Atoms and Molecules

This is correct. In a pure chemical compound, elements are always present in a definite proportion by mass. This is known as the law of definite proportions.
In this case, if we decompose 100 grams of pure water by passing electricity through it, then we get 11 grams of hydrogen and 89 grams of oxygen, showing that hydrogen and oxygen are combined together in water in the same constant proportion of 1 : 8 by weight.

This is the correct answer. All the statements are correct.

This is the correct answer. The combining capacity of an element is called its valency.

This is the correct answer as none of the statements are incorrect.

This is the correct answer as both water and carbon dioxide are triatomic molecules.

Atomic mass of zinc = 65.37 u
1u = 1.67 x 10^-24 g.
Atomic mass of zinc = 65.37 u = 65.37 x 1.67 x 10^-24 = 1.09 x 10^-22 g

This is the correct answer as the correct matching is:
Ag is the symbol for silver.
Fe is the symbol for iron.
Pb is the symbol for lead.

1 mole of H₂O atoms contains 2 moles of H atoms.
= 2 X 6.023 X 10²³ hydrogen atoms
So, 0.25 moles of H₂O contains 2 x 6.023 X 10²³ X 0.25 H atoms.
= 3.01 X 10²³ H atoms.
Thus, this is the correct answer.

This is the correct answer as both these statements are incorrect. Atoms of most elements are not able to exist independently. A molecule is, in general, a group of two or more atoms that are chemically bonded together, i.e. tightly held together by attractive forces.

 Molecular mass of water = 2 + 16 = 18 g
1 mole of water (6.023 X 10²³ molecules) weighs 18 g.
Weight of 1 molecule of water = 18 / 6.023 X 10²³
= 2.99 X 10 ^-23

Thus, this is correct.

Molecular mass of Cr₂S₃ = 52 x 2 + 32 x 3 = 200
200 g Cr2S3 has Cr = 104 g
85 g of Cr = 104 / 200 x 85 = 44.2 g

Thus, this is the correct answer.

This is the correct answer.
Daltons atomic theory of matter states that when two elements combine to form two or more compounds, then the weight of one of these elements which combines with the fixed weight of the other element, bears a simple ratio to one another. 
Though the assertion statement is correct, yet Dalton's theory fails to explain why substances like charcoal, graphite and diamond have different properties when all of these are formed from carbon atoms.

A is incorrect and R is correct.
A: In water, 1 gram of hydrogen combines with 8 grams of oxygen and in hydrogen peroxide, 1 gram of hydrogen combines with 16 grams of peroxide.
R: The law of multiple proportions defines that when two elements combine to form more than one compound, the mass of one element that combines with the given mass of the other element in different compounds is in the ratio of small whole numbers.

This is the correct answer as both these statements are correct.

This is the correct answer. Both the statements (a) and (b) are correct.

This is incorrectly paired.
Marble chips - Calcium carbonate CaCO₃
Lime water - Calcium hydroxide Ca(OH)₂

The correct matching is: 
Magnesium hydroxide - Mg(OH)₂
Potassium nitrate - KNO₃
Silver nitrate - AgNO₃
Thus, this is the correct answer choice.

This is the correct answer as all the given compounds are made up of charged ions.