Question

: The total number of electrons present in ${\mathbf{18}}\;{\mathbf{ml}}$ water is:
A.$6.02 \times {10^{23}}$
B.$6.02 \times {10^{22}}$
C.$6.02 \times {10^{24}}$
D.$6.02 \times {10^{25}}$

Answer 1

In this question, we have to calculate the number of electrons in water. We shall first calculate the mass of the water from its density. Then, calculate the number of moles of water present and thus the total molecules of water. Then, we shall multiply the number of electrons present in one molecule of water to the total number of molecules.
The formula used:
$\,{\text{No}}{\text{. of moles = }}\dfrac{{{\text{Mass of the Substance in grams}}}}{{{\text{Molar mass of a Substance}}}} = \dfrac{{{\text{Number of Atoms or Molecules}}}}{{6.022 \times {{10}^{23}}}}$

Complete step by step answer:
The number of the electron in one molecule of ${H_2}O = 2 + 8 = 10$
The density of water is ${\text{1g}}\,{\text{m}}{{\text{l}}^{{\text{ - 1}}}}$.
$\therefore 18\;{\text{ml}}$ means ${\text{18\;g}}$
No. of moles of water = $\dfrac{{18}}{{18}} = 1$
This means that the no. of molecules will be $= 1 \times {{\text{N}}_{\text{A}}} = {{\text{N}}_{\text{A}}}$
The total number of electrons in this amount of water will be $= 1 \times {{\text{N}}_{\text{A}}} \times 10 = 10{{\text{N}}_{\text{A}}} = 6.02 \times {10^{24}}$

Hence, the correct answer to this question is option C.

Note:
The concept that a mole of any substance contains the same number of particles was formed out of research which was conducted by Italian physicist Amedeo Avogadro.
Avogadro constant can be defined as the number of molecules, atoms, or ions in one mole of a substance: $6.022 \times {10^{23}}$ per mol. It is derived from the number of atoms of the pure isotope $^{{\text{12}}}{\text{C}}$ in 12 grams of that substance and is the reciprocal of atomic mass in grams.
Now the mole concept can be applied to ions and formula units. The mathematical formula to calculate the number of moles present is: ${\text{moles = }}\dfrac{{{\text{mass}}}}{{{\text{molar mass}}}} = \dfrac{{{\text{number of particles}}}}{{6.022 \times {{10}^{23}}}}$
The molar is defined as the mass of 1 mole of any substance and is different for each element and compound. For ex – molar mass of hydrogen is 1, while molar mass of oxygen is 16.