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Avogadro’s Number

Avogadro’s Number. The Mole. Avogadro’s Number N A. Amadeo Avogadro (1766-1856) never knew his own number; it was named in his honor by a French scientist in 1909. Its value was first estimated by Josef Loschmidt, an Austrian chemistry teacher, in 1895. . Atom’s & Molecule’s Small Size.

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Avogadro’s Number

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  1. Avogadro’s Number The Mole

  2. Avogadro’s Number NA • Amadeo Avogadro (1766-1856) never knew his own number; it was named in his honor by a French scientist in 1909. Its value was first estimated by Josef Loschmidt, an Austrian chemistry teacher, in 1895.

  3. Atom’s & Molecule’s Small Size • Owing to their tiny size, atoms and molecules cannot be counted by direct observation. • Many scientists worked on the problem of counting atoms, formula units and molecules.

  4. Avogadro • The Avogadro constant, NA, is a fundamental physical constant that relates any quantity at the atomic scale to its corresponding macroscopic scale, inspired by the kinetic gas theory Avogadro proposed his hypothesis in 1811, in order to describe chemical reactions as an atomic process between atoms or molecules.

  5. Loschmidt • The review of methods aimed at finding a value for NA starts with the calculations made by Loschmidt (1865; NA 72×1023 mol-1) who evaluated the number of molecules in a given gas volume.

  6. Perrin • Consideration of Brownian motion (a random movement of microscopic particles suspended in liquids or gases resulting from the impact of the fluids surrounding the particles) led to some more accurate determinations of NA around the beginning of the 20th century (Perrin (1908); NA 6.7×1023 mol-1).

  7. Millikan • Other methods developed in the following years are based on Millikan's oil drop experiment (1917, NA 6.064(6)×1023 mol-1).

  8. Rutherford • Rutherford counted the of alpha particles emitted from radium or uranium. (Rutherford (1909); NA 6.16×1023 mol-1).

  9. Nuoy • Nuoy’s Investigations of molecular monolayers on liquids refined Avogadro’s number even further (Nuoy (1924); NA 6.004×1023 mol-1).

  10. Final Result • The combination of data from several independent measurements of the unit cell and the molar volumes has led to a value for the Avogadro constant of NA = 6.022 1335(30)×1023 mol-1 (De Bièvre et al 2001) recommended by the national metrology institutes involved in this research project (Becker 2001).

  11. Using Avogadro’s Number • A mole is a number. Like the number 12 for a "dozen“. They are both dimensionless (no units). • It is a huge number, far greater in magnitude than we can visualize. • Its practical use is limited to counting very tiny things like atoms, molecules, formula units, electrons, or photons.

  12. Comparing Moles & Dozens • One dozen roses = _____ roses. • ½ dozen roses = _____ roses. • 3.5 dozen roses = _____ roses. • 10 dozen roses = _____ roses. • One mole of roses = _____ roses. • ½ mole of roses = _____ roses. • 3.5 moles of roses = _____ roses. • 10 moles of roses = _____ roses.

  13. Conversion Factors & the Mole • If we want to find the number of roses in 7.5 dozen roses, we set up a conversion factor. A conversion factor is used to convert between units. Conversion factors always equal one. • 7.5 dozen roses x ___12 roses___ = one dozen roses • Since one dozen is equal to12, our conversion factor equals one.

  14. The Mole • If the mole relates to the microscopic world of atoms and molecules, how is it useful in the macroscopic world we live in? • The mole, commonly abbreviated mol, is the SI unit used to measure the amount of a substance.

  15. A Mole of Atoms • A mole is the number of particles in exactly 12 grams of a particular isotope of carbon (C-12). If you have 12.0 g of C-12 (carbon 12) you have 6.022 x 1023 carbon atoms. • How many moles of C do you have if you have 6.00 g of C?

  16. Elements & the Mole • For all elements, a mole is the atomicmassof the element expressed in grams. • If you have one mole of magnesium, how many Mg atoms do you have? • If you have 24.31 g of Mg, how many atoms of Mg do you have? • If you have one mole of Mg, what mass of Mg do you have?

  17. Compounds & the Mole • A mole of a compound is 6.02 x 1023 units of that compound (molecules or formula units). • The compound magnesium chloride is written ________. • How many formula units of MgCl2 would one mole of MgCl2 have?

  18. Conversion Factors & the Mole • We know one mole of any substance is 6.02 x 1023 particles of that substance. If you wanted to know how many Mg atoms 7.5 moles of Mg contained: • 7.5 moles Mg x 6.02 x 1023 atoms = 1 mole Mg

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