Question

What is the number of neutrons in a drop of water $ \left( 20drops=1ml \right) $ at $ 4{}^\circ C~ $ ?

Answer 1

Hint: We know that we have already learnt that the number of electrons in an atom is equal to the number of protons. And the number of protons along with the number of neutrons is equal to the mass number of an atom.

Complete answer:
The basic thing that we know is the number of electrons in an atom is equal to the number of protons. And the number of protons along with the number of neutrons is equal to the mass number of an atom or we can say that the number of neutrons in an atom can be calculated by subtracting the proton number from its atomic mass. To calculate the number of neutrons, we can subtract the mass number of the atom from the atomic number of that atom. We also know that atomic number is represented as $ Z $ and atomic mass as $ A, $ neutrons are represented as $ N $ and protons are represented as $ P. $
Given, $ 20drops\text{ }=1ml $ thus $ 1\text{ }drop=\dfrac{1}{20}\text{ }ml $ and Density of water at $ 4{}^\circ C=\text{ }1gm/ml $
Thus $ \dfrac{1}{20}ml\text{ }=\dfrac{1}{20}gm $
We know that Molecular Mass of $ ~{{H}_{2}}O $ is $ 18\text{ }gm $
 $ 1\text{ }gm\text{ }{{H}_{2}}O=\dfrac{1}{18}moles $
Thus, $ \dfrac{1}{20}\text{ }gm\text{ }{{H}_{2}}O=\dfrac{1}{18}moles\times \dfrac{1}{20}moles $
Here we know that, $ 1mole=6.023\times {{10}^{23}}molecules $
 $ \dfrac{1}{18}\times \dfrac{1}{20}moles=6.023\text{ }\times {{10}^{23}}\times \dfrac{1}{18}\times \dfrac{1}{20}molecules $
Thus, one molecule of $ {{H}_{2}}O $ has $ 8 $ neutrons.

Note:
Remember that the neutrons are tiny particles found within the nucleus in all atoms except hydrogen. There are Neutron stars present that are the result of a massive collapsed star. They are the smallest and dense stellar objects known. The first neutron star that was discovered with planets is the PSR.