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Assignment - Atom and Molecule, Class 9, Science PDF Download

Important Points

While doing numerical problems of mole concept, the following relations should be remembered.

Key for symbols

Number of moles = n
Given mass = m
Molar mass = M
Given number of particle = N
Avogadro number of particle = N0

Key Formula

1 mole of atoms = 6. 022 × 1023 atoms = Gram atomic mass or Molar mass of element

Number of moles =  Mass of element / Molar Mass

Number of moles = Given number of atoms / Avogadro number

or m = n × M and N = n × N0

Let us apply these formula to solve numerical problems. 

Q. In a reaction 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide. 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

[NCERT]

Sodium carbonate + ethanoic acid  sodium ethanoate + carbon dioxide + water.

Q. Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

[NCERT]

Q. Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?

[NCERT]

(b) Law of constant Proportions / Law of definite proportions :

Q. Which postulate of Dalton's atomic theory can explain the law of definite proportions?

[NCERT]

Q. Define the atomic mass unit.

[NCERT]

Q. Why is it not possible to see an atom with naked eyes?

[NCERT]

Q. Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.

[NCERT]

Q. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u,
K = 39 u, C = 12 u and O = 16 u

[NCERT]

Q. Calculate the molar mass of the following substances.

[NCERT]

  1. Ethyne, C2H2
  2. Sulphur molecule, S8
  3. Phosphorous molecule, P4(Atomic mass of phosphorus = 31)
  4. Hydrochloric acid, HCl
  5. Nitric acid, HNO3

Q. What is meant by the term chemical formula?

[NCERT]

Q.  How many atoms are present in a

(i) H2S molecule and

(ii)   ?

[NCERT]

Q. What are polyatomic ions? Give examples

[NCERT]

Q. Give the names of the elements present in the following compounds.

  1. Quick lime
  2. Hydrogen bromide
  3. Baking powder
  4. Potassium sulphate

[NCERT]

Q. Write down the formulae of

  1. sodium oxide
  2. aluminium chloride
  3. sodium sulphide
  4. magnesium hydroxide

[NCERT]

Q. Write down the names of compounds represented by the following formulae :

[NCERT]

  1. Al2(SO4)3
  2. CaCl2
  3. K2SO4
  4. KNO3
  5. CaCO3

Q. Write the chemical formulae of the following.

  1. Magnesium chloride
  2. Calcium oxide
  3. Copper nitrate
  4. Aluminium chloride
  5. Calcium carbonate

[NCERT]

Q. If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?

[NCERT]

Q. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of
Na = 23u, Fe = 56u)?

[NCERT]

Q. A 0.24 g sample of 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.

[NCERT]

Q. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.0 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

[NCERT]

Q. What is the mass of –
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2SO3)?

[NCERT]

Q. Convert into mole.
(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide

[NCERT]


Q. What is the mass of :
(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

[NCERT]

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FAQs on Assignment - Atom and Molecule, Class 9, Science

1. What is an atom?
Ans. An atom is the smallest unit of matter that retains the chemical properties of an element. It consists of a nucleus, which contains positively charged protons and uncharged neutrons, and negatively charged electrons orbiting around the nucleus.
2. What is a molecule?
Ans. A molecule is a group of two or more atoms held together by chemical bonds. It is the smallest unit of a compound that retains the chemical properties of that compound. Molecules can be made up of atoms of the same element or different elements.
3. How are atoms and molecules related?
Ans. Atoms are the building blocks of molecules. Molecules are formed when atoms chemically bond together. The type and arrangement of atoms in a molecule determine its chemical properties and behavior.
4. What are the differences between atoms and molecules?
Ans. The main difference between atoms and molecules is that atoms are individual particles, whereas molecules are composed of two or more atoms bonded together. Atoms are the basic units of elements, while molecules are the basic units of compounds.
5. Can atoms exist independently without forming molecules?
Ans. Yes, atoms can exist independently without forming molecules. Some elements, such as noble gases like helium and neon, exist as individual atoms in their natural state. These atoms are stable and do not readily form chemical bonds with other atoms.
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