Mass of element Moles of elementatoms of element 1 mole 6.02 x
10 23atomsPT mass 1 mole OR PT mass 1 mole 1 mole 6.02 x 10
23atoms
3. Percent Composition
% mass of each element in a compound
Example:H 3 PO 4= 98.00 g/mol
H = 3.03 g / 98.00 g = 3.09%
P = 30.97 g / 98.00 g = 31.60%
O = 64.00 g / 98.00 g = 65.31%
Putting this in reverse, you can calculate the empirical
formula
4. Empirical Formula
H = 3.09 g x (1 mol/1.01 g) = 3.06 mol H
P = 31.60 g x (1 mol/30.97 g) = 1.020 mol P
O = 65.31 g x (1 mol/16.00 g) = 4.082 mol O Divide all mol #s
by smallest mol #
H = 3.06 / 1.020 = 3.00
P = 1.020 / 1.020 = 1.000
O = 4.082 / 1.020 = 4.002
These become the subscripts H 3 PO 4
Remember, you can only round 0.1 and 0.9.Otherwise, multiply by
a factor to get whole number integers
5.
Molecular Formula:how many atoms of each element are present in
a compound
1.Calculate molecular mass of empirical formula
2.Divide given molecular mass by the calculated molecular mass
of empirical formula.(Whole #)
Example:The molecular mass of ascorbic acid is 176 g/mol.The
empirical formula is C 3 H 4 O 3= 88.07 g/mol
176 g/mol/88.07 = 2 Double subscripts to get C 6 H 8 O 6
6. Chemical Equations
Formulas for substances
Balance equations
Add state symbols (s, l, g, aq)
If you know
1.moles of substance present
2.balanced equation
You can calculate moles of another substance in equation
7.
How to calculate moles
1.write equation
2.moles present
3.use coefficients to find reacting ratio
8.
Volumetric Analysis (Molarity and Molality)
M =mol Molarity L
m =mol Molalitykg
9. Dilutions
Dilution solutions prepared by adding water to more
concentrated ones (stock solutions)
M 1 V 1= M 2 V 2 (molarity)(volume) = (molarity)(volume)
The volume of water that must be added to the concentrated
solution is the difference between the volume of the final
(diluted) solution and the volume of the concentrated solution
Example What volume of water must be added to prepare 2L of 3M
KOH from an 8M KOH solution?
10. Spectroscopy and Beer-Lambert Law
Spectroscopy is the study of the interaction of electromagnetic
radiation and matter (most common are UV, Visible, and
Infrared)
Beer-Lambert Law
Used to relate the concentrations of colored solutions to the
amount of light they absorb
The amount of absorbance is calculated using the formula: A =
bc
A = absorbance
= molar absorptivity (constant that depends on nature of
material during absorption)
b = path length (length of sample that light passes
through)
c = concentration
11.
When absorbance measurements are made at a fixed wavelength in
a cell of constant path length,and b are constant and A will be
directly proportional to c.
If a solution of a compound obeys the Beer-Lambert Law, a plot
of aborbance (y-axis) vs. concentration (x-axis) gives a straight
line with a slope of b.
The y-intercept is zero (the line will pass through the origin
of the graph)
One can use the graph to read corresponding concentrations and
absorption values
12.
A plot of absorption against wavelength can be used to
determine the exact color of a solution
The point at which the greatest absorption is observed can be
used to determine, via a color wheel, which wavelength is being
reflected and, therefore, the color of the solution.
The color that is observed is due to the wavelengths of light
that the sample did not absorb
For example, if a sample absorbs light in the orange region of
the visible spectrum, then the solution will appear blue.
13.
A color wheel can be used to relate absorbed and transmitted
colorsthe transmitted color being the complement color of the
absorbed light.
Although, theoretically the spectrophotometer can be used at a
number of different wavelengths, because of the limitation of
electronics, the optimal wavelength is one where the absorbance is
highest
14.
Performing an experiment at the point of highest absorbance
offers 2 advantages:
1.Beers Law linear relationship between concentration and
absorbance is most likely to hold around this point
2.When diluting the solution in order to investigate other
lower concentrations, it is likely that if one starts at a point of
maximum absorbance, that the absorbance will still remain
significant and, therefore, detectable at the lower
concentrations.
15. Basic Spectrophotometer 16. Spectrophotometer Diagram 2 17.
Typical Beers Law Graph 18. Color Wheel 19. Analysis of
Hydrates
Hydrates are formula units with water associated with them
CuSO 4 5H 2 O Copper (II) sulfate pentahydrate
Strong heating evaporates the water
When water is removed, the salt is called anhydrous
Example:8.69 g of CuSO 4 xH 2 O is heated and left a residue of
5.56 g.What is the formula?
20. Limiting Reactant
Determines if a certain reactant will be completely used up
during a reaction
Example:Phosphorus reacts with chlorine according to the
equation P 4 (s) +6 Cl 2 (g) 4 PCl 3 (l)
If 215 g of P 4are allowed to react with 725 g of Cl 2 , how
much PCl 3will be formed and what is the limiting reactant?