Question

20 grams of water contains how many number of molecules

Answer 1

Molecular mass:-

$\\ \sf\longmapsto H_2O$

$\\ \sf\longmapsto 2(1u)+16u$

$\\ \sf\longmapsto 2u+16u$

$\\ \sf\longmapsto 18u$

Now

1mole of water contains 18g of mass

Thus 20g of water will contain :-

$\\ \sf\longmapsto \dfrac{1}{18}\times 20moles$

$\\ \sf\longmapsto \dfrac{20}{18}moles$

Thus No of molecules:-

$\boxed{\sf No\:of\:molecules=No\:of\:moles\times Avagadro\:Number}$

$\\ \sf\longmapsto No\:of\:molecules=\dfrac{20}{18}\times 6.022\times 10^{23}$

$\\ \sf\longmapsto No\:of\:molecules=6.69\times 10^{23}molecules\:of\:H_2O$