Molar mass of ammonia (NH₃) = 14 + 3
= 17
Mass percentage of Nitrogen = \(\frac{Mass \ of \ nitrogen \ in \ ammonia }{Molar \ mass \ of \ the \ ammonia}\) x 100
= \(\frac{14}{17}\) x 100 =\(\frac{1400}{17}\) = 82.35 %
The Mass percentage of nitrogen in ammonia is 82.35%

The total number of atoms present in the molecule is called its atomicity.
\(\text { Atomicity }=\frac{\text { Molecular mass }}{\text { Atomic mass }}\)

(i) Relative atomic mass of an element is the ratio between the average mass of its isotopes to \(\frac{1}{12^{\text {th }}}\) part of the mass of a carbon-12 atom.
(ii) It is denoted as A_r.
(iii) It is otherwise called "Standard atomic weight".

i) H₂O
Atomic masses of H = 1, O = 16
Gram molecular mass of H₂O
= (1 × 2) + (16 × 1)
= 2 + 16
Gram molecular mass of H₂O = 18 g
ii) CO₂
Atomic masses of C = 12, O = 16
Gram molecular mass of CO₂
= (12 × 1) + (16 × 2)
= 12 + 32
Gram molecular mass of CO₂ = 44 g
iii) Ca₃ (PO₄)₂
Atomic masses of Ca = 40, P = 30, O = 16.
Gram molecular mass of Ca₃ (PO₄)₂
= (40 × 3) + [30 + (16 × 4)] × 2
= 120 + (94 × 2)
= 120 + 188
Gram molecular mass of Ca₃(PO₄)₂ = 308 g

(i) An atom is no longer indivisible.
(ii) Atoms of the same element may have different atomic masses (isotopes ₁₇Cl³⁵, ₁₇Cl³⁷)
(iii) Atoms of different elements may have the same atomic masses (isobars ₁₈Ar⁴⁰, ₁₇Ca⁴⁰)
(iv) Atoms of one element can be transmuted into atoms of other elements. So atom is no longer indestructible. It is called artificial transmutation.
(v) Atoms may not always combine in a simple whole number ratio [Eg: Glucose C₆H₁₂O₆, sucrose C₁₂H₂₂O₁₁)
(vi) Atom is the smallest particle that takes part in a chemical reaction.
(vii) The mass of an atom can be converted into energy (E = mc²).

Molecular mass of water (H₂O) = H₂O = H x 2 + O x 1
= 1 x 2 + 16 x 1
= 2 + 16
=18
No of moles = \(\frac{Given \ Mass}{Molecular \ Mass}\)
Number of moles = \(\frac{0.18}{8}= \frac{0.18 \times 100}{18 \times 100}=0.01\)
No of moles = \(\frac{Number\ of \ molecules}{Avogadro's \ number}\)
Number of molecules = No of moles x Avogadro's number
= 0.01 x 6.023 x 10²³
= 0.06023 x 10²³
Number of molecules in 0.18 g of water = 6.023 x 10²¹
Alternative method
Gram molecular mass of water = H₂O
= 1 x 2 + 16 x 1 = 2 + 16
= 18 g
Number of molecules \(=\frac{\text { Avogadro's number } \times \text { given mass }}{\text { Gram molecular mass }}\)
Number of molecules \(=\frac{6.023 \times 10^{23} \times 0.18}{18}\) 
= \(6.023 \times 10^{23} \times 0.01\)
Number of molecules of 0.18 g of water = 6.023 x 10²¹ molecules.