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

Molar Volume

 STP?

• standard temperature (0°C) or (273.15 k) and pressure (1 atmosphere)  
• where one mole of gas occupies 22.4L 

Molar Volume?

• the volume occupied by one mole of gas

 How to find Molar Volume

Conversion from Moles to Molar Volume

Conversion from Molar Volume to Moles

Isn't that a simple task!?

Here is an online worksheet with some questions! don't look at the answers before you're finished so you can challenge yourself! (:

Molar Volume worksheet!

Do questions 6, 9, 10, 11 and 12

Here's a game you can play with Molar Volume!

FUN GAME! (:

Molar Mass

• The mass (grams) of 1 mole of a substance is called the molar mass
• It can be determined from the atomic mass on the periodic table
• Measured in g/mol

So what exactly is Molar Mass? 
 Molar mass is the atomic weight of an element expressed in grams is the mass of 1 mole of element.


How do you determine Molar Mass in a Compound?
To determine the molar mass of a compound you must ADD all the mass of the atoms together

Here's an explanation that will help you understand what Molar mass is:
 Follow these steps to find molar mass! What is the molar mass of H20? (water)

1. Identify how many atoms of each of the elements are in the chemical formula. In the example there are 2 atoms of Hydrogen and there is 1 atom of Oxygen.
2. Look at the periodic table and search for the elements given & then look at each of their atomic masses. The elements in H20 are Hydrogen and Oxygen. The atomic mass for Hydrogen is 1.0 amu and the atomic mass for Oxygen is 16.0 amu
3. Now multiply the number of atoms found in the chemical formula with the atomic mass.                        2 atoms of Hydrogen --->2(1.0) + 1atom of Oxygen-----> 16.0= 18.0 g/mol

So now that you have a basic foundation of what Molar Mass is now its YOUR TURN to do some practice problems! Answers will be provided at the bottom of the last question. ( but don't look straight into the answers you need to TRY some problems and then check if u understand it)  :)

• What is the molar mass of NO2?

• What is the molar mass of NaCl?

• What is the molar mass of FeO?

• What is the molar mass of NaNO3?

Answers:
NO2: 14.0 + 2(16.0)= 142.1 g/mol
NaCl: 23.0 + 35.5= 58.5 g/mol
FeO: 55.8 + 16.0= 71.8 g/mol
NaNO3: 23.0 + 14.0 + 3|(16.0)= 85 g/mol


Mole Conversions
To convert between moles & mass we use molar mass as the conversion factor. You must also be sure to cancel the appropriate units.

Here's an example: How many grams is there in 1.5 mol of 02?


1.5 Mol x     32.0   g   = 48.0 g
                     1 mol

Now you TRY!

How many moles are there in 115g of Fe203?


A sample of HCL contains 0.54 mol. How many grams of HCl is this?