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3. ATOMS AND MOLECULES
medium
Calculate the molecular masses of $C_ 2H_4$, $NH_3$ and $CH_3OH$.
A
$28\,u $, $17\,u $ and $32\,u $
B
$33\,u $, $18\,u $ and $30\,u $
C
$44\,u $, $36\,u $ and $30\,u $
D
$4\,u $, $18\,u $ and $24\,u $
Solution
$(i)$ Molecular mass of $C_2H_4$ (ethene)
$=($ Atomic mass of carbon $\times 2 $ ) $+$ $($ Atomic mass of hydrogen $\times 4)$
$=(12 \times 2)+(1 \times 4)=24+4=28\,u $
$(ii)$ Molecular mass of $NH_3$ (ammonia)
$=($ Atomic mass of nitrogen $\times 1)+($ Atomic mass of hydrogen $\times 3)$
$=(14 \times 1)+(1 \times 3)=14+3=17\,u $
$(iii)$ Molecular mass of $CH_3OH$ (methanol or methyl alcohol)
$=($ Atomic mass of carbon $\times 1)+$ (Atomic mass of hydrogen $\times 3)+$ (Atomic mass of oxygen $\times 1$ ) + (Atomic mass of hydrogen $\times 1$ )
$=12+3+16+1=32\,u $
Standard 9
Science
