1. Chapter 8: Chemical composition
Chemistry 1020: Interpretive chemistry
Andy Aspaas, Instructor

2. Atomic masses
Atomic mass unit, amu: a very small unit of mass in which masses of atoms and molecules are given
A single carbon-12 atom has mass of 12 amu
But, since there are different naturally occurring isotopes of carbon (carbon-12, carbon-13, carbon-14), the average mass of a carbon atom is larger than 12 amu
Average atomic mass (atomic weight) of carbon is amu
The weighted average of all isotopes in a natural sample

3. The mole
2 objects = 1 pair, 12 objects = 1 dozen
Grouping can allow for large quantities to be more easily counted
Laboratory-sized samples of chemicals contain a very, very large number of atoms or molecules
The “mole” is a unit chemists use to represent very large numbers of particles (atoms, molecules, ions)
Could be used for anything though, just like pair, dozen

4. What is a mole?
1 mole (mol) defined as number of atoms in exactly 12 g of carbon-12
Avogadro’s number, NA = x 1023 particles/mole
Just another quantity like
Pair = 2 objects
Dozen = 12 objects
Gross = 144 objects
Ream = 500 objects
Mole = 6.02 x 1023 objects

5. How big is Avogadro’s number?
1 mole of chemistry textbooks would cover the surface of the earth to a depth of 300 km
If you won 1 mole of dollars when you were born and spent a billion dollars per second, % would still be left at 90 years old
A mole of pennies placed side by side would stretch more than a million light years

6. Converting between particles and moles
Avogadro’s number is the same no matter the type of particles
Use 1 mol = x 1023 particles as a conversion factor
Number of particles in mol carbon?
(Same as 5.00 mol Fe, H2O, or anything!)
Number of moles in 5.21 x 1024 Al atoms?

7. Converting between moles and mass
Since 1 mol = number of atoms in 12 g of carbon-12,
1 mol of natural carbon atoms has a mass of g
The mass of 1 mol of any element is equal to its atomic weight (from the periodic table) in grams!
1 mol Fe = g, 1 mol Na = g, etc
Use values like this from the periodic table as conversion factors

8. Molar mass
Idea extends past just atoms
Once can calculate mass of 1 mol of any molecule too!
Just add up atomic weights of all atoms in the molecule, and you get the molar mass of that compound
Molar mass of H2O = 2(1.008) = g/mol

9. Mass percent
Sometimes useful to know composition of a compound in terms of the masses of elements involved
Mass percent: mass of 1 element in 1 mol of the compound divided by mass of 1 mol of the entire compound, times 100%
Percents are just fractions multiplied by 100
Practice: mass percentages of each element of ethanol, C2H5OH

10. Empirical and molecular formulas
Empirical formula: formula that describes the simplest ratio of elements in a compound
Molecular formula: formula that describes the actual number of atoms of different elements in a single molecule
Ex. Butane
Molecular formula: C4H10 (actual number of atoms in molecule)
Empirical formula: C2H5 (simplest whole-number ratio)

11. Calculating empirical formulas
If given masses of elements in a compound, convert each mass to moles
Then divide all mole values by the smallest one to get the ratios of atoms in the compound
If all the ratio values are integers, they become subscripts in the empirical formula
If there are non-integer values, multiply them all by the smallest integer to make them all integers

12. Empirical formula calculation practice
Determine the empirical formula of the following:
A sample of phosphoric acid contains g hydrogen, g phosphorus, and g oxygen
A sample of para-dichlorobenzene contains g carbon, g hydrogen, and g chlorine.

13. Empirical formula from percent composition
If you’re given percent composition, assume you have a 100-g sample, and convert percentages directly to grams
Ex. 52.5% carbon becomes 52.5 g carbon
All should add up to 100 g since all percentages must add up to 100%
Then, work as before by converting to moles and finding ratios

14. Calculation of molecular formulas
If you know the molar mass, you can convert an empirical formula into a molecular formula
Calculate empirical formula mass, and divide by molar mass
(molar mass) / (empirical formula mass) = n
Multiply n by subscripts in empirical formula to get molecular formula
Ex. empirical formula = CH, molar mass = 78