Homework:
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Read Pages 58-61
Do all of % composition worksheet
Mole problems 3 Packet
Next Quiz: Monday Feb 25 or Tues
Feb 26
• Test: March 11th (Monday) or
March 12th (Tuesday)
Percent Composition
Lesson 3
• Determining the formula of a
compound % by mass of each
element in a compound
% Composition
If 8.20 grams of magnesium
combines with 5.40 grams of
oxygen to form a compound ,
what is the % composition of
this compound?
% composition
9.03 grams of Mg combine
completely with 3.48 grams of
nitrogen
% Composition of a known
compound
% Mass of Element

grams of element
in 1 mol of cmpd.
MM of compound
% composition of C3H8
% composition of each element in
sodium bisulfate
Homework
• Read: pages 59-61
• Worksheet: Finding Empirical and
Molecular Formulas
• More Mole problems WS II # 1-6
• Test: March 11th and 12th
Mole Problems 3
1.
2.
3.
4.
5.
5.6 Liters
3.00 moles
8.03 grams
38.5 Liters
1.3 g/L
Mole Problems 4
1.
2.
3.
4.
5.
170 grams
2.53 grams
22.8 grams
2.75*1023 atoms
106 grams
Mole Problems 5
1.
2.
3.
4.
9.21*1022 molecules NaOH
40.9 grams
2.07 L
3*10-23 grams
Percent Composition
1. HCN
3.74% H
44.43% C
51.83% N
2. H2O
11.21% H
88.79% O
3. Ca(NO3)2
24.42% Ca
17.07% N
58.50% O
4. Al2(CO3)3
23.06% Al
15.40% C
61.54% O
More Mole Problems
Question 1
9.20 x 10-17 g
Question 2
0.5648 moles
Question 3
1.694 moles
Question 4
Question 5
2.11 x 1023 atoms
Question 6
area = 0.00645 cm3
= 0.00645 mL
3.89 x 1020 atoms
1.02x1024 atoms
a) 2.1 x 1023 molecules
b) 183 g
More Mole Problems
Question 7
a) 5.1 mol
b) 1.86 x 1025 atoms
c) 182 mL
Question 8
4.36x1016 molecules
Question 9
Finding Empirical and Molecular
Formulas
Question 1
a) C3H4O3
b) CH2O
c) CH2O
Question 2
Question 4
CH2Cl
C2H4Cl2
Question 5
C4H5N2O
CH5N empirical CH5N molecular
Question 3
P2O5
Question 6
SN empirical;
S4N4 molecular
More Mole Problems Worksheet No.3 Answers
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2) STP: 0°C, 1 atm (760 mmHg, 101.3 kPa)
3) 1.96 g/L
4) 6.19 x 1022 molecules
5) 20 moles
6) 19.97%
7.a) 5.78 x1022 atoms
7.b) 0.384 g
7.c) air: 1.25 g/L; bag: 1.365 g/L,  bag will not
float
• 8) 4.70%
• 9) 5 glasses
Empirical Formula
• Lowest whole number ratio of
atoms of the elements in a
compound.
Calculate the empirical formula of a
compound that is 25.9% nitrogen and
74.1% oxygen
• Step 1: find mole ratio
• Step 2: divide by least # of moles
• Step 3: multiply to get a whole number
Molecular Formulas
• Molecular formulas are whole number
multiples of the empirical formulas
• The empirical formula and the MM of
the molecular formula is needed!
What is the empirical formula of a
compound that is 27.3% C and 72.7% O?
Calculate the molecular formula given the
empirical formula is CO2 and has a molar
mass of 132.03 grams/mol
1. Find the MM of the empirical formula
2. Divide the MM of the molecular formula
by the MM of the empirical formula
3. Multiply all elements by the whole # to
get the molecular formula