ncert 9th standard science textbook chapter 3 question answers
Answers
Explanation:
A pump empties a swimming pool in 40 minutes at the rate of 1400 L/min. If the rate of pumping is 2000 L/min, how long does it take to empty the swimming pool?
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Answer:
Answers of Science NCERT Class 9 Chapter 3 – Atoms and Molecules (All in text and Exercise Questions solved)
Class 9 Science Chapter 3 Exercise-3.1 Questions with Answer
Exercise-3.1 Page: 32
1. In a reaction, 5.3g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.
Sodium carbonate + acetic acid → Sodium acetate + carbon dioxide + water
Solution:
Sodium carbonate + acetic acid → Sodium acetate + carbon dioxide + water
5.3g 6g 8.2g 2.2g 0.9g
As per the law of conservation of mass, the total mass of reactants must be equal to the total mass of
products
As per the above reaction, LHS = RHS i.e., 5.3g + 6g = 2.2g + 0.9 g + 8.2 g = 11.3 g
Hence the observations are in agreement with the law of conservation of mass.
2. 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?
Solution:
We know hydrogen and water mix in the ratio 1: 8.
For every 1g of hydrogen, it is 8g of oxygen.
Therefore, for 3g of hydrogen, the quantity of oxygen = 3 x 8 = 24g
Hence, 24g of oxygen would be required for the complete reaction with 3g of hydrogen gas.
3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Solution:
The postulate of Dalton’s Atomic theory which is a result of the law of conservation of mass is,
“Atoms can neither be created nor destroyed”.
4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Solution:
The postulate of Dalton’s atomic theory that can explain the law of definite proportions is – the
relative number and kinds of atoms are equal in given compounds.
Exercise-3.2 Page: 35
1. Define the atomic mass unit?
Solution:
An atomic mass unit is a unit of mass used to express weights of atoms and molecules where one
atomic mass is equal to 1/12th the mass of one carbon-12 atom.
2. Why is it not possible to see an atom with naked eyes?
Solution:
Firstly, atoms are miniscule in nature, measured in nanometers. Secondly, except for atoms of noble
gasses, they do not exist independently. Hence, an atom cannot be visible to the naked eyes.
Exercise-3.3-3.4 Page: 39
1. Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium sulphide
(iv) magnesium hydroxide
Solution:
The following are the formulae:
(i) sodium oxide – Na2O
(ii) aluminium chloride – AlCl3
(iii) sodium sulphide – Na2S
(iv) magnesium hydroxide – Mg (OH)2
2. Write down the names of compounds represented by the following formulae:
(i) Al2(SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3.
Solution:
Listed below are the names of the compounds for each of the following formulae
(i) Al2(SO4)3 – Aluminium sulphate
(ii) CaCl2 – Calcium chloride
(iii) K2SO4 – Potassium chemical formula of hydrochloric acid is .
Solution
.