Apr. 4 Percent Yield & Percent Purity

 Percent Yield


What is percent yield? It is the ratio.  A ratio of what? It is a ratio of amount of product OBTAINED to the amount of product EXPECTED by calculation.  It must be expressed at a %. 

Here is an example:
By using this reaction: C3H6Cl2 + 2 KSCN -> C3H6(SCN)2 + 2KCL

The percent yield is 52.0%.  How many grams of C3H6(SCN)2 will be produced if 1.5 g of C3H6Cl2 is reacted with excess KSCN?

First, we got to calculate the amount of C3H6(SCN)2 expected to be produced when 120 g of C3H6Cl2 reacts.

1.5 g C3H6Cl2 x 1 mol C3H6Cl2   x 1 mol C3H6(SCN)2  x  158 g C3H6(SCN)2
                            113 g C3H6Cl2     1 mol C3H6Cl2             1 mol C3H6(SCN)2

= 2.10 g of C3H6(SCN)2

Now that we have the amount of C3H6(SCN)2 expected, we have the bottom of the fraction, the predicted mass of the product.  We know that the end percent yield is 52%, we will have to solve for the top.

% yield = C3H6(SCN)2 obtained  x 100% = 52.0 % yeild
                2.10 g of C3H6(SCN)2

52.0 x 2.10 = 109.2/100 = 1.09 g of C3H6(SCN)2 obtained.

Percent Purity
It is the ratio of the mass of impure substance to the mass of impure substance.  The fraction is just like percent yield but on top is mass of pure substance over mass of impure substance all multiplied by 100.

Example:
If a 5.00 g sample of iron ore contains 0.25 g of iron metal, what is the percent purity?

Mass of pure substance     x 100 = percent purity
Mass of impure substance

0.25 g Iron         x 100% = 5.00%
5.00 g Iron ore

Percent Yield:

Mar. 3 Stoichiometry with grams to grams and moles to moles

We can use stoichiometry when doing calculations involving moles and grams.  We just need to know what to multiply and divide.

Ex. How man moles of O_{2 will be formed when 4.1 moles of} H₂O is decomposed?

4.1 mole H₂O  x 1 mole O_{2            =  2.1 moles} O<sub>2
                                     2 moles of H2O

For these questions you always put the number of moles decomposed in the front of the equations and divide it by the number of moles of the element you are looking from the compound.  In this example we are looking for how many moles of</sub>  O_{2 was formed.  We know that there is 4.1 moles of water and that there are 2 moles of} O<sub>2 in water so we divide by two. We get 2.1 moles as the answer.


We can do calculations involving particles -moles - mass


Ex. How many grams of Zinc will be needed to react with 5.20 moles of hydrochloric acid?


First we must write out the equation and we must balance it.


1 Zn + 2 HCL -> 1 ZnCl2 + 1 H2


Then we can do the calculation


5.20 moles HCI x 1 mole Zn     x  65.4 g Zn
                            2 mole HCL     1 mole Zn

= 170 g Zn


*when converting to grams to grams, we must go through this pattern: grams to moles to moles to grams


Ex.How many grams of NO is produced if 20 g of O2 is combined with excess ammonia?
4 NH3 + 5 O2 -> 6 H2O + 4 NO


20 g O2 x 1 mole O2  x  4 mol NO  x  30.01 g NO
                 32 g O2         5 mol O2      1 mol NO


= 15 g  
As you can see the fractions are placed in such an order that the tops and bottoms will cancel out and at the end cancel out to what you are looking for.  In the case, it cancels out in the end to NO.</sub>



Here is a video showing the calculations:

Mar. 1 Stoichiometry?!?!?!??!

NEW CHAPTER!!!!!!!!!!!!! We are starting with Stoichiometry.  It is the part of chemistry where we deal with the quantitative relationship that exists between the reactants and products in a chemical reaction. Normally in balanced equations, the relationships between the reactants and products form ratios of whole numbers.  
Eg. N₂ + 3H₂ → 2NH_{3
The ratio for this would be 1:3:2
1 molecule of nitrogen (N2) reacts with 3 molecules of hydrogen (H2) to produce 2 molecules of ammonia (NH3)}

Here is a link to a stoichiometry exercise:http://schools.cbe.ab.ca/b858/dept/sci/teacher/yancey%5CReview.book%5Cmole-quantityproblems.htm

Here is a YouTube video: