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18 chemistry Thus, the formula mass of sodium chloride is This number of entities in 1 mol is so atomicmassofsodium+atomicmassofchlorine important that it is given a separate name and = 23.0 u + 35.5 u = 58.5 u symbol. It is known as ‘Avogadro constant’, or Avogadro number denoted by N A in honour Problem 1.1 of Amedeo Avogadro. To appreciate the Calculate the molecular mass of glucose largeness of this number, let us write it with (C6H12 )6molecule. all zeroes without using any powers of ten. 602213670000000000000000 Solution Molecular mass of glucose (C H O ) Hence, so many entities (atoms, molecules or 6 12 6 any other particle) constitute one mole of a = 6 (12.011 u) + 12 (1.008 u) + particular substance. 6 (16.00 u) = (72.066 u) + (12.096 u) + We can, therefore, say that 1 mol of hydrogen (96.00 u) atoms = 6.022 ×  1023atoms = 180.162 u 1 mol of water molecules = 6.022 × 10 23water 1.8 M oleconceptandMolarMass es molecules Atoms and molecules are extremely small 1 mol of sodium chloride = 6.022 ×1023formula in size and their numbers in even a small units of sodium chloride amount of any substance is really very large. To handle such large numbers, a unit of Having defined the mole, it is easier to know the mass of one mole of a substance convenient magnitude is required. Just as we denote one dozen for 12 items, or the constituent entities.The mass of one score for 20 items, gross for 144 items, we mole of a substance in grams is called its use the idea of mole to count entities at the molar mass. The molar mass in grams is microscopic level (i.e., atoms, molecules, particles, electrons, ions, etc). numerically equal to atomic/molecular/ formula mass in u. In SI system, mole (symbol, mol) was Molar mass of water = 18.02 g mol –1 introduced as seventh base quantity for the amount of a substance. Molarmassofsodiumchloride=58.5gmol –1 The mole, symbol mol, is the SI unit of amount of substance. One mole contains 1.9 P ercentage Composition 23 exactly 6.02214076 ×  10 elementary entities. So far, we were dealing with the number of This number is the fixed numerical value of entities present in a given sample. But many the Avogadro constant, N , Ahen expressed a time, information regarding the percentage in the unit mol –1and is called the Avogadro of a particular element present in a compound number. The amount of substance, symbol n, of a system is a measure of the number of is required. Suppose, an unknown or new compound is given to you, the first question specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. It may be emphasised that the mole of a substance always contains the same number of entities, no matter what the substance may be. In order to determine this number precisely, the mass of a carbon–12 atom was determined by a mass spectrometer and found to be equal to 1.992648 × 10 –23g. Knowing that one mole of carbon weighs 12 g, the number of atoms in it is equal to: 12g/mol 1C ▯23 12 1.992648 1▯ g/ Catom ▯ 6.0221367 ▯10 atoms/mol Fig. 1.11  One mole of various substances Reprint 2026-27