Measurement of Matter
Short Questions
1. What is the Law of Conservation of Matter?
- It states that the total mass of reactants equals the total mass of products in a chemical reaction.
2. What is the Law of Constant Proportion?
- It states that the proportion by mass of elements in a compound is always fixed, regardless of the source.
3. Who proposed the Law of Conservation of Matter?
- Antoine Lavoisier proposed this law in 1785.
4. What is the proportion by mass of hydrogen and oxygen in water?
- The proportion by mass of hydrogen to oxygen in water is 1:8.
5. What is the atomic radius measured in?
- The atomic radius is measured in nanometers (nm).
6. What determines the mass of an atom?
- The mass of an atom is determined by the protons and neutrons in its nucleus.
7. What is the atomic mass number?
- It is the total number of protons and neutrons in an atom’s nucleus.
8. What is the current reference atom for atomic mass?
- The carbon-12 atom is the reference, with a relative atomic mass of 12.
9. What is the unit of atomic mass today?
- The unit of atomic mass is Dalton (u), where 1 u = 1.66053904 × 10⁻²⁷ kg.
10. What is molecular mass?
- It is the sum of the atomic masses of all atoms in a molecule, measured in Daltons (u).
11. What is the formula for molecular mass of a compound?
- Molecular mass = Sum of atomic masses of all atoms in the molecule (e.g., H₂O = 2(1) + 16 = 18 u).
12. What is a mole?
- A mole is the amount of a substance whose mass in grams equals its molecular mass in Daltons.
13. What is Avogadro’s number?
- Avogadro’s number (Nₐ) is 6.022 × 10²³, the number of molecules in one mole of a substance.
14. What is the formula for the number of moles?
- Number of moles (n) = Mass (g) / Molecular mass (g/mol).
15. How many moles are in 66 g of CO₂ (molecular mass 44 u)?
- n = Mass / Molecular mass = 66 / 44 = 1.5 moles.
16. What is valency?
- Valency is the number of electrons an atom gives or takes to form a bond.
17. What are basic radicals?
- Basic radicals are positively charged ions (cations) like Na⁺ or Ca²⁺.
18. What are acidic radicals?
- Acidic radicals are negatively charged ions (anions) like Cl⁻ or SO₄²⁻.
19. What is a composite radical?
- A composite radical is a group of atoms with a charge, like SO₄²⁻ or NH₄⁺.
20. How many molecules are in 36 g of water (molecular mass 18 u)?
- n = 36 / 18 = 2 moles; Molecules = n × Nₐ = 2 × 6.022 × 10²³ = 1.2044 × 10²⁴ molecules.
Long Questions
1. What is the Law of Conservation of Matter, and how was it verified in Activity 1?
- The Law of Conservation of Matter states that mass remains constant during a chemical reaction, meaning total mass of reactants = total mass of products. In Activity 1, 56 g CaO + 18 g H₂O (total 74 g) formed a product with the same mass (74 g), proving no mass change occurs.
2. Explain the Law of Constant Proportion with an example of water.
- The Law of Constant Proportion states that the mass ratio of elements in a compound is fixed, like hydrogen and oxygen in water being 1:8. For example, 9 g of water always contains 1 g hydrogen and 8 g oxygen (1:8), regardless of its source, as shown by J.L. Proust.
3. How is the atomic mass of an element determined using a reference atom?
- Atomic mass is determined by comparing an atom’s mass to a reference atom, initially hydrogen (mass 1) and now carbon-12 (mass 12). For example, nitrogen’s mass is 14 times hydrogen’s, so its relative atomic mass = 14 on the hydrogen scale, or 14 on the carbon-12 scale.
4. What is molecular mass, and how is it calculated for NaCl (Na = 23 u, Cl = 35.5 u)?
- Molecular mass is the sum of atomic masses of all atoms in a molecule, measured in Daltons (u). For NaCl, it’s Na + Cl = 23 + 35.5 = 58.5 u, showing the total mass of one molecule.
5. What is a mole, and how many molecules are in 1 mole of CO₂ (molecular mass 44 u)?
- A mole is the amount of a substance whose mass in grams equals its molecular mass, with 1 mole containing 6.022 × 10²³ molecules (Nₐ). For CO₂, 1 mole = 44 g, so it has 6.022 × 10²³ molecules, as per Avogadro’s number.
6. How is the number of moles calculated, and find the moles in 90 g of water (molecular mass 18 u)?
- The number of moles (n) is calculated as n = Mass / Molecular mass, which determines the quantity of a substance. For 90 g of water, n = 90 / 18 = 5 moles, meaning 90 g contains 5 moles of water.
7. What is valency, and how is it determined for sodium and chlorine in NaCl?
- Valency is the number of electrons an atom gives or takes to form a bond; for sodium (Na), it’s 1 as it loses one electron to form Na⁺ (2,8,1 to 2,8). Chlorine (Cl) has valency 1 as it gains one electron to form Cl⁻ (2,8,7 to 2,8,8), creating the ionic bond in NaCl.
8. What are radicals, and how are basic and acidic radicals different?
- Radicals are ions that participate in reactions; basic radicals (e.g., Na⁺, Ca²⁺) are positively charged cations often from metals, while acidic radicals (e.g., Cl⁻, SO₄²⁻) are negatively charged anions from non-metals. They differ in charge and origin, with exceptions like NH₄⁺ (basic) and MnO₄⁻ (acidic).
9. How is the chemical formula of calcium carbonate (CaCO₃) deduced using valency?
- Calcium (Ca²⁺) has valency 2, and carbonate (CO₃²⁻) has valency 2; cross-multiplying valencies (Ca: 2, CO₃: 2) gives Ca₁(CO₃)₁, so the formula is CaCO₃. This ensures the compound is neutral, as the charges balance (2+ and 2-).
10. How many molecules are in 8.8 g of CO₂ (molecular mass 44 u), and what formula is used?
- First, calculate moles: n = Mass / Molecular mass = 8.8 / 44 = 0.2 moles; then, molecules = n × Nₐ = 0.2 × 6.022 × 10²³ = 1.2044 × 10²³ molecules. This uses the formula: Number of molecules = (Mass / Molecular mass) × Avogadro’s number.
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