SOLUTION Stoichiometry

Welcome to our next unit, Solution Stoichiometry!!!!!! We have just finished Quantitative stiochiometry, but according to our resident chemist, there are many different kinds of stoichiometry.

Before we start, you need to know some things................

• Solutions are homogenous mixtures composed of a solute and a solvent
• The solute is the chemical that is present in a lesser amount and dissolves.
• The solvent is the chemical present in larger amounts that has the solute dissolved in it

Chemicals that are dissolved in water( solutes, remember?) are called aqueous and we put a little aq beside their formulas.
ex. NaBr(aq)

Molarity

Molarity is basically a fancy way of saying concentration. There are many ways to express molarity including...g/L, mL/L, % by volume, % by mass, and most importantly for us mol/L. This formulas is represented by a capital m like this         M
Now lets do some examples!!!



Example 1: What is the concentration of .47mol of NaBr in 300mL of water????

okay so first we have to change mL into L so we divide by 1000 which gives us .3 L. Yay
next we use the formula
                                                  M= mol
                                                           L

                                                  M= .47mol
                                                          .3L

                                                  M=1.6 mol/L

Now lets try a slightly trickier one...................


Ex 2:  you accidentally poured 30g of salt(NaCl) into the 800mL of water you are using to cook pasta. What is the concentration(M) of NaCl in your pasta water????

Again we have to change the water in to L so we divide by 1000 to get 0.8L of water. thats done now we have to change the NaCl from grams to moles. You know how to do this if you think back.......................

Thats right we have to use the molar mass!!!

                                                   30g x 1 mol = .513 mol NaCl
                                                             58.5g

Now we can do the real problem just like we did the last one
                                              M= mol
                                                     L
                                              M= .513 mol
                                                      0.8 L
                                              M= .64 mol/L


yay now you know Solution Stoichiometry ( at least some......)!!!!!!!!!!!!!!!!!!!!!


heheheheheheheheh......

Percent Yield!!!!

Theoretical yield is basically the amount of a substance that should be produced from a chemical reaction. So if you have x amount of H and x amount of O, how much H2O will be produced?

Percent yield however is based on the actual amount that is produced, versus the theoretical amount. So percent yield is like a measure of how successful your experiment is.

The equation for percent yield is :  Experimental x 100
                                                    Theoretical
Lets try some examples:

Ex 1.  The production of Copper(II) Chloride is given by the equation:
                     
                                       FeCl2 + Cu = Fe + CuCl2

If 34.5g of Copper(II) Chloride and 4g of Copper(II) are produced, determine the theoretical yield of Copper(II).


34.5x   1mol  x 1 x 63.5 = 16.3g
         134.5g    1    1mol

 Percent Yield = 4g x100 =  24%
                          16.3g

Ex 2.  Hydrogen is reacted with Oxygen to produce water. If 1.6 mol of Hydrogen are reacted and 32g of water is produced, what is the theoretical yeild of water?

               2H2 + O2 = 2H2O

1.6mol H2 x 2 x 18g  = 28.8g
                   2    1mol

Percent Yield = 28.8 x100= 90% yield
                         32



And that, Ladies and Gentlemen, is Percent Yield.

Other Conversions (like Volume and Heat)

Here's some more of those good ol' conversions.

Volume at STP can be found using the conversion factor 22.4 L/mol. In chemical reactions, HEAT can be included as a separate term... this is called Enthalpy
Reactions that release heat are exothermic
Reactions that absorb heat are endothermic

Here are some examples of converting to/from volume! It's very very similar to previous conversions... just remember to use 22.4 L/mol!

Ex. 1:
Find the mass of sodium required to produce 5.68 L of hydrogen gas at STP from the reaction described by the following equation:
2Na + 2H2O -> 2NaOH + H2   Here the balanced equation is written out for you

5.68 L H2   x     1 mol    x    2     x    23 g    =   11.7 g       

                            28 g          1          1 mol   
ALWAYS convert the volumes to moles first. You cannot go from volume to mass

Ex 2:
How many litres of Oxygen are necessary for the combustion of 425 g of sulfur, assuming that the reaction occurs at STP?

 S + O2 -> SO2                Write out the balanced equation first.

425 g S  x   1 mol    x   1    x    22.4 L    = 297 L    Remember, convert to moles first!

                     32.1 g       1           1 mol 

Molar Volume Lab (Long Overdue!)

So here's a little something about the Molar Volume Lab. (yes, we did this lab ages ago)

Basically, we had to fill up a sink with nice warm water. Then, we had to take a lighter (that contains Butane gas) and after weighing it, we submerged the lighter under the water and lit it (no, there weren't any flames... cause it was under water!) so that the gas collected into a graduated cylinder. Then we had to dry the lighter in the fancy little oven type thing and weigh the lighter's mass again. We had to use the new mass and subtract it from the old mass to calculate the mass of Butane we'd used. Next, we calculated the molar volume of butane by calculating the volume we'd used over the mols of butane.

