Question 1

According to the modern atomic theory, atoms of the same element must have the same number of:

Accepted Answer

The atomic number (number of protons) uniquely defines an element; neutrons vary in isotopes and electrons vary in ions.

Question 2

What is the empirical formula of a compound containing 40% carbon, 6.67% hydrogen, and 53.33% oxygen by mass?

Accepted Answer

Converting percentages to moles (40/12 : 6.67/1 : 53.33/16 = 3.33 : 6.67 : 3.33) and dividing by the smallest gives 1:2:1 ratio.

Question 3

What is the SI unit of amount of substance?

Accepted Answer

The mole is the fundamental SI unit used to measure the amount of substance in chemistry.

Question 4

How many significant figures are present in the measurement 0.00450 kg?

Accepted Answer

Leading zeros are not significant; only 4, 5, and 0 (trailing zero after decimal) count as significant figures, giving 3 total.

Question 5

If 2.0 moles of oxygen gas (O₂) react completely with hydrogen to form water, how many moles of water are produced? (2H₂ + O₂ → 2H₂O)

Accepted Answer

From the balanced equation, 1 mole of O₂ produces 2 moles of H₂O; therefore, 2.0 moles of O₂ produces 4.0 moles of H₂O.

Question 6

When 2.4 g of carbon completely burns in oxygen, how many moles of CO₂ are produced? (Atomic mass: C = 12, O = 16)

Accepted Answer

Moles of C = 2.4/12 = 0.2 mol; the equation C + O₂ → CO₂ shows 1:1 molar ratio, so 0.2 mol CO₂ is produced.

Question 7

If 5 moles of H₂ react with 2.5 moles of O₂ according to 2H₂ + O₂ → 2H₂O, which reactant is limiting?

Accepted Answer

From stoichiometry, 5 mol H₂ requires (5 × 1)/2 = 2.5 mol O₂, but only 2.5 mol O₂ is available; however, checking: 5 mol H₂ needs 2.5 mol O₂ (exactly matched), so both are consumed equally—recheck shows O₂ becomes limiting when considering the ratio 2:1 means 5 mol H₂ needs 2.5 mol O₂.

Question 8

Which of the following statements best defines the mole concept in chemistry?

Accepted Answer

A mole is defined as Avogadro's number (6.022 × 10²³) of particles, which is the fundamental unit for counting atoms and molecules in chemistry.

Question 9

What is the empirical formula of a compound that contains 40% carbon, 6.67% hydrogen, and 53.33% oxygen by mass?

Accepted Answer

Converting percentages to moles: C(40/12)=3.33, H(6.67/1)=6.67, O(53.33/16)=3.33; dividing by smallest gives ratio 1:2:1, yielding empirical formula CH₂O.

Question 10

Which of the following represents the correct relationship between molarity and molality for a dilute aqueous solution?

Accepted Answer

For dilute aqueous solutions, the density approaches 1 g/mL, making the number of kilograms of solvent approximately equal to liters of solution, thus M ≈ m.

Question 11

What is the oxidation state of sulfur in the compound H₂SO₄?

Accepted Answer

In H₂SO₄, hydrogen is +1 and oxygen is -2; therefore: 2(+1) + S + 4(-2) = 0, giving S = +6.

Question 12

A solution has a pH of 3. What is its hydrogen ion concentration [H⁺] in mol/L?

Accepted Answer

pH is defined as -log[H⁺]; therefore [H⁺] = 10⁻ᵖᴴ = 10⁻³ = 1 × 10⁻³ mol/L.

Question 13

Which law states that the total pressure of a mixture of gases is equal to the sum of partial pressures of individual gases?

Accepted Answer

Dalton's Law of Partial Pressures specifically states that Pₜₒₜₐₗ = P₁ + P₂ + P₃ + ... for a mixture of non-reacting ideal gases.

Question 14

A sample contains 3.011 × 10²³ atoms of carbon. Calculate the mass of carbon in grams. (Atomic mass of C = 12 u, Avogadro's number = 6.022 × 10²³)

Accepted Answer

Number of moles = (3.011 × 10²³) / (6.022 × 10²³) = 0.5 mol; Mass = 0.5 × 12 = 6 g.

Question 15

Which of the following correctly represents the relationship between molarity (M), molality (m), and density (d) for a solution?

Accepted Answer

The molarity-molality relation is derived from definitions of both terms per unit volume and mass of solvent respectively.

Question 16

An element has an electron configuration of [Ar] 3d⁵ 4s². Its position in the periodic table and the number of unpaired electrons in the ground state are:

Accepted Answer

The element is Mn (Z=25); it is in Period 4, Group 7 (IUPAC Group 7), with 5 unpaired electrons in the 3d orbital.

CDS Basic Concepts of Chemistry

Basic Concepts of Chemistry — Practice Questions