Grade 9
Chapter – Atoms and Molecules
1. Case-based Questions
Case 1: Sulphur Dioxide Formation
A laboratory technician burns sulphur in oxygen to form sulphur dioxide (\( \displaystyle \mathrm{SO_2} \)).
- (a) Write the balanced chemical equation for the reaction.
- (b) Using the law of conservation of mass, justify if the total mass of reactants and products will remain the same.
- (c) If \( \displaystyle 32\text{ g} \) sulphur reacts completely with \( \displaystyle 32\text{ g} \) oxygen, how many grams of \( \displaystyle \mathrm{SO_2} \) will be formed?
Case 2: Identification of a Substance
Ramesh has a sample of a compound with formula \( \displaystyle \mathrm{C_6H_{12}O_6} \).
- (a) Interpret what the symbols and numbers in the formula mean.
- (b) How many atoms are present in one molecule of \( \displaystyle \mathrm{C_6H_{12}O_6} \)?
- (c) What is the molecular mass of \( \displaystyle \mathrm{C_6H_{12}O_6} \) (\( \displaystyle \mathrm{H} = 1 \), \( \displaystyle \mathrm{C} = 12 \), \( \displaystyle \mathrm{O} = 16 \))?
2. Competency/Application
1. Water is often written as \( \displaystyle \mathrm{H_2O} \). Prove by calculation that in \( \displaystyle 36\text{ g} \) of water, hydrogen and oxygen are in the mass ratio of \( \displaystyle 1:8 \).
2. A compound contains \( \displaystyle 40\% \) sulphur and \( \displaystyle 60\% \) oxygen by mass. Prove that this is consistent with the compound \( \displaystyle \mathrm{SO_3} \).
3. Calculate the number of molecules in:
- (a) \( \displaystyle 9\text{ g} \) of water
- (b) \( \displaystyle 44\text{ g} \) of \( \displaystyle \mathrm{CO_2} \)
(\( \displaystyle \text{Avogadro’s number} = 6.022 \times 10^{23}\text{ molecules/mol} \))
3. Assertion–Reason
Q1.
- A: Atoms of the same element may combine to form molecules.
- R: Oxygen molecule (\( \displaystyle \mathrm{O_2} \)) consists of two oxygen atoms joined together.
Q2.
- A: The mass of one mole of any substance is equal to the molecular mass expressed in grams.
- R: One mole of a substance contains \( \displaystyle 6.022 \times 10^{23} \) particles.
Q3.
- A: In a balanced chemical equation, the number of atoms of each element before and after reaction remains the same.
- R: Mass is conserved in a chemical reaction.
4. HOTS / Twisted Questions
1. A student insists that “a compound with the same percentage composition by mass must always have the same molecular formula.” Do you agree? Why or why not? Give an example.
2. Imagine if Avogadro’s number was \( \displaystyle 10 \) times smaller (\( \displaystyle \approx 6.022 \times 10^{22} \)). How would this change the definition of a mole and the way we calculate molar masses?
3. Why is formula of hydrogen peroxide written as \( \displaystyle \mathrm{H_2O_2} \) and not \( \displaystyle \mathrm{HO} \)? Relate your answer to law of multiple proportions.
4. If \( \displaystyle 4\text{ g} \) of hydrogen reacts with \( \displaystyle 32\text{ g} \) of oxygen to form \( \displaystyle 36\text{ g} \) of water, calculate the mass of hydrogen needed to react completely with \( \displaystyle 80\text{ g} \) of oxygen. Which law of chemical combination does this illustrate?
A FEW MORE
1. Case-Based
Case 1: Reaction of Hydrogen and Oxygen
In a lab experiment, \( \displaystyle 2\text{ g} \) of hydrogen reacts completely with \( \displaystyle 16\text{ g} \) oxygen to form \( \displaystyle 18\text{ g} \) of water.
- (a) Show how this verifies the Law of Conservation of Mass.
- (b) Express the law of constant proportion using this example.
- (c) If \( \displaystyle 4\text{ g} \) \( \displaystyle \mathrm{H_2} \) reacts with excess \( \displaystyle \mathrm{O_2} \), how much water will be produced?
Case 2: Carbon Compounds
A scientist analyzes two compounds of carbon and oxygen. He finds:
- Compound X: \( \displaystyle 27.3\% \) carbon and \( \displaystyle 72.7\% \) oxygen.
- Compound Y: \( \displaystyle 42.9\% \) carbon and \( \displaystyle 57.1\% \) oxygen.
- (a) Verify whether these two compounds follow the Law of Multiple Proportions.
- (b) Write their simplest empirical formula.
- (c) Name one real pair of compounds that behaves in this way.
2. Competency/Application
1. Calculate the number of oxygen molecules present in a \( \displaystyle 200\text{ mL} \) flask at STP, given that \( \displaystyle 22.4\text{ L} \) of gas at STP contains \( \displaystyle 6.022 \times 10^{23} \) molecules.
2. Sodium carbonate (\( \displaystyle \mathrm{Na_2CO_3} \)) has molar mass \( \displaystyle 106\text{ g} \).
- (a) How many moles are there in \( \displaystyle 53\text{ g} \) of \( \displaystyle \mathrm{Na_2CO_3} \)?
- (b) How many sodium ions are present in this sample?
3. Ammonia (\( \displaystyle \mathrm{NH_3} \)) has \( \displaystyle 82\% \) nitrogen and \( \displaystyle 18\% \) hydrogen by mass. Prove this ratio by calculation from the molecular formula.
3. Assertion–Reason
Q1.
- A: Avogadro’s constant represents the number of atoms in \( \displaystyle 12\text{ g} \) of carbon-12.
- R: It is a fixed number used to relate the macroscopic mass to microscopic particles.
Q2.
- A: The law of definite proportions holds good only for pure compounds.
- R: A pure sample of a compound always contains the same elements in fixed ratio by mass.
Q3.
- A: The molecular mass of \( \displaystyle \mathrm{CO_2} \) is \( \displaystyle 44\text{ u} \).
- R: One mole of \( \displaystyle \mathrm{CO_2} \) weighs \( \displaystyle 44\text{ g} \) and contains \( \displaystyle 6.022 \times 10^{23} \) molecules.
4. HOTS / High-Level Twisted
1. If \( \displaystyle 18\text{ g} \) of water contains \( \displaystyle 6.022 \times 10^{23} \) molecules, calculate the mass of one molecule of water in grams. Reflect on why such a small number is important in chemistry.
2. The molecular formula of glucose is \( \displaystyle \mathrm{C_6H_{12}O_6} \).
- (a) How will its empirical formula differ?
- (b) Why is it sometimes more useful to know the empirical formula than the molecular one?
3. A researcher claims:
“If two gases are taken at the same temperature and pressure, the one with higher molar mass must contain fewer molecules.”
Do you agree? Justify using Avogadro’s Law.
4. Calculate:
- (a) The number of atoms present in \( \displaystyle 5.6\text{ L} \) of \( \displaystyle \mathrm{O_2} \) gas at STP.
- (b) If all atoms of this \( \displaystyle \mathrm{O_2} \) gas were arranged in a single line (O atoms touching each other, each atom \( \displaystyle \approx 1 \times 10^{-10}\text{ m} \) diameter), what would be the approximate length of this line?
(Hint: High-order estimation problem, linking mole concept with atomic scale)
5. Imagine a world where Avogadro’s constant was half the current value.
- (a) How would it affect the definition of a mole?
- (b) Would atomic masses of elements change, or just the scale of counting? Give reasoning.