December 13 Lab Greg Sra

So in todays class we did a lab. The lab consisted of us melting down anhydrous salt in a crucible at finding out how much water evaporated, and what changes occurred during the heating. We found out that once the salt was heated it changed from dark blue colour to green like colour.

Anhydrous Salt

calculating empirical formula of organic compounds

This is the balanced chemical equation for the burning of CxHy

CxHy + 2 O2 -> CO2 + y/2 H20

We need to find x and y, which is the amount of moles of C in CO2 and H in H20

We are trying to find the empirical formula of a compound that burns to produce 16.9 g of CO2 and 3.46 grams of H20.

First, convert the mass ( grams)  of CO2 and H20 into moles.  You should get 0.384 moles of CO2 and 0.192 moles of H20

Secondly, we need to find out how many moles of C are in CO2 and how many moles of H are in H20.   There are 0.384 moles of carbon and hydrogen.  Because they are the same number, when you divide them you get 1. So, the empirical formula is CH.

There are review sheets with plenty of questions for you to practice.  There is a quiz next class.  Here are some videos with empirical formula and molecular formula

Percentages - Ahmad Kilani

Brief Summary:

So percentages are how much each element represent of the whole compount by mass.  Like H2O for example, has 88.9% oxygen, and 11.1% hydrogen.

Example:

- octane has a molar mass of 114g/mol... 96g/mol is carbon and 18g/mol is hydrogen:

(96/114) x 100% = 84.2% Carbon
(18/114) x 100% = 15.8% Hydrogen

December 1st- Nick Kim

 Today we have learned about empirical formula and molecular formula.
First empirical formula is the simplest whole number ratio of  atoms present in compound

How do we get empirical formula?
In order to find empirical formula we need to follow few steps
Let just say you have a compound containing 40% carbon, 53.3% oxygen and 6.7% hydrogen by mass

1,Start with the number of grams of each element, given in the problem. 

If percentages are given, assume that the total mass is 100 grams so that 
the mass of each element = the percent given.

Thus, 40% of carbon is 40g of carbon, and same process for all other elements
40g of carbon
53.3g of oxygen
6.7g of hydrogen

2,Convert the mass of each element to moles using the molar mass from the periodic table
(we have learned  molar conversion last week)
40g of carbon x 1mole/12.0g = 3.3333....... mole of carbon
53.3g of oxygen x 1mole/16.0g= 3.33125 mole of oxygen
6.7g of hydrogen x 1mole/1.0g= 6.7 mole of hydrogen

3,Divide each mole value by the smallest number of moles calculated.
3.3333....... mole of carbon
3.33125 mole of oxygen-smallest
6.7 mole of hydrogen

3.3333....... /3.3333....... =1
3.33125 /3.3333....... =1
6.7/3.3333....... =2

4,Round to the nearest whole number, if number is decimal or fraction.  This is the mole ratio of the elements and is  represented by subscripts in the empirical formula.

Empirical formula: CH2O

here is some practice questions
1, A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and 0.675 g H.  What is the empirical formula of the compound?
answer: Ca(OH)2

2,NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O.  Calculate the empirical formula of NutraSweet and find the molecular formula.  (The molar mass of NutraSweet is 294.30 g/mol)
answer: C14H18N2O5

Molecular formula
Molecular formula is a multiple of the empirical formula and shows th actual number of atoms that combine to form a molecule

molecular formula= molar mass of the compound/molar mass of the empirical formula

Molar mass of the compund: usually this number is given
Molar mass of the empirical formula: need to use the method that is above

Ex, Molecular formula of a compound is 92.0 g/mol and there are 40g of carbon,53.3g of oxygen and 6.7g of hydrogen

First you need to get emirical formula
do same thing as above information

40g of carbon x 1mole/12.0g = 3.3333....... mole of carbon
53.3g of oxygen x 1mole/16.0g= 3.33125 mole of oxygen
6.7g of hydrogen x 1mole/1.0g= 6.7 mole of hydrogen

3.3333....... mole of carbon
3.33125 mole of oxygen-smallest
6.7 mole of hydrogen

3.3333....... /3.3333....... =1
3.33125 /3.3333....... =1
6.7/3.3333....... =2

Empirical formula: CH2O
 To get molar mass, we simply need to look at periodic table
12.0+(1.0)2+6.0
=20.0g/mol

back to furmula
molar mass of the compound/molar mass of the empirical formula
92.0 g/mol/20.0g/mol
=4.6
4.6(CH20)
usually there arent decimal. Sorry lol

here are some videoes that will help
http://www.youtube.com/watch?v=r2Log6-voWo
http://www.youtube.com/watch?v=nslC7lOSc7Y