Answer

(i) Molecular mass of $H 2$ (hydrogen)

$ \begin{aligned} & =\text{ Atomic mass of hydrogen } \times 2 \\ & =1 \times 2=2 u \end{aligned} $

(ii) Molecular mass of $O 2$ (oxygen)

$=$ Atomic mass of oxygen $\times 2$

$=16 \times 2=32 u$

(iii) Molecular mass of $Cl 2$ (chlorine)

$ \begin{aligned} & =\text{ Atomic mass of chlorine } \times 2 \\ & =35.5 \times 2=71 u \end{aligned} $

(iv) Molecular mass of $CO 2$ (carbon dioxide)

$ \begin{aligned} & =(\text{ Atomic mass of carbon } \times 1)+(\text{ Atomic mass of oxygen } \times 2) \\ & =12+(16 \times 2)=12+32=44 u \end{aligned} $

(v) Molecular mass of $CH 4$ (methane)

$=($ Atomic mass of carbon $\times 1)+($ Atomic mass of hydrogen $\times 4)$

$=12+(1 \times 4)=12+4=16 u$ (vi) Molecular mass of $C2H 6$ (ethane)

$=($ Atomic mass of carbon $\times 2)+($ Atomic mass of hydrogen $\times 6)$

$=(12 \times 2)+(1 \times 6)=24+6=30 u$

(vii) Molecular mass of $C 2 H 4$ (ethene)

$=($ Atomic mass of carbon $\times 2)+($ Atomic mass of hydrogen $\times 4)$

$=(12 \times 2)+(1 \times 4)=24+4=28 u$

(viii) Molecular mass of $NH 3$ (ammonia)

$=($ Atomic mass of nitrogen $\times 1)+($ Atomic mass of hydrogen $\times 3)$

$=(14 \times 1)+(1 \times 3)=14+3=17 u$

(ix) Molecular mass of $CH 3 OH$ (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 $\times 1)$

$=12+3+16+1=32 u$

Answer

(i) Formula unit mass of $ZnO$ (zinc oxide) $=$ $65+16=81 u$

(ii) Formula unit mass of $Na 2 O$ (sodium oxide) $=$

$(23 \times 2)+(16 \times 1)=46+16=62 u$

(iii) Formula unit mass of $K 2 CO 3$ (potassium carbonate) $=$

$(39 \times 2)+(12 \times 1)+(16 \times 3)=78+12+48=138 u$