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Mole Concept

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Published in: Physics
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Molecular mass,formula mass explained with examples

Mayank K / Delhi

3 years of teaching experience

Qualification: M.Sc. (Chemistry) IIT DELHI

Teaches: Chemistry

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  1. Molecular Mass and Formula Unit Mass ' An atom contains three types of particles i.e. electrons, protons, and neutrons. Out of these three particles, the mass of electrons is negligible as compared to that of protons and neutrons. Thus, the mass of an atom is equal to the mass of the total number of neutrons and protons present in it. ' Molecules are formed when two or more atoms combine chemically in a fixed proportion.The molecular mass of a substance is the sum of the atomic masses of all the atoms present in a molecule of that substance. This is also called the relative molecular mass and its unit is atomic mass unit (u). Hence, to calculate the mass of molecules i.e. to calculate the molecular mass, the mass of all the atoms present in it are added.
  2. Calculation of molecular mass: ' Molecular mass of sodium hydroxide and potassium sulphate. ' 1) The chemical formula of sodium hydroxide is NaOH ' Atomic mass of Sodium (Na) = 23 u ' Atomic mass of Oxygen (O) = 16 u ' Atomic mass of Hydrogen (H) = 1 u ' Since sodium hydroxide molecule contains one atom of sodium, one atom of oxygen, and one atom of hydrogen. ' Thus, the molecular mass of NaOH = 23 + 16 + 1 , = 40 u
  3. ' 2) The chemical formula of potassium sulphate is K2S04 ' Atomic mass of Potassium (K) = 39 u ' Atomic mass of Sulphur (S) = 32 u ' Atomic mass of Oxygen (O) = 16 u ' Since potassium sulphate molecule contains two atoms of potassium, one atom of sulphur, and four atoms of oxygen. ' The molecular mass of K2S04 = 39 x 2 + 1 + 16 x 4 0=78+32 +64 = 174 u
  4. Formula unit and formula unit mass ' The term formula unit is used for those substances whose constituent particles are ions. The formula unit mass of a substance is the sum of the atomic masses of all the atoms in the formula unit of a compound. ' For example, calcium oxide has a formula unit CaO.
  5. Calculation of formula unit mass Formula unit mass is calculated in the same way as molecular mass. ' formula unit mass of nitric acid and sodium oxide. ' 1) The chemical formula of nitric acid is HN03 ' Atomic mass of Hydrogen (H) = 1 u ' Atomic mass of Nitrogen (N) = 14 u ' Atomic mass of Oxygen (O) = 16 u ' In one formula unit of HN03, there are one hydrogen (H) atom, one nitrogen (N) atom, and three oxygen (O) atoms. ' Thus, the formula unit mass of HN03 = 1 x 1 + 14 x 1 + 3 = 1 + 14 + 48 0=63 u
  6. ' 2) The chemical formula of sodium oxide is Na20 ' Atomic mass of sodium (Na) = 23 u ' Atomic mass of oxygen (O) = 16 u ' In one formula unit of Na20, there are two sodium (Na) atoms and an oxygen (O) atom. ' Thus, the formula unit mass = 2 x 23 + 1 x 16 = 46 + 16 0=62 u
  7. The Mole Concept ' One mole of a substance (atoms, molecules, ions, or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams. One mole of any substance contains 6.022 x 1023 particles (atoms, molecules, or ions). This means that one mole atom of any substance contains 6.022 atoms. Similarly, one mole molecule of any substance contains 6.022 molecules, and one mole ion of any substance contains 6.022 ions. Hence, the mass of a particular substance is fixed. The number 6.022 x 1023 is an experimentally obtained value and is known as Avogadro's number or Avogadro constant (represented by No). It is named after the Italian scientist, Amedeo Avogadro
  8. ' 1 mole atoms of an element has a mass equal to the gram atomic mass of that element. Hence, the mass of 1 mole molecules is equal to its molecular mass in grams. The atomic mass of an element is the mass of its atom in atomic mass units (u). By taking the same numerical value of atomic mass and changing the units from 'u' to 'g', the mass of 1 mole atoms of that element is obtained. The mass of one mole of atoms is known as the molar mass of atoms, gram atomic mass, or gram atoms. For example, the atomic mass of nitrogen is 14 u, and the gram atomic mass of nitrogen is 14 g. Similarly, the molecular mass of a substance gives the mass of a molecule of that substance in atomic mass units (u). Therefore, as discussed earlier, by taking the same numerical value of molecular mass and by changing its units from 'u' to 'g', the mass of one mole molecules of that substance can be obtained. ' Therefore, the mass of one mole molecules of any substance is equal to the gram molecular mass of that substance.
  9. The relationship between the mole, Avogadro's number, and mass is summarized as follows: I mole of any panicle (atom, molecule, ion) 6022 x 1023 Relative mass of those particles 'in number Of atoms/molecules/ions grams
  10. Examples 0 0 000 8) Calculation of the mass of 0.5 mo les of N 2 gas: The molecular mass of nitrogen is 28 u. Mass = Molar mass x Number of moles Mass = 28 g x 0.5 = 14 g 2) Calculation of the number of moles present in 18.066 x 1023 Particles of nitrogen: mole = 6.022 x 1023 given no. of particles no. of moles avogadro number 18.066 x 1023 6.022 x 1023 Therefore. floe ber of no oles is Q

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