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QUANTITTIES IN CHEMISTRY: THE MOLE, MASS AND MOLAR MASS

QUANTITTIES IN CHEMISTRY: THE MOLE, MASS AND MOLAR MASS. MASS. Mass of an atom = mass of neutrons + mass of protons (mass of electrons can be ignored because their mass is relatively extremely small ) The mass of an electron is 10 000 times smaller then that of a proton or neutron.

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QUANTITTIES IN CHEMISTRY: THE MOLE, MASS AND MOLAR MASS

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  1. QUANTITTIES IN CHEMISTRY: THE MOLE, MASS AND MOLAR MASS

  2. MASS Mass of an atom = mass of neutrons + mass of protons (mass of electrons can be ignored because their mass is relatively extremely small) The mass of an electron is 10 000 times smaller then that of a proton or neutron. To find the mass of an object, enough weights are placed on the other arm to achieve a balance We compare the mass of an object with a standard mass: the kilogram

  3. The Mass of One Atom • Mass of individual atom cannot be measured the same way simply because they are too small to be seen and grabbed • We need a different standard mass to measure mass of an atom. • In the past, the mass of a hydrogen atom (having 1 proton, 1 electron and no neutron) was chosen as the standard mass for comparison and was set as 1

  4. The Mass of One Atom • If the mass of Hydrogen is set as 1, the atomic mass of sulphur that is relative to hydrogen is therefore 32. However, hydrogen is NO LONGER used as the standard mass; instead, element carbon is used as the standard mass

  5. Carbon is the Standard for Mass Carbon has 2 naturally occurring isotopes : C -12 and C -13 All Carbon atoms have 6 protons, C -12 has only 6 neutrons C -13 has 7 neutrons The C-12 isotope is chosen as the standard mass to compare the mass of all other elements to, replacing the use of hydrogen

  6. Isotopic Abundance • Isotopic abundance is another way of stating the relative mass of each isotope in sample of a given element. • The isotopic abundance of a sample of an element is given as percentages. Ex. A sample of Chlorine might have… 75.5 % Cl-35 and 24.5 % Cl-37

  7. Calculating Average Atomic Mass Isotopic abundance can be used to calculate the Average Atomic Massof a sample of a particular element such as Chlorine…. If we have 1 000 atoms of chlorine then… 75.5% of 1 000 atoms gives us 755 atoms of Cl-35 & 24.5% of 1 000 atoms gives us 245 atoms of Cl-37 Total Mass = (755 x 35 u) + (245 x 37 u) = 26 425 u + 9065 u = 35 490 u Average Mass = 35 490 u / 1 000 atoms = 35.49 u

  8. Using Terms to Represent Numbers We use the term dozen to represent the number 12……. • One dozen of eggs has 12 eggs • One dozen of tables has 12 tables • One dozen of sheep has 12 sheep • One dozen of anything has 12 items in it

  9. One Dozen = 12 Ex. 12 Eggs:

  10. Other Terms that RepresentA Particular Quantity A Score of Test Tubes = 20 Test Tubes A Pair of Socks = 2 socks

  11. A Dozen isn’t Big Enough • When dealing with atoms, because of their extremely small size, a dozen is no longer convenient to use • Instead, we will use something to represent a much, muchLARGER number… … THE MOLE

  12. How Much is One Mole? A mole is what is used to represent the number 6.022 x10 23 or 602 200 000 000 000 000 000 000 000 • One mole of tables is 6.022 x10 23 tables • One mole of cookies is 6.022 x10 23 cookies in it • One mole of oxygen atom is 6.022 x10 23 oxygen atoms in it • One mole of water is 6.022 x10 23 water molecules in it • One mole of anything has 6.022 x10 23 items in it • Symbol of mole: mol and is used as a unit. Ex. 2.0 mol • This number, 6.022 x10 23is called Avogadro’s number (symbol: NA) • Question:  Which has more atoms, a mole of helium or a mole of copper atoms? • Did you know one mole of eggs will cover the entire surface of Earth to a depth of ~60 km?

  13. How many moles are there…. • if there are 12.044 x10 23 batteries ? • If there are 6.022 x10 23particles of chalk dust on the chalk ledge? • if there 3.011 x10 23 pages in all the books in the school?

  14. How many moles are there…. • if there are 12.044 x10 23 batteries ? Answer: 2 moles • If there are 6.022 x10 23particles of chalk dust on the chalk ledge? Answer: 1 mole • if there 3.011 x10 23 pages in all the books in the school? Answer: 0.5 mole or half a mole

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