Empirical Formulas & Molecular Formulas

* Are the simplest formulas of compounds


Ex. Empirical Molecular
* C4H9 - C8H18
* NO2 - N2O4
* CH7O2 - C3H21O6


* They show only the simplest rations, not the actual atoms.
* Molecular formulas give the actual # of atoms…
* To determine an empirical formula we need to know the ratio of each element. This is why we use the table below; to gather all the information we may need to solve each problem.

Atoms	Mass	Molar Mass	Moles	Mol/ Smallest Mole	Ratio
C	8.4	12.0	0.7	2
H	2.1	1.0	2.1	6
O	5.6	16.0	0.35	1

Step # 1. Fill in the chart with what’s already given to you (the names and masses of the three atoms, plus their molar masses which you can find on the periodic table).

Step #2. Calculate the number of moles simply by following the conversion chart.
* 8.4g x 1mol / 12g = 0.7 mol
* 2.1g x 1mol / 1g = 2.1mol
* 5.6g x 1mol / 16g = 0.35mol

Step # 3. Find the smallest mole (0.35mol in this case) and divide it into itself and the rest of the mole #’s.


* 0.7 / 0.35 = 2
* 2.1 / 0.35 = 6
* 0.35 / 0.35 = 1
* If your answers are not “whole #’s you must multiply everything by a common #. For example:
* Say you end up with the numbers 1.0, 1.5, and 6.0… you would use the number 3 to change 1.5 into a whole # while still keeping the other two numbers whole as well. Thus you would end up with 2.0, 3.0, and 12.

= C2H6O

Atom	Mass	Molar     Mass	Moles	Mole/ Smallest mole	Ratio
Pd	42.56	106.4	0.3992	1
H	0.80	1.0	0.80	2

Step # 3.  You must be given the compound’s molar mass (in this case it’s 216.8 g/mol) in order to figure out its molecular formula.  Besides the molar mass, you also know from your previous calculations that the empirical formula is PdH2. You then calculate the empirical formula’s molar mass (by looking at the periodic table).  In this case it is 108.4.  You can then divide 216.8 by 108.4 to figure out what number to use for the molecular formula’s subscripts.  It is 2… thus you change Pd into Pd2 and H2 into H4.  And there you have your molecular formula!  


Informational and VERY helpful video:

Density

* Density is a measure of mass / volume
* When calculating density the following chart makes the task a lot easier to understand…


Density
* (D = m / v)
(Molar Mass) (Molar Volume)
Mass ------ Moles ------ STP

*
(Avogadro’s #)

Molecules
*
(Subscripts)
*
Atoms


* Here are some examples / solutions to density problems:


Ex. Compound “X” has a density of 3.0 g/ml. Determine the mass of 12 mL of X. How many moles are in 12 mL of X…?

Step # 1. 12 mL x 3g /1 mL = 36g


Step # 2. H2O ~ 2(1.0) + 16.0 = 18.0g


Step # 3. 36g x 1 mol / 18.0g = 2.0 mol


Ex. An unknown compound has a molar mass of 73.0 g/mol. If 0.15 mol occupies a volume of 50 mL, determine the compound’s density.

Step # 1. 0.15 mol x 73.0g / 1mol = 10.95g

Step # 2. 10.95g / 50mL = 0.219 g/mL ~ 0.22 g/mL

Cool Video :)

Moles to Atoms

To go to atoms from moles you must use Avogrado's number (6.02 x 10^23)

1. How many atoms are in 1.5 mol of Iron:

     1.5 mol x 6.02x 10^23 atoms = 9.0 x 10^23 atoms
                           1 mol

2. How much water molecules are there in 0.65 mol?

    0.65 mol x 6.02 x 10^23 molec  = 3.9 x 10^23 molec
                            1 mol

3. How many H2, Hydrogen atoms are there? How many Oxygen atoms? In H20

   i) 3.9 x 10^23 molec x 2 atoms = 7.8 x 10^23 atoms
                                         1 molec

  ii) 3.9 x 10^23 molec x 1 atoms  = 3.9 x 10^23 atoms
                                         1 molec

4. A cylinder of Helium contains 4.6 x 10^30 atoms. How many moles of Helium?
      4.6 x 10^ 30 atoms x 1mol   = 7.6 x 10^6 mol
                                       6.02 x 10^23 atoms


How many C atoms? How many H atoms? How many O atoms? How many in total? in CH3COOH?

1. 3.0 x 10^24 molec x 2 C atoms  = 6.0 x 10^24 C atoms
                                       1 molec

2. 3.0 X10^24 molec x 4 H atoms  = 1.2 x 10^25 H atoms
                                       1 molec

3. 3.0 x 10^24 molec x 2.0 atoms = 6 x 10^24 O atoms

4. 6.0 x 10^24 + 1.2 x 10^25 + 6 x 10^24 atoms = 2.4 x 10^25 atoms

Mole Conversions

Mole Conversions:
(Converting between Grams and Moles)


* To convert between moles & mass we use molar mass as the conversion factor
* Be sure to cancel the appropriate units!
* Use the chart below to help you with your conversions

Density
* (D = m / v)
(Molar Mass) (Molar Volume)
Mass ------ Moles ------ STP
*
(Avogadro’s #)

Molecules
*
(Subscripts)
*


Atoms

Ex. How many grams are there in 3.0 mol of O2?
* O2 = 2 x 16.0 ~ 36g / mol
* 3.0 mol x 32g / 1mol = 96.0g


Ex. Determine the # of moles of C5H12 that are in 362.8g of the compound.
* 5(12.0) + 12(1.0) = 72g / mol
* 362.8g x 1mol / 72g = 5.039mol


Ex. How many moles of magnesium bromide contain 5.38 x 10^24 formula units?
* 5.38 x 10^24molec x 1mol / 6.02 x 10^23molec = 8.39mol


Ex. Find the mass of 0.89mol of CaCl2.
* 40.1 + 2(35.5) = 111.1g / mol
* 0.89mol x 111.1g / 1mol = 99g


Ex. Find the mass of 0.159mol of SiO2.
* 28.1 + 2(16.0) = 60.1g / mol
* 0.159mol x 60.1g / 1mol = 9.56g

Percent Composition

Don't worry! we have easy instructions on how to plow through percent composition and become a professional at it! (:

Percent Composition - The percent composition of a component in a compound is the percent of the total mass of the compound that is due to that component.

****The percentage by mass of an element in a compound is always the same*****

To find the percentage by mass, determine the mass of each element present in one mole.

The percent composition (percentage composition) of a compound is a relative measure of the mass of each different element present in the compound.




To calculate the percent composition of a component in a compound:


1. Find the molar mass of the compound by adding up the masses of each atom
in the compound using the periodic table.
2.Calculate the mass due to the component in the compound you are for which you are solving by adding up the mass of these atoms.
3.Divide the mass due to the component by the total molar mass of the compound and multiply by 100.

Remember this!

Percent composition = mass due to specific component/ total molar mass                                                    of compound x 100%

here's a video on percent composition