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3. ATOMS AND MOLECULES
medium
Calculate the molecular masses of $CO_2$, $CH_4$ and $C_2H_6$.
A
$33\,u$, $18\,u$ and $30\,u$
B
$6\,u$, $18\,u$ and $36\,u$
C
$9\,u$, $36\,u$ and $84\,u$
D
$44\,u$, $16\,u$ and $30\,u$
Solution
$(i)$ Molecular mass of $CO_2$ (carbon dioxide)
$=($ Atomic mass of carbon $\times 1)+($ Atomic mass of oxygen $\times 2)$
$=12+(16 \times 2)=12+32= 44\,u$
$(ii)$ Molecular mass of $CH_4$ (methane)
$=($ Atomic mass of carbon $\times 1)+($ Atomic mass of hydrogen $\times 4)$
$=12+(1 \times 4)=12+4=16\,u$
$(iii)$ Molecular mass of $C_2H_4$ (ethene)
$=($ Atomic mass of carbon $\times 2 $ ) $+$ $($ Atomic mass of hydrogen $\times 6)$
$=(12 \times 2)+(1 \times 6)=24+6=30\,u$
Standard 9
Science
