CHEMICAL CALCULATIONS Chptr 4
MOLECULAR WEIGHTS OR MOLECULAR MASS p 83
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 pg 84
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
MWMoles = 17
84Moles = 0.20
Step Two
1 mole = 0.20
6.02 X 10 23 Xcross 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 pgs 90 - 94
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Chemical |
arithmetic is calculation of the quantities | |
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of chemicals |
that react with each other. | |
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I have 1 bowling |
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ball and I pasted two ping pong balls on |
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If I had 100 |
pounds of bowling balls, would I need |
|
|
I do have a 2 to 1 ratio |
why doesn't that work? |
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. Multipy 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 equations3 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
MW5 = 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?
Problems at end of Chapter 4 - 4.17; 4.18; 4.19; 4.20; 4.21; 4.23; 4.25; 4.31; 4.32; 4.33; 4.34; 4.39