Question

How to make chemical formula for merging cation and anion.

Answer 1

Ionic compounds are those compounds in which cation and anion are combined together to form an ionic bond. Among cation and anion, the electrostatic force of attraction is present which is one of the strongest forces of attraction among the primary forces between the molecules.

Complete step by step answer:
In nature elements exist either in native form or in combined form. In combined form they either share electrons or transfer their electrons to form a chemical bond. Those elements which transfer the electron form an ion and the combination of an ion forms an ionic compound or molecules.
There are two types of ions which are formed during the transfer of electrons. An element which loses electrons forms a positive charge species called cation while the element which gains electrons forms a negative charge species called anion. When the cation and anion combine together then they form a compound called an ionic compound.
The charge present on cation depends upon the number of electrons lost by an element and the charge present on anion depends upon the number of electrons gained by an element.
For example,
a. Sodium atoms contain one electron in its valence shell so it readily loses one electron to form sodium cation.
$\operatorname{Na} \to N{a^ + } + {e^ - }$
b.Chlorine atom contains seven electrons in its valence shell and it requires only one electron to complete its octet and forms a chloride anion.
$\operatorname{Cl} + {e^ - } \to C{l^ - }$
Finally sodium cation and chloride anion combine to form an ionic compound and due to this process large amounts of energy are also released which is known as Lattice energy.
Final answer:
The number of water molecules in $2.5$ mole of water $= 15.06 \times {10^{23}}$ water molecules.

Note:
Mole can also be find out by many ways
Number of moles $=$ mass of given atom / gram atomic mass
Number of moles $=$ mass of given molecule/ gram molecular mass
Number of moles $=$ Given volume of gas at STP / molar volume of $22.4$ Liter