Questions and Answers NCERT Science Class 9, Chapter “Atoms and Molecules”
Chapter 3: Atoms and Molecules
In-Text Questions:
- Define the atomic mass unit.
– Answer: An atomic mass unit (amu) is defined as one-twelfth of the mass of a carbon-12 atom. It is a standard unit used for expressing atomic and molecular weights.
- Why is it not possible to see an atom with the naked eye?
– Answer: Atoms are extremely small, with sizes in the range of picometers (1 pm = (10^{-12}) meters). This makes them impossible to be seen with the naked eye or even with optical microscopes.
Textbook Exercises:
- A 0.24 g sample of a compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
– Answer:
– Percentage of boron = ((\frac{0.096}{0.24}) \times 100 = 40\%)
– Percentage of oxygen = ((\frac{0.144}{0.24}) \times 100 = 60\%)
- When 3.0 g of carbon is burnt in 8.0 g of oxygen, 11.0 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.0 g of carbon is burnt in 50.0 g of oxygen? Which law of chemical combination will govern your answer?
– Answer: When 3.0 g of carbon is burnt in excess oxygen, 11.0 g of carbon dioxide will be produced. This is governed by the Law of Definite Proportions which states that a chemical compound always contains exactly the same proportion of elements by mass.
- What are polyatomic ions? Give examples.
– Answer: Polyatomic ions are ions that consist of more than one atom. Examples include:
– Ammonium ion ((NH_4^+))
– Sulfate ion ((SO_4^{2-}))
– Nitrate ion ((NO_3^-))
- Write the chemical formula of the following:
– Answer:
– Magnesium chloride: (MgCl_2)
– Calcium oxide: (CaO)
– Copper nitrate: (Cu(NO_3)_2)
– Aluminium chloride: (AlCl_3)
– Calcium carbonate: (CaCO_3)
- A 12.2 g sample of X reacts with a sample of Y to form 19.9 g of XY. What is the mass of Y that reacted?
– Answer: Mass of Y = Mass of XY – Mass of X
– Mass of Y = 19.9 g – 12.2 g = 7.7 g
- Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
– Answer: The postulate that atoms are indivisible particles which can neither be created nor destroyed in a chemical reaction is a result of the law of conservation of mass.
- Why is it necessary to balance a chemical equation?
– Answer: It is necessary to balance a chemical equation to satisfy the law of conservation of mass. This ensures that the number of atoms of each element is the same on both sides of the equation, indicating that mass is conserved in the reaction.
- How can atoms exist?
– Answer: Atoms can exist in the form of:
– Individual atoms (e.g., noble gases like Helium, Neon)
– Molecules (e.g., (O_2), (N_2))
– Ions (e.g., (Na^+), (Cl^-))
- What is meant by the term chemical formula?
– Answer: A chemical formula is a symbolic representation of a compound indicating the types and numbers of atoms present in a molecule of the compound. For example, (H_2O) represents water, which has two hydrogen atoms and one oxygen atom.
- How many atoms are present in a (H_2S) molecule and a (PO_4^{3-}) ion?
– Answer:
– (H_2S) molecule has 3 atoms (2 hydrogen and 1 sulfur).
– (PO_4^{3-}) ion has 5 atoms (1 phosphorus and 4 oxygen).
Additional Questions:
- What is an ion?
– Answer: An ion is an atom or a group of atoms that has a positive or negative charge due to the loss or gain of one or more electrons.
- Give examples of molecules of elements and molecules of compounds.
– Answer:
– Molecules of elements: (O_2) (oxygen), (N_2) (nitrogen)
– Molecules of compounds: (H_2O) (water), (CO_2) (carbon dioxide)
Feel free to ask if you need detailed explanations or have any more questions about this chapter!