Complete combustion of 3g of ethane gives x × 10 ^ 22 molecules of...

${C}_{2}{H}_{6} + \dfrac{7}{2}{O}_{2} \to 2{CO}_{2} + 3{H}_{2}O$

${MW}_{{C}_{2}{H}_{6}} = 30g$

$30gof{C}_{2}{H}_{6}$ gives rise to $3$ moles of ${H}_{2}O$

$3g$of ${C}_{2}{H}_{6}$ gives rise to $0 . 3$moles of ${H}_{2}O$

No. of molecules $= 0 . 3 \times 6 . 023 \times {10}^{23}$
$= 18 . 069 \times {10}^{22}$