i) 2 moles of hydrogen molecule, H₂
Mass = number of moles x molecular mass
= 2 x 2
Mass of 2 moles of H₂ = 4g
ii) 3 moles of chlorine molecule, Cl₂ 
mass = number of moles x molecular mass
= 3 x 71
Mass of 3 moles of Cl₂ =213 g
iii) 5 moles of sulphur molecules, S₈ 
Mass = number of moles x molecular mass
=5 x 256
Mass of 5 moles of S₈=1280 g
iv) 4 moles of phosphorous molecule, P₄ 
Mass = number of moles x molecular mass
= 4 x 123.88 = 495.52
Mass of 4 moles of P₄ = 496 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).

(i) 27 g of AI
Number of moles = \(\frac{Given\ Mass}{Atomic \ Mass}=\frac{27}{27}=1\)
Number of moles of 27 g of Al is 1 mole
(ii) 1.51 x 10²³ molecules of NH₄Cl
Number of moles = \(\frac{Number \ of \ molecules}{6.023 \times 10^\text{23}}\)
= \(\frac{1.51 \times10^{23}}{6.023 \times10^{23}}\) 
\(\frac{1.51}{6.023}\)
Number of moles of 1.51 x 10²³ molecules of NH₄Cl = 0.25 mole

Mass of 1 mole of nitrogen \(\left(N_{2}\right)=2 \times N=2 \times 14=28 \mathrm{~g}\)
Mass of 3 moles of hydrogen \(\left(\mathrm{H}_{2}\right)=3[2 \times \mathrm{H}]\)
=3[2 x 1]=3[2]
=6 g
Mass of 2 moles of ammonia \(\left(\mathrm{NH}_{3}\right)=2\left[\mathrm{NH}_{3}\right]\)
=2[1 x N + 3 x H] 
=2[1 x14 + 3 x 1] 
=2[14 + 3] 
=2[17] = 34 g
\(\left.\begin{array}{c}1 \text { mole of nitrogen }(28 \mathrm{~g}) \\ + \\ 3 \text { mole of hydrogen }(6 \mathrm{~g})\end{array}\right\}=\text {2 moles of ammonia} (34 \mathrm{~g})\)

\(\text { Molar mass of } \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}=\mathrm{Al} \times 2+3[\mathrm{~S} \times 1+\mathrm{O} \times 4]\)
\(27 \times 2+3[32 \times 1+16 \times 4]\)
= 54 + 3[32 + 64]
= 54 + 3 [96]
Molar Mass = 54 + 288 = 342
\(\text { Mass } \% \text { of an element }=\frac{\text { mass of that element in the compound }}{\text { molar mass of the compound }} \times100\)
\(\text { Mass } \% \text { oxygen in } \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}=\frac{192}{342} \times 100 \)
\(=0.5614 \times 100=56.14 \% \)
\(\text { Mass } \% \text { of oxygen in } \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \text { is } 56.14 \%\)