The mole and chemical composition
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The Mole and Chemical Composition. Relative Atomic Mass and Chemical Formulas. Average Atomic Mass. Why are the atomic masses of most elements on the periodic table not exact whole numbers?. These are relative atomic masses

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The Mole and Chemical Composition

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The mole and chemical composition

The Mole and Chemical Composition

Relative Atomic Mass and Chemical Formulas.


Average atomic mass

Average Atomic Mass

  • Why are the atomic masses of most elements on the periodic table not exact whole numbers?

  • These are relative atomic masses

  • Most have isotopes and are a mixture of these differently massed species.


Most elements are mixtures of isotopes

Most Elements are Mixtures of Isotopes

  • The periodic table reports Average Atomic Mass, a weighted average of the atomic mass of an element’s isotopes.

  • Example: If I said there was a larger abundance of C-12 than C-13, which value would the average atomic mass be closer to?


Calculating average atomic mass

Calculating Average Atomic Mass

  • If you know the abundance of each isotope, you can calculate the average atomic mass of an element.

  • The mass of a Cu-63 atom is 62.94 amu, and that of a Cu-65 atom is 64.93 amu. Abundance of Cu-63 is 69.17% and Cu-65 is 30.83%. Find the average atomic mass of Cu.


More practice

More Practice

  • Calculate the average atomic mass for gallium if 60.00% of its atoms have a mass of 68.926 amu and 40.00% have a mass of 70.925 amu.

  • Calculate the average atomic mass of oxygen. Its composition is 99.76% of atoms with a mass of 15.99 amu, 0.038% with a mass of 17.00 amu, and 0.20% with a mass of 18.00 amu.


Molar masses

Molar Masses

  • Review:

  • Calculate the molar mass for each of the following compounds:

    CsIC12H22O11HC2H3O2

    CaHPO4I2Mg3(PO4)2


Percent composition

Percent Composition

  • A Percent Composition is the percentage by mass of each element in a compound.

  • Just like your grade on a test

  • First, calculate the molar mass of the ENTIRE compound (total points possible).

  • Then, divide that by the mass of the part you are trying to find the percent for (your score).


Practice with percent composition

Practice with Percent Composition

  • Calculate the percent composition of iron in iron(III)oxide.

  • Calculate the percent of Nitrate in magnesium nitrate.


Empirical formulas

Empirical Formulas

  • The Empirical Formula is a chemical formula that shows the composition of a compound in terms of the relative numbers and kinds of atoms in the SIMPLEST RATIO.

  • How do we find this simplest ratio?


Empirical formulas1

Empirical Formulas

  • To calculate the empirical formula, you must first know the percent composition.

  • Using the percents, assume a 100g sample.

    • How will this help?

    • What units will this leave your percents in?


Empirical formulas2

Empirical Formulas

  • Now that we are in grams of each component (from the percent compositions), we need to convert these to moles.

  • Last step, divide each number by the LOWEST number to determine what the lowest ratio values will be.


Lets practice

Lets Practice!!

  • Chemical analysis of a liquid shows that it is 60.0% C, 13.4% H, and 26.6% O by mass. Calculate the empirical formula of this substance.


More practice1

More Practice!!

  • A dead alkaline battery is found to contain a compound of Mn and O. Its analysis gives 69.6% Mn and 30.4% O.

  • A compound is 38.77% Cl and 61.23% O.

  • A liquid compound is 18.0% C, 2.26% H, and 79.7% Cl.


Molecular formulas

Molecular Formulas

  • Molecular Formulas are multiples of Empirical Formulas.

  • A molecular formula is a chemical formula that shows the number and kinds of atoms in a molecule, but not the arrangement of the atoms.


Molecular formulas1

Molecular Formulas

  • The molar mass of a compound is equal to the molar mass of the empirical formula times a whole number, n.

  • Fill in the following table:


Practice with molecular formulas

Practice with Molecular Formulas

  • The empirical formula for a compound is P2O5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound.


Practice with molecular formulas1

Practice with Molecular Formulas

  • The empirical formula for a compound is P2O5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound.


More practice2

More Practice

  • A compound has an experimental molar mass of 78 g/mol. Its empirical formula is CH. What is its molecular formula?

  • A brown gas has the empirical formula NO2. Its experimental molar mass is 46 g/mol. What is its molecular formula?


The mole and chemical composition

Lets work on Page 253-254, 54-72 even. Odds would be excellent practice also.


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