Concept of mole

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Concept of mole

  1. 1. Concept of Mole 1. A mole is defined as the amount of substance which contains the Avogadro Number of particles. 2. The Avogadro Number (or Avogadro Constant) is defined as the number of atoms in 12 g of the carbon-12 isotope, which is equal to 6.02 x 1023. 1 mole = 6.02 x 1023 Example: 1 mol of atoms = 6.02 x 1023 atoms ½ mol of atoms = 3.01 x 1023 atoms ¼ mol of atoms = 1.505 x 1023 atoms. Note: The particles in a material can be atoms, molecules or ions. Therefore 1 mol potassium atom = 6.02 x 1023 potassium atom 1 mol carbon dioxide molecules = 6.02 x 1023 of carbon dioxide molecules. 1 mol sulphate ions = 6.02 x 1023 sulphate ions. Example: Find the number of atoms in: a. 2 molferum b. 3.6 molzink c. 2.8 molzink d. ¼ molferum Answer: a. 2 mol iron = 2 x 6.02 x 1023 = 1.204 x 1024 iron atoms b. 3.6 molzink = 3.6 x 6.02 x 1023 = 2.167 x 1024 zink atoms c. 2.8 molzink = 2.8 x 6.02 x 1023 = 1.686 x 1024 zink atoms d. ¼ mol iron = ¼ x 6.02 x 1023 = 1.505 x 1023 iron atoms. Mole and mol 1. Mole is the unit of amount of substance. 2. “mol” is the symbol of mole.
  2. 2. Number of Mole and Number of Particles 1. We have just learn that, mole is a quantity, and it is equal to 6.02 x 1023. The number 6.02 x 1023 is called the Avogadro constant. 2. Therefore, if we are given the number of mole of substance, and asked to find the number of particles (atoms, molecules or ions) in it, we multiply the number of mole by the Avogadro constant. 3. Likewise, if we are given the number of particles, and asked to find the number of mole of the particles, we divide the number of particles by the Avogadro constant. Example Which contains more atoms, 1 mol of helium or 1 mol of uranium? Which has a greater mass? [ RAM: He=4; U=238 ] Answer: 1 mol of helium and 1 mol of uranium has equal number of atoms. The mass of one Uranium atom is greater than the mass of one helium atom. Example Find the number of atoms in 2.5 mol of gold. Answer: Number of atoms = Number of mole x Avogadro constant = 2.5 x 6.02 x 1023 = 1.505 x 1024 Example How many moles of magnesium that contain 2.76 x 1023 of magnesium atom? Answer: Number of mole = Number of atoms ÷ Avogadro constant
  3. 3. = 2.76 x 1023 ÷ 6.02 x 1023 = 0.46 mol Number of Mole of Atoms and Number of Mole of Molecules 1. All molecules contain more than one atom. 2. For example, in a carbon dioxide molecule (CO2), there are 3 atoms - 1 carbon atom and 2 oxygen atoms. 3. Therefore, in 5 carbon dioxide molecules, there will be 15 atoms - 5 carbon atom and 10 oxygen atoms. 4. Similarly, in 1 mole carbon dioxide molecules, there will be 3 mole atoms - 1 mole carbon atoms and 2 mole oxygen atoms, and in 3 mole carbon dioxide molecules, there will be 9 mole atoms - 3 mole carbon atoms and 6 mole oxygen atoms, so on and so forth. Example: a. How many hydrogen molecules can be made with 20 mol of hydrogen atoms? b. How many hydrogen atoms are there in 20 mol of hydrogen molecules? Answer: a. Each hydrogen molecules (H2) consist of 2 hydrogen atoms. Therefore, 20 mol of hydrogen atoms can make 10 mole of hydrogen molecules. b. Number of mole of hydrogen atoms in 20 mole of hydrogen molecules = 2 x 20 mole = 40 mole. Example: a. Find the number of carbon atom in 0.75 mol of carbon dioxide. b. Find the number of oxygen atom in 0.75 mol of carbon dioxide. Answer: a. Each carbon dioxide molecules (CO2) consist of 1 carbon atom and 2 oxygen molecules. Therefore, 0.75 mole of carbon dioxide contain 0.75 mole carbon atoms.
