Answer: Option C. -> Formula unit mass
:
C
The word formula unit is used for the substances of which constituent particles are ions.
For example, magnesium chloride has a formula unit $MgC{l}_2$
Its formula unit mass can be calculated as:
$(1\times Atomicmassofmagnesium)+(2\times Atomicmassofchlorine)$
=$(1\times 24)+(2\times 35.5)=95u$

Answer: Option A. -> 36.132×1023
:
A
Molecularmass of $HN{O}_3$ = atomic mass of hydrogen + atomic mass of nitrogen + 3$\times$ atomic mass of oxygen = 1 + 14 + (3 $\times$ 16) = 63 u.
Number of moles = $\frac{Givenmass}{Grammolecularmass}$
Moles of $HN{O}_3$= $\frac{126}{63}$ = 2 moles
$\bullet$ In 1 molecule of $HN{O}_3$we have 3 atoms of oxygen.
$\bullet$ 1 mole of $HN{O}_3$ contains 3 moles of oxygen atoms. So, in2 moles of$HN{O}_3$ we will have 6 moles of oxygen atoms.
$\bullet$ One mole of a substance contains$6.022\times {10}^{23}$ particles. So, the number of atoms in 126 g of $HN{O}_3$= $6\times 6.022\times {10}^{23}$ = $36.132\times {10}^{23}$ atoms.

Answer: Option B. -> False
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B
Atomicity means the number of atoms present in one molecule. Oxygen exists as $O_2$. The atoms in one molecule of oxygen is2. Hence, it is diatomic and the given statement is false.

Answer: Option A. -> 3:1
:
A
Mass of one carbon atom = 12u
Mass of each hydrogen atom = 1u
Mass of 4 hydrogenatoms = 4u
Mass ratio of carbonand hydrogen atomsin methane $=12:4=3:1$

Answer: Option A. -> Hydrogen
:
A
Dalton was the first scientist to use the symbols for elements in a very specific sense. The symbols are the unique pictorial representation of each element. The given symbol was used for representing the element "hydrogen".

Answer: Option A. -> Dalton's atomic theory
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A
Daltons' atomic theory was based on the laws of chemical combinations i.e.,law of conservation of mass and the law of definite proportions. The two postulates which supports the law of combinations are:

• Atoms combine in the ratio of small whole numbers to form compounds.


• Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction

:
Each point: 1.5 Marks
1. Atomic mass of Na = 23 u (Given)
Then, gram atomic mass of Na = 23 g
Now, 23 g of Na contains = 6.022 x 10²³ g number of atoms
Thus, 100 g of Na contains = $6.022\times \frac{{10}^{23}}{23}\times 100$ number of atoms
= 2.6182 x 10²⁴ number of atoms.
2. Again, atomic mass of Fe = 56 u (Given)
Then, gram atomic mass of Fe = 56 g
Now, 56 g of Fe contains = 6.022 x 10²³ g number of atoms
Thus, 100 g of Fe contains =$6.022\times \frac{{10}^{23}}{56}\times 100$ number of atoms
= 1.0753 x 10²⁴ number of atoms.

:
Each step: 1 Mark
1. Valency of aluminium = 3
2. Valency of chlorine =1
3. By crossing over thevalencies we have$Al{Cl}_3$.

:
Statement : 1 Mark each
The two laws of chemical combination are
1. Law of conservation of mass:
Thelaw of conservation of massstates thatmassin an isolated system is neither created nor destroyed by chemical reactions or physical transformations. According to thelaw of conservation of mass, themassof the products in a chemical reaction must equal themassof the reactants.
2. Lawof definiteproportions:
Thelawof definiteproportions, also called Proust'slawor the lawof definite composition, orlaw of constantcomposition states that a given chemical compound always contains its component elements in fixed ratio (by weight) and does not depend on its source and method of preparation.

:
Definition: 1 Mark
Example: 1 Mark
A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.
For example oxygen molecule is written as (O₂)