Question

If the mass of one drop of water is 0.1 g then what is the number of molecules present in it?​

Answer 1

Given that the mass of one drop of water is 0.1g.

$\longrightarrow\sf{m=0.1\ g}$

The no. of moles present in it is given by the no. of moles in the given amount of water multiplied by the Avogadro Number, $\sf{N_A=6.022\times10^{23}.}$

And the no. of moles is the mass of the water taken divided by the gram molecular mass (GMM) of water.

$\longrightarrow\sf{No.\ of\ moles=\dfrac{Given\ mass\ (in\ grams)}{GMM}}$

$\longrightarrow\sf{n=\dfrac{m}{M}}$

GMM of water $\sf{(H_2O)}$ is $\sf{18\ g.}$

$\longrightarrow\sf{M=18\ g}$

Then no. of moles is,

$\longrightarrow\sf{n=\dfrac{0.1}{18}}$

Or,

$\longrightarrow\sf{n=\dfrac{1}{180}}$

Then the no. of molecules is,

$\longrightarrow\sf{N=n\times N_A}$

$\longrightarrow\sf{N=\dfrac{1}{180}\times6.022\times10^{23}}$

$\longrightarrow\sf{\underline{\underline{N=3.346\times10^{21}}}}$