3.2 Mole Concept

Short Answer Questions

1. What is the standard unit of measurement for the amount of a substance in chemistry?
   Answer
   The mole (mol).
2. How many elementary entities are contained within one mole of any substance?
   Answer
   6.02 × 10²³ elementary entities.
3. What is the approximate value of Avogadro’s constant?
   Answer
   6.02 × 10²³ mol⁻¹.
4. How is the mole defined in terms of carbon-12?
   Answer
   The mole is defined as the amount of substance that contains as many elementary entities as there are atoms in 12 grams of carbon-12.
5. Define the term molar mass.
   Answer
   Molar mass is the mass of one mole of a substance.
6. What are the units of molar mass?
   Answer
   Grams per mole (g/mol).
7. How does the molar mass of an element relate to its relative atomic mass?
   Answer
   The molar mass of an element is numerically equal to its relative atomic mass expressed in grams.
8. How is the molar mass of a compound calculated from its formula?
   Answer
   The molar mass of a compound is the sum of the relative atomic masses of all atoms in the chemical formula, expressed in grams per mole.
9. What is molar volume?
   Answer
   Molar volume is the volume occupied by one mole of a gas at a specified temperature and pressure.
10. What is the approximate molar volume of an ideal gas at STP?
    Answer
    22.4 dm³.
11. What is the approximate molar volume of a gas at room temperature and pressure (RTP)?
    Answer
    24 dm³.
12. Is molar volume applicable to solids and liquids?
    Answer
    No, molar volume is only applicable to gases.
13. State the formula that relates moles to mass.
    Answer
    Number of moles = Mass / Molar mass.
14. State the formula that relates moles to number of particles.
    Answer
    Number of moles = Number of particles / Avogadro's constant.
15. State the formula that relates moles to gas volume at a specified temperature and pressure.
    Answer
    Number of moles = Volume of gas / Molar volume.
16. If you have 1 mole of sodium chloride, how many particles are present?
    Answer
    6.02 × 10²³ particles.
17. How many grams are there in 0.5 moles of carbon dioxide (CO₂)? (Molar mass of CO₂ = 44 g/mol)
    Answer
    22 grams. Calculation: **mass = moles × molar mass = 0.5 mol × 44 g/mol = 22 g**.
18. How many moles of gas do you have if you have 11.2 dm³ of the gas at STP?
    Answer
    0.5 moles. Calculation: **moles = volume/molar volume = 11.2 dm³ / 22.4 dm³/mol = 0.5 mol**.
19. What quantity is used to relate moles of different reactants and products in a chemical equation?
    Answer
    Mole ratios from the balanced chemical equation.
20. If you have 12 grams of carbon-12, how many moles of carbon-12 do you have?
    Answer
    1 mole.

Explanation Questions

1. Explain the significance of the mole in chemical calculations.
   Answer
   The mole provides a bridge between the microscopic world of atoms and molecules and the macroscopic world of grams and litres. It allows for the quantification of chemical substances and the consistent measurement of reactants and products in chemical reactions using relative atomic masses and balanced equations, enabling accurate stoichiometric calculations.
2. Why is Avogadro’s constant so large, and why is it needed in chemistry?
   Answer
   Avogadro's constant is large because atoms and molecules are extremely small. A large number is needed to make the connection between the tiny scale of individual atoms and molecules, and the scale of grams and litres that we measure in the laboratory. It allows us to count particles by weighing them and to convert between numbers of particles and moles.
3. Explain the relationship between molar mass and relative atomic mass, and relative molecular mass.
   Answer
   Molar mass is numerically equal to the relative atomic mass (RAM) of an element, or the relative molecular mass (RMM) of a molecule, or relative formula mass (RFM) of a compound, expressed in grams per mole. The RAM is the average mass of an atom of an element on a scale where an atom of carbon-12 has a mass of exactly 12. The RMM is the sum of the RAMs of all atoms in a molecule.
4. Why does the molar volume of any gas equal the same value at STP?
   Answer
   The molar volume of any gas is the same at STP (22.4 dm³) because equal volumes of all gases at the same temperature and pressure contain the same number of molecules (Avogadro's Law). The space between gas molecules is much greater than the size of the molecules themselves. This means the volume occupied by the gas is determined by the number of molecules and not by the size or mass of the individual molecules.
5. Why is it necessary to specify temperature and pressure when discussing the molar volume of a gas?
   Answer
   The volume of a gas is highly dependent on temperature and pressure. According to the ideal gas law, the volume of a gas is directly proportional to its temperature and inversely proportional to its pressure. Therefore, to have a standard value for molar volume, the conditions (STP) of temperature and pressure must be specified.
6. How can you calculate the number of moles of a substance from its given mass?
   Answer
   To calculate the number of moles of a substance from its given mass, you divide the mass of the substance in grams by its molar mass in grams per mole. The formula is: **Number of moles = Mass (g) / Molar mass (g/mol)**.
7. How can you determine the number of particles present if you know the number of moles of a substance?
   Answer
   To determine the number of particles present, multiply the number of moles by Avogadro's constant (6.02 × 10²³). The formula is: **Number of particles = Number of moles × Avogadro's constant**.
8. Describe the step-by-step process of converting from grams of a substance to the number of particles.
   Answer
   1. Find the molar mass of the substance using the periodic table.
   2. Convert the mass of the substance to moles using the formula: **Number of moles = Mass (g) / Molar mass (g/mol)**.
   3. Convert the number of moles to the number of particles using the formula: **Number of particles = Number of moles × Avogadro's constant**.
   
9. Explain the concept of stoichiometry and how mole ratios from balanced equations are used to calculate amounts of reactants and products.
   Answer
   Stoichiometry is the study of the quantitative relationships between reactants and products in chemical reactions. Mole ratios from balanced equations provide the exact proportions in which reactants combine and products are formed. By converting known quantities (like mass or volume) to moles, using the mole ratios from the balanced equation, and converting back to desired units, we can determine the quantities of other substances involved in the reaction.
10. How can you determine the volume of a gas produced by a reaction given the number of moles of a reactant?
    Answer
    
    1. Convert the given amount of the reactant to moles using the appropriate formula.
    2. Use the mole ratios from the balanced chemical equation to find the moles of the gas produced.
    3. Use the molar volume at the given temperature and pressure to convert the moles of the gas to volume. For example, at STP, use **22.4 dm³/mol** as the molar volume. The formula is: **Volume of gas = Number of moles × Molar volume**.