e: moles and molar masses the mole: the most important concept you’ve never heard about a mole of...
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E: Moles and Molar Masses
• The Mole: The most important concept you’ve never heard about
A mole of objects contains the samenumber of objects as there are in 12.0 gof Carbon-12
6.022x1023 somethings/mole = Avogadro’s Number
• 1 mole of any element always has 6.0221x1023 atoms in it
• 1 mole of elephants always has 6.0221x1023 elephants in it.
• In equations, you will frequently see the number of moles represented by the variable “n”
• 1 kmol = 1000 mol
• 1 mol = 1x10-6 mol• # of objects = n(NA) where NA is Avogadros’ number
Molar Mass• The molar mass is the mass of one mole of
material– Also referred to as molecular weight or formula weight
for compounds
• The molar mass of an element is the mass of one mole of its atoms
• The molar mass of a compound is the mass of a mole of its molecules.
• Molar mass (M) is usually given in g/mol
Mass of Sample = nM
• Remember: An element is a collection of isotopes present in different amounts
• The molar mass of an element takes this into account and is really an average of the different isotope masses scaled by their percentage abundance.
• We can use this to determine the percent composition of the elements of to calculate the average molar mass
M = matom averageNA
• To calculate the average molar masses of molecular and ionic compounds, we use the molar masses of the elements present
The Molar Mass of a Compound is the
sum of the molar masses of the elements
that make up the molecule or the formula
unit of the ionic compound
Determination of Chemical Formulas
• For hundreds of years, people knew that drinking willow bark tea would belp cure a headache
• When scientists identified a compound form Willow bark that was biologically active, they had to determine its structure before they could synthesize it.
• Determining the chemical formula is the first step in the structure determination process…
Chemical Formulas
• We will deal with 2 types of chemical formulas:
1. Empirical formula: Shows the relative numbers of each element present in the compound
2. Molecular formula: The precise number of atoms of each element present in the compound
Mass Percentage Composition
• The mass percentage composition of a compound tells us the percentage (by mass) that a given element comprises of the compound.
• We frequently determine the mass % composition using combustion analysis
Mass % Composition Summary:
Mass % composition is found by calculating the fraction of the total mass contributed by each element present in a compound and expressing the fraction as a percentage.
Determining Empirical Formula from Mass % Data
• To convert the mass % composition obtained from a combustion analysis into an empirical formula, we must convert the mass % of each type of atom into the relative number of atoms.– To do this, assume that we have 100g of
sample– The mass % will then be in grams
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