Question

Calculate the molar mass of the following substances.
(a) Ethyne, $C_2H_2$
(b) Sulphur molecule, $S_8$
(c) Phosphorus molecule, $P_4$ (Atomic mass of phosphorus= 31)
(d) Hydrochloric acid, $HCl$
(e) Nitric acid, $NO_3$

Answer 1

**(a) Molar mass of **$\bf{C_2H_2}$ = (2 × Atomic mass of C) + (2 × Atomic mass of H)
= (2 × 12) + (2 × 1) = 26 u

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**(b) Molar mass of **$\bf{S_8}$ = (8 × Atomic mass of S)
= 8 × 32 = 256 u

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**(c) Molar mass of **$\bf{P_4}$ = 4 × Atomic mass of P
= 4 × 31 = 124 u

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**(d) Molar mass of **$\bf{HCI}$ = Atomic mass of hydrogen + Atomic mass of CI
= 1 + 35.5 = 36.5 u

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**(e) Molar mass of **$\bf{HN0_3}$ = Atomic mass of H + Atomic mass of N + (3 × Atomic mass of 0)
= 1 + 14 + (3 × 16)
= 15 + 48 = 63 u