Q1: What is a mole? A1: A mole is the amount of substance that contains as many elementary entities (atoms, molecules, ions) as there are atoms in exactly 12 grams of carbon-12. This number is called Avogadro's number.
Q2: What is Avogadro's number? A2: Avogadro's number is 6.022 × 10²³. It represents the number of particles (atoms, molecules, ions) in one mole of any substance.
Q3: What is GAM? A3: GAM stands for Gram Atomic Mass. It is the atomic mass of an element expressed in grams. For example, if the atomic mass of carbon is 12u, then its GAM is 12g.
Q4: What is GMM? A4: GMM stands for Gram Molecular Mass (also called Molar Mass). It is the molecular mass of a compound expressed in grams.
2. Mole Calculations
Q5: How do you calculate the number of moles from mass? A5: Number of moles = Given mass in grams / GAM (for elements) or GMM (for compounds) Formula: n = m/M
Q6: If you have 23g of sodium (Na), how many moles are present? (Atomic mass of Na = 23) A6: Number of moles = 23g / 23g/mol = 1 mole
Q7: How do you find the number of atoms/molecules in a given sample? A7: Number of atoms/molecules = Number of moles × Avogadro's number = n × 6.022 × 10²³
3. Gas Laws and Molar Volume
Q8: What is STP? A8: STP stands for Standard Temperature and Pressure. Standard conditions are:
- Temperature = 273K (0°C)
- Pressure = 1 atmosphere
Q9: What is the molar volume of a gas at STP? A9: At STP, one mole of any gas occupies 22.4 liters. This is called the molar volume.
Q10: How do you calculate the volume of a gas at STP? A10: Volume at STP = Number of moles × 22.4 L V = n × 22.4 L
4. Percentage Composition
Q11: How do you calculate the percentage of an element in a compound? A11: Percentage of element = (Mass of element in compound / Molecular mass of compound) × 100
Q12: What is the percentage of oxygen in water (H₂O)? A12: Molecular mass of H₂O = (2×1) + (1×16) = 18g Mass of oxygen = 16g Percentage of O = (16/18) × 100 = 88.89%
5. Empirical and Molecular Formulas
Q13: What is an empirical formula? A13: An empirical formula shows the simplest whole number ratio of atoms of different elements in a compound.
Q14: What is a molecular formula? A14: A molecular formula shows the actual number of atoms of each element in one molecule of the compound.
Q15: How are empirical and molecular formulas related? A15: Molecular formula = (Empirical formula) × n Where n = Molecular mass / Empirical formula mass
6. Stoichiometry
Q16: What is stoichiometry? A16: Stoichiometry is the calculation of quantities in chemical reactions based on the balanced chemical equation and mole concept.
Q17: How do you solve stoichiometry problems? A17: Steps:
- Write the balanced chemical equation
- Convert given quantities to moles
- Use mole ratios from the balanced equation
- Convert back to required units
Q18: In the reaction 2H₂ + O₂ → 2H₂O, how many moles of water are formed from 4 moles of hydrogen? A18: From the equation: 2 moles H₂ produce 2 moles H₂O Therefore: 4 moles H₂ will produce 4 moles H₂O
7. Practice Problems
Q19: Calculate the number of molecules in 18g of water. A19:
- Molecular mass of H₂O = 18g/mol
- Number of moles = 18g / 18g/mol = 1 mole
- Number of molecules = 1 × 6.022 × 10²³ = 6.022 × 10²³ molecules
Q20: What volume will 3.2g of oxygen gas occupy at STP? A20:
- Molecular mass of O₂ = 32g/mol
- Number of moles = 3.2g / 32g/mol = 0.1 mole
- Volume at STP = 0.1 × 22.4L = 2.24L
Important Formulas Summary
- Number of moles: n = m/M (mass/molar mass)
- Number of particles: N = n × 6.022 × 10²³
- Volume at STP: V = n × 22.4L
- Percentage composition: % = (mass of element/total mass) × 100
- Molecular formula: = Empirical formula × n
Key Points to Remember
- 1 mole contains 6.022 × 10²³ particles
- 1 mole of any gas at STP occupies 22.4L
- GAM is atomic mass in grams
- GMM is molecular mass in grams
- Always balance chemical equations before stoichiometric calculations
- Mole ratios from balanced equations are crucial for stoichiometry
Tips for Exam Preparation
- Practice converting between mass, moles, and number of particles
- Memorize Avogadro's number and molar volume at STP
- Master percentage composition calculations
- Practice empirical and molecular formula problems
- Solve stoichiometry problems step by step
- Always check if your final answer makes logical sense