1 mol is equal to molecular mass of molecule.so 1mol is equal to 18gm here and also 1 mol contain 6.02x10^23 molecules. hence 18 gm of H2O also contain 6.02x10^23 molecules.

Calculation of the number of molecules in 18mg of water in terms of Avogadro number NA

Introduction
The Avogadro constant, denoted by NA, is the number of atoms or molecules in one mole of substance. One mole of a substance is defined as the amount of that substance that contains as many entities (atoms, molecules, or other particles) as there are atoms in 12 grams of pure carbon-12. The value of NA is approximately 6.022 x 10^23 entities per mole.

Step 1: Calculation of the molecular weight of water
The molecular weight of water is the sum of the atomic weights of its constituent atoms. The molecular formula of water is H2O, which means it contains two hydrogen atoms (atomic weight = 1) and one oxygen atom (atomic weight = 16). Therefore, the molecular weight of water is:

Molecular weight of water = 2 x Atomic weight of hydrogen + Atomic weight of oxygen
= 2 x 1 + 16
= 18 g/mol

Step 2: Calculation of the number of moles of water in 18mg
To calculate the number of molecules of water in 18mg, we first need to convert 18mg to grams. There are 1000 milligrams (mg) in one gram (g). Therefore:

18mg = 18/1000 g = 0.018g

The number of moles of water in 0.018g can be calculated using the formula:

Number of moles = Mass / Molecular weight

Number of moles of water = 0.018g / 18g/mol = 0.001 moles

Step 3: Calculation of the number of molecules of water
The number of molecules of water can be calculated using the formula:

Number of molecules = Number of moles x Avogadro constant

Number of molecules of water = 0.001 moles x 6.022 x 10^23 molecules/mol = 6.022 x 10^20 molecules

Therefore, there are approximately 6.022 x 10^20 molecules of water in 18mg.

Conclusion
In conclusion, the number of molecules in 18mg of water in terms of Avogadro number NA is approximately 6.022 x 10^20 molecules. This calculation can be applied to any substance to determine the number of molecules present in a given mass of that substance.