Question

The density of water at room temperature is 1.0 g/mL. How many molecules are there in a drop of water if its volume is 0.05 mL?

Answer 1

D=m/v
so,D×v=m
1×0.05=m
m=0.05
so no of molecules=0.05/18×6.022×10^23

Answer 2

"From the given,

At room temperature, density of water = 1.0 g/ml

Volume of a drop of water = 0.05 ml

Mass of a drop of water $=\quad volume\quad \times \quad density$

$=\quad (0.05\quad ml)\quad \times \quad (1.0\quad g/ml)$

$Mass\quad =\quad 0.05\quad g$

$Gram\quad molecular\quad mass\quad of\quad water =\quad 2\quad \times \quad 1\quad +\quad 16\quad =\quad 18g$

$18 g\quad of\quad water = 1 mol$

$\therefore \quad 0.05\quad g\quad of\quad water\quad =\quad 1\quad mol\quad \times \quad (0.05\quad g)\quad =\quad 0.0028\quad mol$

1 mole of water contains molecules $= 6.022\quad \times \quad { 10 }^{ 23 }$

The number of molecules in 0.0028 mole of water =$\quad (6.022\quad \times \quad { 10 }^{ 23 })\times 0.0028\quad mol$

$=\quad \quad 1.68\quad \times \quad { 10 }^{ 21 }\quad molecules$"