p.204 #1 Acetylene, C2H2, contains an equal number of carbon and hydrogen atoms. Explain why acetylene is not 50% carbon by mass.
p.201#2
Find the % composition of a pure substance that contains
7.22 g nickel, 2.53g phosphorous and 5.25g oxygen and nothing else
p.204 #7
What is the mass percent of oxygen in potassium nitrate?
Empirical and Molecular Formulas
glucose hydrogen peroxide Molecular formula C6H12 O6 H2O2 Empirical formula CH2O HO
hydrazine water N2H4 H2O Is it possible for the empirical and molecular formula to be the same?
H2O Molecular Formula
The actual number of each type of atom in a molecule.
H
C
C
describes actual structure
H
Empirical Formula
The relative number of each type of atom in a molecule. It represents the formula of a compound in simplest form. ratio of each type of atom
When would you use an Empirical formula?
1) For ionic compounds or large covalent compounds with a network structure (like diamonds). These compounds are described using an Empirical formula since there is no discrete molecule. 2) To find the molecular formula. You need to know the percentage composition and molar mass of a substance to calculate the molecular formula.
Check
Molecular Empirical
Finding the Empirical Formula from Percent Composition
1) Use a chart to organize your work. This rhyme may help you remember what goes in the chart.
Percent to mass
Mass to mole
Divide by small
Multiply till whole
2) Assume you have a 100 g sample so that your mass % is the same as your mass in grams.
3) To get a whole number at the end
If the decimal ends in... Fraction Multiply by...
__.33
__.25
__.5
__.67
__.75
Ex. Find the empirical formula of Vitamin C if it contains 40.5% C, 4.6% H, and 54.5% O.
Assume 100g Sample
Percent to mass Mass to mole Divide by small Multiply till whole
Atom Mass Mass M % (g) (g/mol)
moles
(mol)
C 40.5 40.5 12.01 3.3722 H 4.6 O 54.5 54.5 4.6 Smallest Ratio Whole Number 3.3722
3.3722 1.01 4.5545 16.00 Find the number of moles for each element:
nc = 40.5 g x 1 mol = 3.37 mol of C
12.01 g
nH = 4.6 g x 1 mol = 4.55 mol of H
1.01 g
nO = 54.5 g x 1 mol = 3.41 mol of O
16.00 g
3.4063
The empirical formula of Vitamin C is C3 H4 O3 .
Try p. 211 #13
An oxide of chromium is made up of 68.4% chromium and 31.6% oxygen. What is the empirical formula of this oxide?
Percent to mass Mass to mole Divide by small Multiply till whole
Element %
Mass Molar Mass Mole
(g/mol)
(g)
(mol)
Divide by small
Whole #
Finding Molecular Formula
The molecular formula (actual formula) for a compound can only be found if you know
*the molar mass of the molecule
AND
*Empirical formula (or percent composition) 1) Find the molar mass of the empirical formula. 2) Use the following formula to find the ratio of the molar masses of the compound and empirical formula.
Ratio = Mmolecular formula
Mempirical formula
3) Multiply the empirical formula subscripts by the ratio to obtain the molecular formula.
ex. The empirical formula for Vitamin C is C3H4O3 and its molar mass is 176.17 g/mol. Find the molecular formula.
Homework
p.211 # 14,15,16 Empirical Formula from % Composition
p.214 # 3 Empirical formula from moles or grams
p.218 # 17,18,19 Molecular formula from Empirical formula
p.201#20 % composition, Empirical formula and molecular formula
Exit Card
Don't FORGET SHARE LENT
APOLLO 13 QUIZ
To find LR 1) Find mol of each 2) Divide by coefficient
Extra Example