Percent Composition

Percent Composition: The percent in mass that a certain element in a compound contains.  In other words, the percentage of the molar mass that is taken up by a certain element.

          % Composition =    mass of element      *100
                                      mass of compound


What percentage of glucose (C6H12O6) is oxygen?

Molar mass of Glucose:
        6  C @ 12g/mol =     72g/mol
       12 H @   1g/mol =     12g/mol
        6  O @ 16g/mol =  + 96g/mol
                                        180g/mol

Calculate Percent Composition

  96g/mol    *100 = 53.3 %
180g/mol  

Now add the step.  How about if there is more then just one molecule?  Pull out you mole map because this is where you will need it!

What is the percent composition of 5.00g of Vitamin A (C20H30O)?

Step 1:  Molar mass:
      20  C  @  12g/mol = 240g/mol
      30  H  @    1g/mol =   30g/mol
        1  O  @  16g/mol = +16g/mol 
                                        286g/mol

Step 2:  Percentage of total molar mass
  240g/mol C     *100= 83.9%              30g/mol H     *100= 10.5%             16g/mol O     *100= 5.6%                  
286g/mol total                                    286g/mol total                                   286g/mol total

Step 3: Check your work
(always be sure to check that the percentages that you came up with add up to 100%)

  83.9%
  10.5%
 + 5.6%
  100.0%

Now one question is done.  Yes, I know it is a long process.  Going through all the steps is very helpful though and reminds you what is going on.  Do them even when it does start feeling tedious!


Now that you have seen some examples, try out this quiz!!!  Make sure you have some scrap paper and a periodic table at hand!

Percent Composition Quiz

Calculating the Empirical & Molecular Formula

What is Empirical formula:

• Empirical formula gives the LOWEST TERM RATIOS of atoms or moles
• All formula of ionic compounds are empirical
• e.g. NaCl, MgO, Al2O3

How to determine the Empirical formula given the mass of each atoms?

2 simple steps: 

1. Convert all the mass of elements (in grams) to moles
2. Find the simplest whole number ratio between these amounts

e.g. Determine the empirical formula of a 100 g compound of phosphorus and oxygen that contains a 43.64% of phosphorus by mass.

Solution:

In 100 g, there are 43.64 g of phosphorus and 56.36 g (100 - 43.64 g) of oxygen.

Amount of phosphorus = 43.64 g / 30.97 g mol-1 = 1.409 mol

Amount of oxygen = 56.36 g / 16.00 g mol-1 = 3.523 mol

The whole number ratio of phosphorus to oxygen bay be found by dividing through by the smaller number:

Ratio of P : O = 1.409 : 3.523 = 1 : 2.5 (dividing by 1.409) = 2 : 5 (multiplying by 2)

Therefore, the empirical formula is: P2O5

What is Molecular formula:

• Molecular formula gives all atoms which makes up a molecule
• Ionic or covalent compound formulas can be molecular
• e.g. C3H8, Na2CO3

IMPORTANT EQUATIONS!!!

Molecular formula (MF) = Empirical formula (EF) x Whole Number (N)

MF mass = EF mass x N

Mass of one mole = EF mass (in grams) x N

Tool kit

Here is an awesome online empirical formula calculator.  You can check if your answers are correct by typing in the mass percentage of element and the name of the element.

http://firstyear.chem.usyd.edu.au/calculators/empirical_formula.shtml

Practice questions

If you want to practice on solving empirical formula problems, check out these:

http://kurtniedenzu.cmswiki.wikispaces.net/file/view/Empirical+Formula+Worksheet+5.pdf

http://www.fremont.k12.ca.us/cms/lib/CA01000848/Centricity/ModuleInstance/6112/Empirical_WS.pdf

Two Step Mole Conversions

Two Step Conversions: Grams --> Formula Units/particles/molecules/atoms
The process is much the same except now you have to go two steps.  If you want to go from grams to formula units/particles/molecules/atoms then you multiply the number by 1 mol/grams and by 6.022 x 10^23/1 mol.

Ex.  There are 250g worth of pennies.  How many molecules of atoms of copper are present?
Atomic mass= 58.9
molar mass= 58.9g/mol

250g x   1 mol    x 6.022 x 10^23 -- 2.6 x 10^24 molecules C
           58.9g/mol         1 mol

Ex.  An Olympic sized pool has 2.5 x 10^9 g of water in it.  How many atoms of oxygen are present in an Olympic sized pool?
water=H2O
molar mass= 2 O @ 16.0
                     1 H @   1.0
                                        =33g/mol

2.5 x 10^9g x 1mol x 6.022 x 10^23 x   2 atoms   -- 9.1 x 10^31 atoms O
                      33.0g         1 mol            1 molecule  

Two Step Conversions: Formula Units/particles/molecules/atoms --> Grams
If you want to go from formula units/particles/molecules/atoms to grams you go through a similar process.  This time you multiple the number of molecules by 1 mol/6.022 x 10^23 and then by grams/1 mol.  If it is atoms and you have to account for multiple atoms in each molecule, you have to account for that and divide as necessary.

