10 Minute Chemistry – The Mole and Molar Conversions Section 2: Relative Atomic Mass and Chemical Formulas Objectives: • Use a periodic table or isotopic composition data to determine the average atomic masses of elements. • Infer information about a compound from it chemical formula. • Determine the molar mass of a compound from its formula. Key Term: • Average Atomic Mass: Key Concepts and Outline Topics 1. Average Atomic Mass and the Periodic Table The masses on the periodic table are averages of the isotopes of each element. These averages are based on a percent abundance of that element. (See Chemistry Resources – List of the Elemental Isotopes) These isotopes are then averaged together and then weighed against carbon-12 in order to get a relative atomic mass. These relative atomic masses are in units of amu(atomic mass units) which we can think of as atom masses based on the number of protons, electrons and neutrons. a. Most Elements are mixtures of isotopes When elements are formed we know that they exist with a specific number of protons and depending on their charge more or less electrons. We also know that there exists another subatomic particle called the neutron – which until now has been left out of discussion. We are now able to bring back the neutron into discussion as it plays a large role in the determination of the ‘isotope’ of an element. A series of isotopes are a family of the same element and the share the same number of protons but will differ in the number of neutrons and exhibit different properties. We utilize carbon-12 as the reference atom and ‘declare’ it to have an atomic mass of 12 amu. We have tables that allow us to determine the relative atomic masses which we then factor in the percent abundance and sum the products to determine the average atomic mass. Example 1: Average Atomic Mass of Hydrogen Relative Atomic mass(RAM) of H-1 = 1.0078250321 amu, % abundance(RIC) 99.9885% Relative Atomic mass(RAM) of H-2 = 2.0141017780 amu, % abundance(RIC) 0.0115% RAM = Relative Atomic Mass, RIC = Relative Isotopic Composition (RAM H-1)*(RIC H-1) + (RAM H-2)*RIC H-2) = Average Atomic Mass of Hydrogen (1.0078250321)*(99.9885%) + (2.0141017780)*(0.0115%) = 1.0079430754 amu This answer does not have the correct number of significant figures, the accepted values are 1.008 amu or 1 amu(estimation purposes only) with 1.00794 is a generally accepted precise value.

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10 Minute Chemistry – The Mole and Molar Conversions 2. Chemical Formulas and Moles Chemical formulas are expression for the composition of a compound by telling us what is in a compound and how many of each atom is in the compound. It also tells us the general structure through a standardized formulation dependent on the subject area. We will see various nomenclature methods that differs from organic chemistry to inorganic chemistry as we are focused on different aspects of the molecule and we have more variation in the inorganic molecules.

a. Formulas express composition The chemical formula is a series of letters and numbers each with a specific purpose with brackets or parentheses thrown in every so often. They can be broken down by function: • Atomic Symbol • Charge • Functional Group or Polyatomic ions • Quantity

10 Minute Chemistry – The Mole and Molar Conversions closest noble gas is to the right. Many times brackets are used with superscripts to indicate overall charge on an the ion. We then use this overall charge to calculate the individual charges of each atom which is indicated by superscripts. Quantity We have subscripts that determine the quantity of the preceding atom or atomic group. Subscripts that exist on the outside of parentheses are distributed via multiplication to those atoms inside. If there are additional subscripts applied to individual atoms inside the parentheses they are multiplied with those outside. The absence of a subscript indicates there is only one atom. Example 1: Magnesium Chlorate, Mg(ClO3)2 is a herbicide that is relatively toxic

Mg(ClO3)2 Atomic Symbol for Magnesium (Capital M, Lowercase g)

Note () indicating the polyatomic ion ClO3

Subscript, that indicates the quantity of the oxygen atom (3)

Subscript outside the () indicates the quantity of the polyatomic ion. We can derive the qty of the individual atoms by distributing the subscript to each atom in the () using multiplication.

Formation of Ammonium Acetate, CH3COONH4

CH3COOH + NH3 → CH3COONH4 Functional Group

CH3COO- +NH4+ → CH3COONH4 Polyatomic Ion Charges

Note that even though there are polyatomic ions, they are not always indicated by parentheses. It is important to recognize them as they will help indicate a potential pathway to formation(reaction mechanism). This work is licensed under the Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA. Digital Versions can be found at crhsteacherpages.com under Chemistry A

10 Minute Chemistry – The Mole and Molar Conversions b. Formulas give ratios of polyatomic ions The chemical formulas that utilize polyatomic ions will use parentheses to indicate multiple groupings of polyatomic ions. This was seen in the example above in magnesium chlorate. The polyatomic ion chlorate has a -1 charge compared to the 2+ charge on the magnesium. In order for a stable product to form there must be a net neutral charge which is obtained by adding another chlorate ion. We could as easily neutralized the charge by adding another 1- ion however we would not obtain the compound magnesium chlorate. We differentiate the compound magnesium chlorate with the formula Mg(ClO3)2 from magnesium chloride with the formula MgCl2. The difference between is the addition of oxygen which changes the ending of chlorine from –ide, to –ite, to –ate depending on the quantity of oxygen atoms that are added to get to the neutral magnesium. c. Formulas are used to calculate molar masses Chemical formulae are used to calculate individual molar masses as they indicate the quantity of atoms within a compound. We utilize subscripts to increase the quantity of an indicated atom and multiply the quantity of an atom by its average atomic mass which is obtained from a periodic table. The Steps to Determining Molar Mass (units will always be grams/mole(g/mole)) • Atom Inventory(list the elements left to right, and then the qty of each) • Assign the atomic masses to each atom • Sum the series, with each series being the product of the qty of an atom by its atomic mass Example 1: Calculate the Molar mass of water, H2O 1. Atom Inventory H2O H – 2(atomic mass) O – 1(atomic mass) 2. Assign the atomic mass for each element H – 2(1.00794 g/mole) = 2.01588 g/mol O – 1(15.9994 g/mole) = 15.9994 g/mol = 18.01528 g/mole When reporting the molar mass of a compound it is essential to have the correct unit of grams/mole(g/mol) and that you report a reasonable number of significant digits. As a rule of thumb we will have 2 digits after the decimal point.

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