Plan, Ppt09(PS4)
• % Mass
– From mass data OR chemical formula (Tro, 3.8)
• Empirical Formula (Tro, 3.9)
– Concept/definition
– From mass data OR % mass data
– Note: Use “ratio of moles = ratio of FUs” idea
– How to reduce a “non-whole-number” ratio to a
whole-number ratio
• Difference between Empirical and Molecular
Formula (Tro, 3.9)
Ppt09(PS4)
1
% Mass
Partial Mass
% Mass 
x 100
Total Mass

If mass data for a particular sample are
provided, use those.

If a formula is provided, assume you have
a sample of one mole of the substance
 Use molar masses from Periodic Table
Ppt09(PS4)
2
From Green Handout Sheet
(PS4 & 5 Practice)
1. What is the mass percent of P in P4O10?
mass of P (in some sample of P4 O10 )
%P 
x 100
total mass of the sample
Pick one mole (for your sample size). It contains:
4 moles of P and
___
10 moles of O
___
4 mol P x 30.97 g/mol = 123.88 g P
10 mol 0 x 16.00 g/mol = 160.0 g O
283.88 g/mol P4O10
123.88 g P
%P 
x 100  43.638...  43.64%
283.88 g P4 O10
3
Types of Chemical Formulas*
• Chemical formula (review):
– A formula in which the subscripts indicate the exact
composition of a single formula unit of the substance.
• Molecular formula: (MF)
– Same as chemical formula in my class (though this annoys me)
• Empirical formula: (EF)
– A type of chemical formula in which the subscripts
are in the lowest whole number ratio.
• Quick quiz: Glucose is C6H12O6 Is this an EF?
CH2O
1:2:1 ; EF = _____
– No. 6:12:6 can be reduced to _____
* “Formula” in a chemistry context means something completely different
than a mathematical formula (e.g., D = m/V).
4
EF and MF are not always
different for a substance
• SO3 is both the MF and the EF for sulfur
trioxide (1:3 cannot be reduced)
• Na2S is the MF and the EF for sodium
sulfide
• But consider dinitrogen tetroxide:
N2O4
– Chemical formula is _____
NO2
– Empirical formula is _____
 Represents exact # of
each atom in one FU
 Represents only the
ratio of each atom
NOTE: Most ionic compound formulas are already
EFs. Not so for most molecular compounds!
5
From Green Handout Sheet
(PS4 & 5 Practice)
1(b) What is the empirical formula for P4O10?
Answer: P2O5
(In 4: 10, if you divide both
#’s by 2, you get 2 : 5)
Ppt09(PS4)
6
Why do we have empirical formulas
at all? Don’t we want MFs?
• Empirical:
– “derived from or guided by experience or experiment.”
(dictionary.com)
• In the lab, you cannot “see” the molecules of a
molecular compound! How can you “know” that
the molecules are N2O4?
• Empirical formula is what you can get with ONLY
“masses of elements” data (see next slide!).
• NOTE: To get the molecular formula, you need
other info
– For us, that info is the molar mass (example later)
Ppt09(PS4)
7
Determining Empirical Formula
from mass data of a sample
(i.e., knowing grams of elements A, B, C, etc.)
masses (of A, B, C, …)
moles (of A, B, C, …) atoms
ratio (of moles)
reduced ratio (whole-number ratio of moles)
Empirical formula (symbols with subscripts)
Ppt09(PS4)
8
From Green Handout Sheet
(PS4 & 5 Practice)
2. An 8.26-g sample of a compound is chemically
separated to form 3.31 g of sulfur and 4.95 g of
oxygen.
What is the empirical formula?
What is the mass % of O in the compound?
3.31 g S x
1 mol S
 0.1032... mol S
32.07 g S
1 mol O
4.95 g O x
 0.3093... mol O
16.00 g O
mol O 0.3093... mol O

 2.997...  3  3 : 1 ratio  EF is SO3
mol S 0.1032... mol S
Ppt09(PS4)
9
From Green Handout Sheet
(PS4 & 5 Practice)
2. An 8.26-g sample of a compound is chemically
separated to form 3.31 g of sulfur and 4.95 g of
oxygen.
What is the mass % of O in the compound?
(two ways; see board)
Ans. 59.9% OR 59.95%
2 (follow up): If the molar mass of this compound is
(separately) determined to be 160. g/mol, what is
the molecular formula? [Note: this example is made-up]
•
Approach?
Ppt09(PS4)
10
To get MF from EF and MM, find
“empirical mass” (EM)
• Since the MF must be a “multiple” of the
EF, the MM must be a multiple of the EM
– Last example:
• EF was SO3; thus EM was ~80. g/mol
• MF must be (SO3)n
• Thus, if MM is 160. g/mol, n must be 2.
– Generally speaking, you can find n by dividing
MM by EM! (Don’t memorize, just think about it!!)
Ppt09(PS4)
11
A Note About Ratios
• A ratio is a fraction—take one thing and
divide it by another.
– If you do the division, you’ll get ONE
NUMBER—this number is actually the ratio of
the numerator to one of the denominator
• See example on prior slide; moles of O : moles of
S is basically 3 (to 1).
– If the number you get is not extremely
close to an integer value, you’ll need to
reduce further (next slide)
Ppt09(PS4)
12
Reducing a non-whole-number ratio to
a whole-number ratio.
[NOTE: Never forget that in an empirical formula problem,
you need the ratio of moles not grams!!!]
One approach:
1) Divide all mole values by the smallest. (That
value will become a “1”)
2) If all other resultant values are not extremely
close to a whole number, multiply all (new)
values by a small whole number (2, 3, 4,
etc.) until a whole number ratio is obtained
•
To take some of the “randomness” out of this approach, look
for decimals that equate to simple fractions (0.25, 0.333,
0.5, 0.666, 0.75, etc.; SEE NEXT SLIDE )
13
2nd Step “Multiplication” Help
…multiply by ____ to make the
number a whole #
Since….
0.333 =
0.666 =
0.500 =
0.250 =
0.750 =
0.200 =
0.400 =
0.800 =
1
3
2
3
1
2
1
4
3
4
1
5
2
5
4
5
3 (1/3 x 3 = 1)
3 (2/3 x 3 = 2)
2 (1/2 x 2 = 1)
4 (1/4 x 4 = 1)
4 (3/4 x 4 = 3)
5 (1/5 x 5 = 1)
5 (2/5 x 5 = 2)
5 (4/5 x 5 = 4)
Ppt09(PS4)
14
2nd Step “Multiplication” Examples
If after dividing through by the
lowest # moles you get:
…multiply (all
#’s) by ____
to obtain (after
rounding)
C1H2.66
3
C3H8
Al1Cr1.51O6
2
Al2Cr3O12
C1.334H2.999O1
3
C4H9O3
X2.01Y1Z2.75
4
X8Y4Z11
Ppt09(PS4)
15
From Green Handout Sheet
(PS4 & 5 Practice)
3. You have determined that in a certain
compound, the ratio of moles of C : H : O
is: 2.89 moles : 5.78 moles : 2.17 moles.
What is the empirical formula of the
compound?
Ppt09(PS4)
16
PS Sign-Posting
• The concepts and skills related to
problems 13, 14, 16, and 17 on PS4
have been covered in this PowerPoint
(and almost #15). Give those problems
a try now!
Ppt09(PS4)
17