Transcript File
UNIT 2
Moles and Molar Mass
The Mole represents a specific amount of any substance.
Specifically it represents 6.02x10
23 particles such as atoms and molecules.
The mole is based on the fact that 12 grams of Carbon-12 has a mole of atoms. A mole means you have 6.02x10
23 particles.
Molar Mass (aka gram formula mass or molecular mass) is the mass of one mole of a substance. Element ’ s molar masses are reported on the periodic table.
1.
What is the molar mass of iron?
55.8 g/mol 2.
What is the molar mass of copper?
63.5 g/mol
1. What is the molar mass of water?
H2O = 2(1.0) + 16.0 = 18.0g/mol 2. What is the gram-formula-mass of calcium chloride?
CaCl2 = 40.1 + 2(35.5) = 111.1 g/mol
Calculating Moles
Use the formula on table T: number of moles = given mass (g) _ gram-formula mass (Given mass will be your answer.)
1.
Calculate the mass of 6.70 moles of carbon.
6.70 = x _ 12.og/mol x = 80.4 g
Calculate the mass of 0.023moles of lithium.
0.023 = x _ 6.9 g/mol x = 0.16 g
Calculate the mass of 25.02 moles of calcium phosphate.
25.02 = x _ 310.3/mol x = 7764 g
Use the same formula to calculate the moles, placing the number in the question on the numerator:
number of moles = given mass (g) gram-formula mass
4. Determine the number of moles in 8.0 grams of Boron.
x = 8.0 g _ 10.8g/mol x = 0.74 moles
Determine the number of moles in 0.567 grams of Helium.
x = 0.567 g _ 4.00 g/mol x = 0.142 moles
Find the number of moles in 1230 grams of magnesium sulfate.
x = 1230 g _ 120.4 g/mol x = 10.2 moles
Moles to Coefficients
Mix + 2 Eggs + 1 cup water = Cake Reactants Products 2 Mix = 2 Cakes 6 Eggs = 3 Cakes
4Al + 3O
2
2Al
2
O3
reactants products Coefficients: How many moles of the substance are needed in a reaction.
To relate moles of one substance to another, simply create a proportion: 4Al + 3O 2 2Al 2 O3 1.
2.
3.
4.
If 3 moles of oxygen react, how many moles of Aluminum oxide form? 2 If 4 moles of aluminum react, how many moles of oxygen are needed? 3 If 4 moles of aluminum oxide are formed, how many moles of oxygen were used?
6 If 8 moles of aluminum react, how many moles of oxygen are needed? 6
16 Al + 3S 8 8Al 2 S 3 1.
If 2.50 moles of sulfur react, how many moles of aluminum sulfide form?
2.50 mole S = x moles Al 2 S 3 3 8 x = 6.67 moles
16 Al + 3S 8 8Al 2 S 3 If 3.75 moles of aluminum react, how many moles of sulfur are needed?
3.75 mole Al = x moles S 8 16 3 x = 0.703 moles
Why do chemists use moles to measure substances? Why aren ’ t grams, liters and molecules enough?
Balancing
In a reaction, atoms and molecules cannot appear or disappear. Mass must stay constant from the beginning to the end of the reaction.
H2 + O2 H2O ___H2 + ___O2 ___ H2O
___ N2 + ____H2 ____ NH3 ___Li + ____O2 ___Li2O
__Pb(NO 3 ) 2 +__K 2 CrO 4 ___PbCr 2 O4 + ___KNO 3 ___C4H8 + ___O2 ___CO2 + ___H2O
1.
2.
3.
4.
5.
Synthesis: Decomposition: Combustion: Single Replacement: A + 2B AB 2 CH 4 + O 2 AB + C AB 2 A + 2B CO 2 Double Replacement: AB + CD + H 2 O CB +A AD + CB * Notice synthesis and decomposition are opposites. Also, combustion can have any carbon compound as a reactant.
Empirical and Molecular Formulas
Empirical formula refers to any molecular formula in it ’ s reduced form.
Are these empirical? If not, reduce them: 1.
2.
3.
4.
C 2 H 2 C 6 H 12 O 6 NO 2 Na 2 (OH) 2 1. CH 2. CH 2 O 3. NO 2 4. NaOH
Molecular Formulas are some multiple of the empirical formula.
If the empirical formula is CH 4 could be CH 4 , C 2 H 8 , C 3 H 12 etc. a molecular formula A compound whose empirical formula is NH 3 has a mass of 34 g/mol. What is the empirical formula?
NH 3 2NH 3 = 17.0 g/mol 34/17 = 2 = N 2 H 6
Find the mass of the empirical formula.
Divide the mass given by the empirical mass.
Distribute your answer through the empirical formula.
If a compound has a mass of 45 g/mol and an empirical formula of CH 3 , what is the molecular formula?
CH 3 3CH 3 = 15 g/mol = C 3 H 9 45/15 = 3
Percent Composition
Nutrition Facts on foods can tell you just how much of a substance you are consuming and how that relates to how much you should eat in a day.
It is equally important to know how much of a element or compound is in a mixture.
% By mass: mass part x 100 total mass 1.
Find the % by mass of phosphoric acid.
H3PO4 3(1.0) x 100 3(1.o)+32.1+4(16.0) = 3.02%
Find the percent by mass of Ca in Ca(OH) 2 .
40.0 40.0+2(16.0)+2(1.0) x 100 = 54.1%
1.
2.
3.
NaClO NaCl NaOH
23.0/74.5 *100 = 23.0/58.5 *100 = 30.9% 39.3% 23.0/40.0 *100 = 57.5%
A sample of a substance containing only magnesium and chlorine was tested in the laboratory and found to be composed of 74.5% chlorine by mass. If the total mass of the sample was 190.2 grams, what is the mass of the magnesium?
0.745 (190.2) =
141.7 grams
For teacher use
I went apple picking and they charge per dozen apples. I was worried it would take forever to count all the apples when I was on line. But the were smart: they weighed the apples. A dozen apples weighs 180 grams.
I have 540 grams of apples, how many dozen do I have?
How much does 2 dozen weigh? How much does 0.25 dozen weigh?
How is this similar for moles?
1.
Find the gram formula mass of: a.
Ne c. NH 3 2.
3.
4.
b.
NaCl d. Mg(OH) 2 Find the number of moles is 230g Mg(OH) 2 Find the mass of 2.50 moles of NH 3 Given: 2NaCl + Mg(OH) 2 2NaOH + MgCl 2 Find the number of moles of NaCl that will react with 3.75 moles of Mg(OH) 2 .