In my group's case, we calculated 38.7 L/mol, and the actual amount should have been 22.4 L/mol... let's just say, we were a tad bit off :)

It's cool to apply all these calculations we've been doing to actual chemistry and experiments. So the things we're learning ARE usable!

Enthalpy and Calorimetry

Enthalpy is the name for all the energy stored in chemical bonds. This energy increases or decreases during chemical reactions, depending on what kind of reaction it is.

The symbol for Enthalpy is H  and the units are in Joules (J)

There are two different kinds of reactions

Exothermic, which produce energy:


and Endothermic which absorb energy:




These two graphs show what happens to the products in each situation and what happens as the level of enthalpy changes.

In order to use these graphs, we need to get into Calorimetry.

To find out the amount of heat released we need to know 3 things

1. Temperature change (^T) in degrees Celcius
2. Mass (m) in grams
3. Specific heat capacity (C)

Once we know these, using the formula

^H=mC^T

we can figure out the change in Enthalpy!!

For example:

500g of water is heated up on a stove from 10* to 100*. The Specific heat capacity of water is C= 4.18

What is the change in Enthalpy (^H)?

^H=mC^T
^H=(500g)(4.18)(90*)
^H= 188100 J

And there you have it!!

Enthalpy and Calorimetry!!!!!!!!

The lab

As usual in our ever so exciting we conducted a thrilling lab. We were conducting this lab to see if stoichiometry, namely the application of stoichiometry to this particular formula
Sr(NO₃)₂ + CuSO₄ -> SrSO₄ + Cu(NO₃)₂ is right. After all, even though we know Mr. Doktor gave us the right equation we still end up testing it out, in the off chance that he's wrong (which we know he isn't).
Anyways, we ended up conducting the lab much like a previous one, we measured out the right amount of Strontium Nitrate and Copper Sulphate. Then we mixed each individually in water, later combining the two chemicals to complete the reaction. After that we poured the mixture into filter paper in a funnel and waited. Once we finished waiting we put the filter paper in the drying oven and cleaned up, by that time class was over. So stay tuned for part two of our lab.
Copper Sulphate

Mass to Mass Conversions

You've done mole to mole conversions, now it's time for mass to mass conversions!

Mass to mass conversions aren't that much different form mole to mole conversions though, you just have to do the correct conversions from last unit.

For Example:

You have 2.0 grams of HCl how many grams of H₂ will you produce?

First of all write out the chemical equation.
2HCl reacts to form H₂ + Cl₂

This is when you start the conversions
2.0g x ((1 mol) / (1.0+ 17.0)) x (1/2) x  ((2)/(1 mol)) = .11 g of H₂

Of course you're probably wondering what I just did. Well let me explain.
In the first step. We were told that there was 2.0 g of HCl so we had to convert it into moles, thus like following the last unit we divided 2 by 18. 
Then we multiplied the number of moles in HCl by what we had over what we needed (we had 1 H and we needed 2HCl) 
After that we converted the moles of H into grams thus giving us an answer of .11 g.
Easy right? Of course if you still don't get it please just don't blurt out any random answer.
 

Stoichiometry

And Just hot off the press in the world of chemistry

Stoichiometry

It deals with the quantitative analysis of chemical reactions.

Basically it is a generalization of mole conversions.
Now for the basics of Stoichiometry, the six types of chemical reactions

Synthesis A+ B reacts to form AB

it is elemnts to compounds
For example 3Cl₂ + 4P reacts to form 2Cl₃P₂

ecompostion AB reacts to form A+B

Compounds to elements

Such as RhAs reacts to form Rh + As

Single Replacement A+ BC reacts to form B+ AC
as you can see the elements move around a bit
An example is K + NaCl reacts to form KCl + Na

Double Replacement AB + CD reacts to form AD + CB
an important thing to remember for both this and single replacement is that the metals and non-metals cannot switch places (ie. AB + CD reacts to form AC + BC)

For example FeS + 2HCl reacts to form H₂S + FeCl₂

Combustion a hydro carbon combustion reacts to form Co₂  and HOH

usually a reaction of hydrocarbon with the air/something in the air

ie. C₁₀H₈ + 12O₂ reacts to form 10CO₂ + 4H₂O

Neuturalization reaction between an acid and a base
Such as Li(OH) + HCl reacts to form LiCl + HOH

Well that's it, the six basic things you need to know about stoichiometry.