CHEMICAL  CALCULATIONS    

MOLECULAR  WEIGHTS OR MOLECULAR MASS     Chapter 6

To find the molecular weights or molecular mass as it is referred to in most chemistry texts, add up all the individual atomic weights  (masses) for each of the elements.

NaCl     Na = 23.0                                H2O     2 H @ 1.0   =     2
             Cl =  35.5                                              1 O @ 16.0 =   16       
                      58.5 AMU                                                             18 AMU

DRILL

NH3

Ca(OH)2

C6H14

How many things are in a dozen?

How many things in a gross?

How many things in a mole?       6.02 x 1023                               6.02x1023
                                                                                                                                         mole

Mole - Avogadro's Number    Page 164

A mole is the formula weight of the compound in grams.

moles = grams                             6.02 x 1023
                  MW                                 mole

1 mole of NaCl would be 58.5 g
1 mole of H2O would be 18.0 g 

Distinguish between atoms and molecules. 

H2O  is a molecule having 3 atoms -  1 oxygen atom and 2 hydrogen atoms. 

18 grams of H2O would have:
                                                1 mole of water
                                                1 mole of water has 6.02 x 1023 molecules of water
                                                3 moles of atoms ( 1 O & 2 H)
                                                18.06 x 1023 atoms or 1.81 x 1024 atoms total

How many molecules in an mole?

How many moles                 in 58.5 g NaCl?
  "      "       molecules           "     "    "    "
  "      "        atoms                 "     "    "    "  

How many moles are there in 32 g CH4? 

How many molecules are there in 32 g CH4? 

How many atoms are there in 32 g CH4? (need to do a) number of moles and b) how many atoms per mole) 

How much would 5 moles of C6H12O6 (glucose) weigh? 

How many atoms of silver are there in 53.935 g of silver (Ag)?
  Find out number of moles first.

Sodium bicarbonate (NaHCO3) is an important component of blood.  How many sodium (Na+) ions are there in 17 g of NaHCO3?

Step 1.  To get the number of atoms or ions we will be using Avogadro's number.  But to use that number we need to know how many moles we have.

FW   NaHCO3       1  Na @ 23  =      23                                                                                      1  H   @ 1    
=        1
                               1 C @ 12    =      12

      3 O @ 16    =       48                                                                                                                
          84 

So if we had 84 g we'd have 1 mole but we've got 17 g which will be less than 1 mole.

   Moles =    grams     
                                             MW  

    Moles =         17  
                                                  84

    Moles   =     0.20

Step Two

    1 mole                      =          0.20  
                           6.02 X 10 23                              X 

 cross multiply

  1 X  =  (.20)(6.02 X 1023)
                              X  =  1.2 X 10 23                  leave the exponent (23) alone! 

   For each molecule of  NaHCO3 we would have 1  Na  ion.
(How many oxygen atoms would be present in  17 grams NaHCO3?)

STOICHIOMETRY     Pages 171 - 172

Chemical arithmetic is calculation of the quantities of chemicals that react with each other.

I have 1 bowling ball and I pasted two ping pong balls on top and made a great selling piece of pop art.  I wanted to make a bunch of them and really rake in the dough.

If I had 100 pounds of bowling balls, would I need 200 pounds of ping pong balls?

 I do have a 2 to 1 ratio, why doesn't that work?

 

 

 Suppose I took 1 oxygen atom and added 2 hydrogen atoms, I would make water.  If I had 1 gram of oxygen would I need 2 grams of hydrogen?

  

2 H   +    1 O  ----->   1 H2O

 We see hydrogen as H2 and oxygen as O2 so we have to use that in our equations.

   O2  +  2 H2               ---->  2 H2O

 HCl  +  NaOH  ----->  NaCl  +  H2O
acid         base                  salt     water 

Is this equation balanced?

 

How many moles of NaOH is needed to react with 1 mole of HCl?

 

If I had 36.5 g of HCl how many NaOH would I need?

