Atoms, Molecules & Stoichometry (I)

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  • Atoms, Molecules & Stoichometry (I)

    1. 1. Relative Atomic Mass Scale <ul><li>What is the Relative Atomic Mass Scale ( 12 Carbon Scale)? </li></ul><ul><li>masses of atoms are expressed relative to </li></ul><ul><li>the mass of a Carbon-12 atom. </li></ul><ul><li>Why is there a need for the scale? </li></ul><ul><li>atoms are too small to be weighed directly. - inconvenient to express masses of atoms in terms of kg. </li></ul><ul><li>So what does it imply? </li></ul><ul><li>- 1 atom of 12 C = 12 - 1/12 the mass of a 12 C atom = 1 </li></ul>© my-chem-tutor.blogspot.com Note: Relative atomic masses have no units (dimensionless) since they are relative to the arbitrary standard (i.e. ratio). However, in more advanced texts, relative masses are represented in terms of atomic mass units (u) 12 C 12 “units”
    2. 2. (1) Relative Isotopic Mass - the mass of one atom of an isotope of an element relative to 1/12 the mass of an atom of carbon-12 isotope. (2) Relative Atomic Mass, Ar - the average mass of one atom of an element relative to 1/12 the mass of an atom of carbon-12 isotope. - the weighted average of the relative isotopic masses of the element e.g. Ar of Cl = 75/100 x 35 + 25/100 x 37 = 35.5 Definitions © my-chem-tutor.blogspot.com
    3. 3. (3) Relative Molecular Mass, Mr - the average mass of one molecule of a compound relative to 1/12 the mass of an atom of carbon-12 isotope. - sum of the individual Ar of the atoms in the molecular formula e.g. Mr of CH 4 = 12 + (1 x 4) = 16 (4) Relative Formula Mass - the average mass of one formula unit of ionic compound relative to 1/12 the mass of an atom of carbon-12 isotope. © my-chem-tutor.blogspot.com CH 4 16 “units” Relative Masses may be visualized as the number of “balls” or “units” required to balance the isotope, atom, molecule or ionic compound. TIP
    4. 4. Mole & Avogadro’s Constant Mole - is the amount of substance which contains the same number of particles ( atoms, molecules, ions, electrons etc ) as there are atoms in exactly 12g of 12 C. Avagadro’s constant, L - is the exact number of carbon atoms in 12g of 12 C. (i.e. L= 6.02 x 10 23 ). Molar Mass - the mass of one mole of a substance (g/mol). © my-chem-tutor.blogspot.com Mole may be visualized as just another quantitative term! TIP 6.02 x 10 23 units A Mole 12 units A Dozen 2 units A Pair

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