ - Molecular mass = $(2 	im...

Question 7

Q1: In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium a...

Accepted Answer

Given: Mass of sodium carbonate = 5.3 g Mass of acetic acid = 6 g Mass of carbon dioxide = 2.2 g Mass of water = 0.9 g Mass of sodium acetate = 8.2 g To Show: The observations are in agreement with the law of conservation of mass. Law of Conservation of Mass: This law states that mass can neither be...

Question 8

Q2: Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydro...

Accepted Answer

Given: The ratio of mass of hydrogen to the mass of oxygen in water is 1:8. Mass of hydrogen gas = 3 g To Find: The mass of oxygen gas required. Solution: According to the law of constant proportions, hydrogen and oxygen combine in a fixed ratio by mass to form water. The ratio is given as: $$\frac{...

Question 9

Q3: Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?

Accepted Answer

Postulate: The postulate of Dalton's atomic theory that is the result of the law of conservation of mass is: "Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction." Explanation: The law of conservation of mass states that mass remains constant during a chemica...

Question 10

Q4: Which postulate of Dalton's atomic theory can explain the law of definite proportions?

Accepted Answer

Postulate: The postulate of Dalton's atomic theory that can explain the law of definite proportions is: "The relative number and kinds of atoms are constant in a given compound." Explanation: The law of definite proportions states that in a chemical substance, the elements are always present in defi...

Question 11

Q1: Define the atomic mass unit.

Accepted Answer

Definition: An atomic mass unit (amu or u) is defined as a mass unit exactly equal to one-twelfth ($1/12^{	ext{th}}$) the mass of one atom of the carbon-12 isotope. $$1 	ext{ u} = \frac{1}{12} 	imes 	ext{mass of one C-12 atom}$$

Question 12

Q2: Why is it not possible to see an atom with naked eyes?

Accepted Answer

Reason: It is not possible to see an atom with naked eyes because atoms are extremely small. Their size is on the order of nanometers ($1 	ext{ nm} = 10^{-9} 	ext{ m}$). This size is far smaller than the wavelength of visible light, making it impossible for the human eye, or even a standard optica...

Question 13

Q1: Write down the formulae of (i) sodium oxide (ii) aluminium chloride (iii) sodium sulphide (iv) magnesium hydroxide

Accepted Answer

Solution: To write the chemical formulae, we use the symbols and valencies (charges) of the ions and then criss-cross the valencies. (i) Sodium oxide - Symbol for sodium is Na, valency is 1+ ($Na^+$). - Symbol for oxide (oxygen) is O, valency is 2- ($O^{2-}$). - Criss-crossing the valencies:   Symbo...

Question 14

Q2: Write down the names of compounds represented by the following formulae: (i) $Al_2(SO_4)_3$ (ii) $CaCl_2$ (iii) $K_2SO_4$ (iv) $KNO_3$ (v) $CaCO_3$

Accepted Answer

Solution: To name the compounds, we identify the cation (positive ion) and the anion (negative ion) and write their names. The cation is named first, followed by the anion. (i) $Al_2(SO_4)_3$ - Cation: $Al^{3+}$ is the aluminium ion. - Anion: $SO_4^{2-}$ is the sulphate ion. - Name: Aluminium sulpha...

Question 15

Q3: What is meant by the term chemical formula?

Accepted Answer

Definition: The term chemical formula refers to a symbolic representation of the composition of a chemical compound. It indicates the types of elements present in the compound and the ratio of the number of atoms of each element in one molecule or formula unit of the substance. For example, the chem...

Question 16

Q4: How many atoms are present in a (i) $H_2S$ molecule and (ii) $PO_4^{3-}$ ion?

Accepted Answer

Solution: To find the total number of atoms, we sum the number of atoms of each element present in the chemical formula. (i) $H_2S$ molecule - The formula $H_2S$ indicates:   - 2 atoms of Hydrogen (H)   - 1 atom of Sulphur (S) - Total number of atoms = $2 + 1 = 3$ atoms. (ii) $PO_4^{3-}$ ion - The f...

Question 17

Q1: Calculate the molecular masses of $H_2, O_2, Cl_2, CO_2, CH_4, C_2H_6, C_2H_4, NH_3, CH_3OH$.

Accepted Answer

Given: Atomic masses: H = 1 u, O = 16 u, Cl = 35.5 u, C = 12 u, N = 14 u. To Find: The molecular mass of the given molecules. Formula: Molecular mass is the sum of the atomic masses of all atoms in a molecule. Solution: - $H_2$: $2 	imes (	ext{Atomic mass of H}) = 2 	imes 1 = 2 	ext{ u}$ - $O_2$...

Question 18

Q2: Calculate the formula unit masses of $ZnO, Na_2O, K_2CO_3$, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Accepted Answer

Given: Atomic masses: Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, O = 16 u. To Find: The formula unit mass of the given ionic compounds. Formula: Formula unit mass is the sum of the atomic masses of all atoms in a formula unit of a compound. Solution: - ZnO:    Formula unit mass = (Atomic mass of Zn)...

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