  4. 4. b.Number of mole of oxygen atoms in 0.75 mole of carbon dioxide = 2 x 0.75 mole = 1.50 moles. Number of Mole and Mass of a Substance Molar Mass 1. The mass of one mole of atoms is its relative atomic mass in grams, and is called a molar mass. 2. A Molar mass is the mass of a substance containing the Avogadro Constant of particles. Element Hydrogen Carbon Oxygen Copper Iron Relative atomic mass 1 12 16 64 56 Mass of 1 mol of particle 1g 12 g 16 g 27 g 56 g Molar Mass 1 g mol-1 12 g mol-1 16 g mol-1 27 g mol-1 56 g mol-1 Example The relative atomic mass of copper is 64, therefore the mass of 1 mole copper is64g 1. The RAM of sodium is 23, therefore the mass of 2 mole sodium is 46g 2. The RAM of nitrogen is 14, therefore the mass of 2.5 mole l nitrogen is 35g 3. The RAM of calcium is 20, therefore the mass of 0.3 mole calcium is 6g Conclusion Note: The relationship between the mass of a substance and the number of mole of
  5. 5. the particles in the substance can be summarised by using the following equation: n=m / Molar Mass If you are given the mass of substance and asked to find the number of mole of the substance (or vice versa), the problem can be solved by using this equation. Example: Find the number of mol of atoms in 4.6g sodium [Relative atomic mass: Na=23] Answer: Number of mole, N=4.6 / 23=0.2mol Example How many moles of each substance are there in 191 g NaOH [Relative atomic mass: Na=23, O=16, H=1] Answer: Relative Formula Mass of NaOH = 23 + 16 + 1 = 40 Number of mole of 191g NaOH, n=191 / 40=4.775mol Question What is the mass of 7.12 molNaI. [Relative atomic gas: Iodine = 131; Sodium = 23] Answer: The relative formula mass of NaI = 23 + 131 = 154. The mass of 7.12 molNaI = Number of mole x Relative Formula Mass of NaI = 7.12 x 154 = 1096.48g
  6. 6. Number of Mole and Volume of Gas Molar Volume of Gas 1. Equal amount of gas occupies same volume of space. 2. Molar volume of a gas is defined as the volume occupied by one mol the gas at certain condition 3. At room temperature(25°C) and pressure (1 atm), one mole of any gas occupies a volume of 24 dm³ (24 000 cm³ ) 4. At standard temperature(0°C) and pressure (1 atm) one mole of any gas occupies a volume of 22.4 dm³ (22400cm³). Number of mole of gasses Volume at s.t.p. (dm3) 1 mol of O2 gas 1 x 22.4 = 22.4 1 mol of H2 gas 1 x 22.4 = 22.4 2 mol of He gas 2 x 22.4 = 44.8 3.5 mol of N2 gas 3.5 x 22.4 = 78.4 Number of mole of gasses Volume at r.t.p. (dm3) 1 mol of O2 gas 1 x 24 = 24 1 mol of H2 gas 1 x 24 = 24 2 mol of He gas 2 x 24 = 48 3.5 mol of N2 gas 3.5 x 24 = 84 Example: What volume (at room temperature and pressure r.t.p.) would 2 moles of
  7. 7. oxygen gas occupy? (Molar Gas Volume at r.t.p. = 24 dm³) Answer: Volume of gas = 2 x 24 = 48 dm³ Example: A sample of ozone gas has a volume of 960cm³ at room temperature and pressure. Find the number of mole of the ozone. [Molar volume at r.t.p. = 24.0dm³] Answer: Number of mole=960cm3 / 24000cm3=0.04 mol

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