Ex. There are 300 atoms in a sheet of aluminum.  What is its mass in grams?
atomic mass= 27.0
molar mass  = 27.0g/mol

300 atoms x         1 mol        x 27.0g -- 1.3 x 10 ^-20g
                     6.022 x 10^23    1 mol

Ex. There are 2500 molecules of table salt (NaCl) in a salt shaker.  How many grams of salt is there present?
molar mass= 1 Na @ 23.0
                     1 Cl  @ 35.5
                                        = 58.5g/mol

2500 molecules x        1 mol        x 58.5g -- 2.4 x 10^-19g
                            6.022 x 10^23   1 mol

Mole Conversions

Particle/Atom/Formula Unit ---> Moles

ex. How many moles of Ag are present in 3.0 x 10¹⁶ atoms of Silver?
3.01 x 10²⁴ Ag atoms x         1 mol Ag               
                                     6.022x10²³ Ag atoms
= 5.0 x 10^-8 mol Ag

Moles ---> Particles/Atoms/Formula Units
ex. How many atoms are present in 2 moles of Potassium?
2 mol K x 6.022 x 10²³ atoms K
                        1 mol K
=1x10²⁴ mol K

Moles ---> Grams
ex. What is the mass in grams of 3.5 moles of Sulfur?
3.5 mol S x 32.1 g S
                   1 mol S
= 110 g S

Grams ---> Moles
ex. How many moles are there in 2.49 grams of Tin?
2.49 g Sn x   1 mol Sn   
                   118.7 g Sn
= 0.0210 mol Sn

The Mole

The mole is a unit, like dozen. It is equal to the number of carbon-12 atoms, in 12 grams of carbon.
1 Mole is also known as Avogadro's number. It creates a simpler way for chemists to count atoms and molecules.
Avogadro's number = 6.022 x 10²³ KNOW THIS NUMBER!!!!
Keep in mind that equal volumes of a different gasses have consistent values.


Relative Mass
Written as a comparison against the mass of another object.
  amu = atomic mass unit
Avogadro's Hypothesis
Explains that equal volumes of different gasses that are currently at the same temperature and pressure will  have the same number of particles.

Li. 7.0                    N. 14.0

=>It therefore states that if they have the same number of particles, the mass ratio is due to the mass of the particles.

Atomic Mass

Mass of the atoms of an element. Generally written in amu's, (or u's, of dalton's)

              Eg: Carbon. 16.0 amu

Formula Mass

Applies only to ionic compounds. Sum of the mass of atoms, written in amu's.

              Eg: NaCl - Na = 23.0  + Cl = 35.5 => 58.5 amu

Molecular Mass

Applies only to covalent compounds. Sum of the mass of atoms, written in amu's.

              Eg: CH4 (Methane) C = 12 + H4 = 1 * 4 = 4 => 16 amu

Molar Mass

Molar mass is the mass of one Mole of each element. (written in grams/mole)

              Eg: 1 Mole of Fe = 55.8 g/mol, 1 mole of Cs = 132.9 g/mol

Can also apply to the atomic/molecular/formula mass of any pure substance.

REMEMBER: ALL HAVE THE SAME NUMBER OF PARTICLES!

Graphing in Excel 2010

A graph represents the relationship between two variables.  The horizontal axis and the vertical axis each show the measurements for the two different variables.

This post will just illustrate the basic steps to making a decent scatter point graph.  

Step 1: Type in your data.  Highlight it with your courser so as all the data you want in your graph is selected. 

Step 2:  Go to the top of the page and look at the menu bar.  In the section that lists the different types of graphs, there is an icon labeled scatter.  Pull your courser over it and you should get a small window showing the options below.

After you select the format that you would like, then you will get a graph. The dots will not necessarily be in a straight line like what is shown below.  The graph will represent the data that you entered.

Step 3: Now that you have a simple graph, you can add a trend line.  If you right click a data point, this menu will come up.  Then you select the trend line option.

Step 4:  Now that you have gotten to this menu, you have quite a few options.  First, you can choose the best tread line for your graph.  If your line is straight, then linear would be best where as if it is curved, you would want to choose polynomial etc.  At the bottom of this first menu, there is also a box.  Beside it says display equation.  If you check this box, you get a slope equation put on your graph.  There is also several options on the side of the menu which you can adjust to make your graph more visually appealing.

Step 5: Your graph now has the absolute basics and maybe even is looking a little bit colorful. Its now time to change your tittle and add axis labels.  To get to the menu that allows you to change the

. Step 6:  Select the label chart button to change the tittle.  The icon for adding axis labels is also located here.

You can also just click on the current tittle and change it to whatever you would like it to be

Step 7:  Click the label axis icon and go down to the horizontal axis.  From here you can add on.  A text box will show up underneath your horizontal axis and you can change the label to what is fitting for the graph you are creating.

Step 8:  Now that you have a horizontal axis label, go back and do the same thing except go to vertical axis.  There are a few different options as to how you can format that (words turned to be written sideways, horizontal beside data, or words written vertically).

Once you have choosen this, you have a basic excel scattered point graph

Lab 2E: Determining the Aluminum Foil Thickness

Objectives
1. to calculate the thickness of a sheet of aluminum foil and express the answer in terms of proper scientific notation and significant figures

Supplies
Equipment
3 rectangular pieces of aluminum foil (minimum 15cm x 15cm)
metric ruler
centigram balance


Procedure
1. measure the length and width of each piece of foil and record the measurements in Table 1
2. use a centigram balance to find the mass of each piece of aluminum foil and record the masses

Table 1