Question

How much positive and negative charge is there in a water molecule ?

Answer 1

We know that water is a polar molecule as it has some amount of positive charge at one end of the molecule and some amount of negative charge at the other end of the molecule. The number of electrons in a neutral atom is equal to the number of protons. Use all these concepts to solve the question.

Complete step by step answer:
Let us assume we have a cup of water. Now we know that the molecular mass of water is $18\,gm$ (as molecular mass oh $H = 1$ and that of $O = 16$ therefore molecular mass of ${H_2}O$ comes out to be $1 + 1 + 16 = 18\,gm$). Now, each molecule of water contains $2 + 8 = 10$ electrons and protons. If we say that in the cup there are $n$ molecules of water then we can write as,
$n = \dfrac{{{N_A} \times m}}{{{m_{{H_2}O}}}}$
where ${N_A} = 6.022 \times {10^{23}}$ is the Avogadro constant.

Assume that the cup is 250 ml. By using this information we can calculate the value of $n$ which comes out to be,
$n = \dfrac{{6.022 \times {{10}^{23}} \times 250}}{{18}} \\\ \Rightarrow n= 8.36 \times {10^{24}}$
Now,
Total number of protons or electrons in that cup of water can be $x$ which equal to $n \times 10 \Rightarrow x = 8.36 \times {10^{25}}$
As the charge has a property which tells that the charge is always quantized, we can calculate the total positive or negative charge by using the formula of quantization.
$q = ne$
$\Rightarrow q= 8.36 \times {10^{25}} \times 1.6 \times {10^{ - 19}}\,C \\\ \therefore q= 1.33 \times {10^7}\,C$

Therefore, a water molecule has an equal amount of positive and negative charge which is $1.33 \times {10^7}\,C$.

Note: Water is formed by the sharing of electrons between two hydrogen atoms and one oxygen atom through the formation of a covalent bond. Due to this kind of sharing, the electrons spend more time moving around the oxygen atom which gives the oxygen side a bit more negative charge in comparison to that on the hydrogen side which makes up the other side of the molecule. Since the electrons are all spending more time with the oxygen atom, the hydrogen atoms are left with a slight positive charge which causes some difference in electronegativity and hence the molecule becomes polar in nature.