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3. ATOMS AND MOLECULES
medium
Calculate the molar mass of the following substances.
$(a)$ Phosphorus molecule, $P_4$ (Atomic mass of phosphorus $=31$ )
$(b)$ Hydrochloric acid, $HCl$
$(c)$ Nitric acid, $HNO_3$
A
$136\,u $, $40\,u $ and $63.5\,u $
B
$140\,u $, $48\,u $ and $63.5\,u $
C
$124\,u $, $36.5\,u $ and $63\,u $
D
$112\,u $, $38\,u $ and $70\,u $
Solution
$(a)$ Molar mass of $P_4$
$=4 \times$ Atomic mass of $P$
$=4 \times 31=124\,u $
$(b)$ Molar mass of $HCl$
$=$ Atomic mass of hydrogen $+$ Atomic mass of $Cl$
$= 1 + 35.5 = 36.5\,u $
$(c)$ Molar mass of $HNO_3$
$=$ Atomic mass of $H$ $+$ Atomic mass of $N$ $+$ $(3 \times $ Atomic mass of $O)$
$= 1 + 14 + (3 \times 16) = 15 + 48 = 63\,u $
Standard 9
Science
