Question

How many water molecules are present in one mL of water (d=1g/mL)?

Answer 1

Step 1: Calculating the moles of water

Given:

$d=1g/mL$

$\text{Volume}=1mL$

Since, $\text{Density}=\frac{\text{Mass}}{\text{Volume}}$Density=MassVolume\text{Density}=\dfrac{\text{Mass}}{\text{Volume}}

$\text{Mass}=Density\times Volume$

$\text{Mass}=1g/mL\times 1mL=1g$

The molecular mass of water $=2\left(1\right)+16=18g$

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molecular mass}}$

$\text{Number of moles}=\frac{1}{18}moles$

Step 2: Calculating the number of molecules in 1mL water

According to Avagadro's hypothesis

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

$\frac{1}{18}$ moles of water contains $=\frac{1\times 6.022\times {10}^{23}}{18}=3.34\times {10}^{22}molecules$

Hence, the no. of molecules of water in 1mL of water $=3.34\times {10}^{22}$