Question

Which postulates of Dalton's atomic theory can explain the Law of definite proportion?
A) All matter is made of very tiny particles called atoms.
B) Atoms of a given element are identical in mass and chemical properties.
C) Atoms are indivisible particles and cannot be created nor destroyed.
D) The relative number and kind of atoms in a given compound remain constant.

Answer 1

Hint: The law of definite proportion states that the atoms of different elements combine in a fixed ratio by mass to give a new compound. The different ration of the atoms results in the different compounds. For example, in $\text{ C}{{\text{H}}_{\text{4}}}$ the carbon and hydrogen combine in $3:1$ mass ratio.

Complete step by step answer:
John Dalton has published a theory about the atoms. These postulates are based on the law of conservation of mass and law of constant proportion.

The postulates of Dalton’s atomic theory are:
1) All matter is composed of the indivisible particles called atoms. Atoms of an element cannot be created neither can be destroyed.
2) Atoms of the same element have similar shape and mass, but the mass and shape can vary for atoms of other elements.
3) The atoms of the different elements combine in fixed and whole ratios to form different compounds of atoms.
4) Atoms of the same element can combine in more than one ratio to form different compounds.
5) The atom is the smallest indivisible unit of matter which takes part in the chemical reaction.

The law of definite proportion is explained based on $\text{ }{{\text{3}}^{\text{rd}}}$ the postulate of Dalton’s atomic theory. According to Dalton’s postulate, the atoms of the same elements combine in fixed and whole ratios to form different compounds of atoms.
Therefore, the law of definite proportions states that the sample of compounds always contains the same proportions of elements by mass. This law is the basis for the stoichiometric studies in chemistry.
Let us consider an example of water $\text{ }{{\text{H}}_{\text{2}}}\text{O }$. Here, a water molecule is composed of hydrogen and oxygen atoms. The hydrogen and oxygen combine in a ratio $\text{ 1:8 }$ by mass. Then, the mass of oxygen required to react completely with the $\text{ 1 g}$ hydrogen is equal to$\text{ }8\text{ g }$.

Let's take another example of ammonia $\text{ N}{{\text{H}}_{\text{3}}}\text{ }$. Ammonia is made of one atom of nitrogen and three atoms of hydrogen. 3 g of hydrogen combines with the 14 g of nitrogen. The mass ratio for ammonia is $\text{ 3:14 }$.the atomic weight of hydrogen is $1\text{ amu}$ and nitrogen is $\text{14 amu}$. The mass percentage of nitrogen and hydrogen can be given as:
$\begin{align}
  & \text{ mass }{\scriptstyle{}^{\text{0}}/{}_{\text{0}}}\text{ of N = }\dfrac{\text{14}}{\text{17}}\text{ }\!\!\times\!\!\text{ 100 = 82 }{\scriptstyle{}^{\text{0}}/{}_{\text{0}}} \\
 & \text{ mass }{\scriptstyle{}^{\text{0}}/{}_{\text{0}}}\text{ of H = }\dfrac{3}{\text{17}}\text{ }\!\!\times\!\!\text{ 100 = 18 }{\scriptstyle{}^{\text{0}}/{}_{\text{0}}} \\
\end{align}$

Any other combination of hydrogen and nitrogen would result in different chemical compounds.
Thus, the postulate which explains the law of definite proportion is “The relative number and kind of atoms in a given compound remain constant”.
So, the correct answer is “Option D”.

Note: Atoms are indivisible particles therefore they always combine in the whole number ratio. Therefore, it makes no sense to write the formula of table salt $\text{ N}{{\text{a}}_{\text{0}\text{.5}}}\text{C}{{\text{l}}_{\text{0}\text{.5}}}$ because atoms can’t be cut into the half or they cannot be combined as the half.