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Some examples from this set of practice questions
1.
Give one relevant reason why scientists choose 1/16 of the mass of an atom of naturally occurring oxygen as the atomic mass unit. Class 9 science atom and molecules
Answer: Initially 1/16th of the mass of naturally occurring oxygen was taken as the atomic mass unit because this unit gave masses of most of the elements as whole numbers.
2.
Distinguish between molecular mass and molar mass.
Answer: The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule, whereas the mass of 1 mole of any substance is called its molar mass.
3.
What is meant by the term chemical formula ?
Answer: The chemical formula of a compound is a symbolic representation of its composition and actual number of atoms in one molecule of a pure substance, may be an atom or a compound.
4.
How many atoms are present in a (i) H2S molecule and (ii)PO3−4ion ? [SAII – 2011]
Answer: (i) In H2S, 3 atoms are present (ii) In PO3−4, 5 atoms are present.
5.
Give two drawbacks of Dalton’s atomic theory.
Answer: Drawbacks of Dalton’s Atomic Theory : (i) According to modern theory, atom is not the ultimate indivisible particle of matter. Today, we know that atoms are divisible, i.e., they are themselves made-up of particles (protons, electrons, neutrons, etc.). (ii) In case of isotopes of an element, the assumption that the atoms of the same element have same mass does not hold good.
6.
How would you differentiate between a molecule of element and a molecule of compound ? Write one example of each type.
Answer: Molecule of an element is made-up of only one kind of atoms, e.g., O2,N2,F2,O3,P4,S8 etc. Molecule of a compound is made-up of two or more different kinds of atoms in a fixed ratio, e.g.,H2O,CS2,H2S,NH3,CH4.
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