Question

How much positive and negative charges are present in a cup of water?

Answer 1

Let us assume that the mass of a cup of water is 250 g. The molecular mass of water is 18 g. This means 18 g of water contains $6.02\times {10}^{23}$ molecules of water. Therefore, the number of molecules in one cup of water is $\frac{250}{18}\times 6.02\times {10}^{23}$.
Now, each molecule of water contains two hydrogen atoms and one oxygen atom, i.e., 10 electrons and 10 protons. Hence the total positive and total negative charges have the same magnitude. It is equal to $\left(\frac{250}{18}\right)\times 6.02\times {10}^{23}\times 10\times 1.6\times {10}^{-19}C=1.34\times {10}^{7}C$.
Here $(1.6\times {10}^{-19}C)$ is the magnitude of charge on each electron.