Monday March 8th, 2010

To start the class we learnt a new term,

Percent Yield

With this term, there are two types of such meaning,

1.Actual Yield: The amount of product that actually forms is carried out in the lab.
2.Theoretical Yield: the maximum amount of product that could formed from given amounts of reagents.

An example: Xgrams + Ygrams= 200g in a theoretical yield,
Xgrams+ Ygrams = 195g as a result of the actual lab, this is the actual yield.

The Percent Yield is used to compare the theoretical yield to the actual yield. And to demonstrate this we use ratios.

Example 1: If your T.Y(theoretical yield) is 200g, and your A.C(actual yield) is 195, your ratio will look like this:

195x 100%=97.5%
200

so your 97.5% is your ratio of T.Y to A.Y

Concentrations of Solutions

Before beginning the class we reviewed the meaning of Concentration

Molarity:measurement of the expressed amount of moles you have disolved into the solution.

To solve questions related to concentration, you will need to use a formula.

Formula: mass = moles=concentration=molarity
volume litres

Example: you have a 1 M solution o NaCl. How many grams of salt are in 500ml of solution?

Step1: calculate how much of molarity you have with 500ml.

1 ml= x molarity cross multiplication
1 lt=.500ml

x=.5ml

Step 2: now calculate how many grams will you get with .5ml.

1ml=58g (NaCl)
.5ml=x grams

x=29grams of salt

Answer is 29 grams of salt

*in some questions it may ask for the molarity were you do the whole same process but backwards.

Note: it is important to capitalize the M for Molarity.

Stefano Pinzon

Monday March 1st, 2010

In today's class, we came upon two new important terms. The first one:

LIMITED REAGENT: it is the reagent that determines the amount of product that can be formed by a reaction. The reaction occurs only until the limited reagent is used up.

The second term is EXCESS REAGENT: the unlimited amount of reactant. But the term is self explanatory.

In our exercises we learnt how to identify the reagents.

For example: using the analogy of recipes there can only be a limited amount of sandwiches produce with 4 slices of bread it is known to be the limited reagent.

Practice #1

Information:Copper reacts with sulfur to form copper (1) sulfide according to the following equation: Atomic Mass

2Cu+S-->Cu2S Cu=63 S=32

What is the limiting reagent when 80g of Cu reacts with 25g of S?

Step #1: If 1 mole of Cu (copper) weighs 63, how many moles are there if you have 80g

your equation should look something among the lines of this

1 MOLE of Cu = 63g using cross multiplication

X MOLES of Cu = 80g

X = 1.269 or 1.27 moles

Step #2: If one mole of S (Sulfur) weighs 32g, how many moles are there for 25g

your equation should look something like this

1 MOLE of S=32g using cross multiplication

X MOLES of S=25g

X=0.781 moles

Step#3: In this step you will be able to compare the amounts and quantities

If there is one mole of Sulfur and gives you 2 moles of Cu as a product, you will be able to calculate the end result with different amounts of moles like this:

1S--->2Cu Cross multiplication 1S--->2Cu

0.78moles of S --->X X moles--->1.27

X= 1.56 moles X=0.635 moles

Step #4: answer the question by comparing the amount of moles from the 80g of Cu (1.27) to the result you got from step #4, and depending on the contrast, in this case the answer from step #1 is less than 1.56 than your are able to say that

Answer: 1.27 moles of Cu is your limited reagent

0.78 moles of S is your excess reagent

Your process you do is not obligatory to do the same, it can be recommended, but the best method is the one you understand.

Stefano Pinzon