Jackson Math Manipulation

1. Take the 2 pieces of the equation you'll be working with and    write them out

 

2. Multiply the molecular weights by the coefficients and put  it under the two pieces of the equation.

  

3. Put how many grams over them

 

                                       36.5 g                                 x grams
                                                        HCl                 +                 NaOH
                      (coeff)(MW)       (1)(36.5)                                  (1)(40)

 

Top is to bottom as top is to bottom

 

    36.5       =    X  
                   36.5             40
                                       x = 40

 Let's try another

      4 NH3 + 3 O2 -----> 2 N2 + 6 H2O

How many grams of oxygen is needed to react with 34 g NH3?

 A.           4 NH3  + 3 O2  MAKE SURE YOU USE THE COEFFICIENTS  

 

 B.                       4 NH3  + 3 O2                      MW   NH3   =  17         MW  O2  =  32                                                                 

   (coeff)(MW)   (4) (17)      (3) (32)  

               4 NH    +    3 O2
                      68               96                 

 

The MW of NH3 = 17 but you have 4 moles of it, so you multiply by (4)(17)=68


 

C.            34                 x
                4 NH3   +    3 O2
                68                 96

34     =    x   
                68          96

68 x =  3,264
                     x = 48 g

3  Fe + 4  H2O -----> Fe3O4  +  4 H2

Calculate

a. number of moles of Fe3O4 produced from 5 moles of Fe 

3Fe -----> 1 Fe3O4    
               5Fe -----> xFe3O4     Draw a line between the two equations

              3 Fe   ----->  1 Fe3O4               3 Fe       =    1 Fe3O4
              5 Fe   ----->   x Fe3O4              5 Fe              x Fe3O4

       Crisscross  multiply   3x  =  5       x   =   1.7

 b. number of moles of H2 produced from 5 moles Fe

  

c. number of grams of Fe to react with 5 moles of H2O. (We are asked to give grams when we were given moles)   

          Moles  =        grams  
                               MW      

             5      =           x  grams 
                                 18

             (5) (18)  =  x             =  90  grams of H2O

Remember to take in account the coefficients! 

d. The number of grams of H2 produced from 5 moles of H2O.   

    Write the section of the equation first!


 

e. The number of grams of Fe3O4 produced 20 grams of Fe

Drill
Using

  Ca3(PO4)2 + 3 SiO2 + 5 C ----> 3 Ca(SiO3) + 5 CO + 2 P 

Find

1. How many moles of phosphorus are formed for each mole of  Ca3(PO4)2 used?  (2)

2. The number of grams of phosphorus formed for each mole of    Ca3(PO4)2 used.     (61.8)

3. The number of grams of phosphorus formed for each gram of    Ca3(PO4)2 used.   (0.19)

4. The number of tons of phosphorus formed for each ton of    Ca3(PO4)2 used.   (0.19)

5. The number of moles of phosphorus produced by 5 moles of   Ca3(PO4)2.  (10)

6. The weight of carbon required to react with 100 lbs. of

   Ca3(PO4)2.  (19.4 lbs)

7. The weight of SiO2 that will react with 10 tons of Ca3(PO4)2.    (5.82 tons)

8. How many moles are contained in

a. 2.45 g H2SO4                                    b. 4.4 x 10-2 g CO2

9. How many molecules are contained in

a. 8 g of oxygen                                      b. 30 g C2H5OH

10. How many molecules are contained in

a. 5.0 g H2O2                                        b. 47.5 g F2

Given the equation      2 C5H11OH + 15 O2 ------> 10 CO2 + 12 H2O

a. How many moles of O2 are needed for the combustion of 1 mole of C5H11OH?
b. How many grams of CO2 are produced for each mole of C5H11OH  burned?
c. How many moles of H2O are formed for each mole of O2 consumed?
d. How many grams of CO2 would you get if you had 64 grams of oxygen?
e. How many moles of the alcohol would you need to get 5 moles of CO2