empirical formula
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Empirical Formulas and Molecular Formulas
CLE.3221.3.3 Explore the Mathematics of Chemical Equations
Empirical FormulasObjective
Define empirical formula
Determine the empirical formula when given percentage composition or mass composition
Determine the molecular formula from empirical formula
Definition
Consists of the symbols for the elements combined in compound
with subscripts showing the smallest whole number ratio of the atoms in compound
Ionic Compounds
ionic compounds
formula unit is usually the empirical formula
NaCl is in the smallest whole number ratio
CaCO3 is in the smallest whole number ratio
Molecular Compounds
empirical formula does not necessarily indicate the actual numbers of atoms present in the molecule
B2H6 can be reduced to BH3
C6H12O6 can be reduced to CH2O
Empirical Formula from Percent Composition
Quantitative analysis shows that a compound contains 32.38% Na, 22.65% S, and 44.99% O. Find the Empirical Formula (EF)
Empirical Formula from Percent Composition
1. assume 100 g sample of substance since percentages are given
32.38 % = 32.38g ; 22.65% = 22.65g; 44.99% = 44.99g
Empirical Formula from Percent Composition
2. Find the moles of each element in the fomula
Na = 32.38 g/ 23.0g = 1.408 mol
S = 22.65 g/ 32.1g = 0.7056 mol
O = 44.99 g/ 16.0g = 2.812 mol
Empirical Formula from Percent Composition
3. Develop small whole number mole ratio by dividing through by smallest number of mols
Na = 1.408 mol/0.7056 mol = 1.99 ~ 2
S = 0.7056 mol/0.7056 mol = 1
O = 2.812 mol/0.7056 mol = 3.98 ~ 4
Empirical Formula from Percent Composition
Mole ratio is 2:1:4
Empirical Formula is Na2SO4
Empirical Formula from Percent Composition
Find the EF of a compound that is 26.56% K, 35.41% chromium, and the remainder oxygen.
Empirical Formula from Percent Composition
K = 26.56g/39.1g = 0.6793 mol/0.6793mol =1
Cr = 35.41 g/52.0g = 0.6810 mol/0.6793mol=1
O = 100g - (26.56 g + 35.41g)/16.0g = 2.377mol/0.6793mol= 3.5
Empirical Formula from Percent Composition
K = 1 Cr = 1 O = 3.5
the number of oxygens is not close to a whole number, multiply by a number that will make 3.5 a whole number
K = 1 x2 = 2
Cr = 1 x 2 = 2
O = 3.5 x 2 = 7 EF = K2Cr2O7
Empirical Formula from Mass Composition
Analysis of a 10.150 g sample contains 4.433g phosphorous and 5.717g oxygen.
P = 4.433g/31.0g = 0.1430 mol/0.1430 mol = 1
O = 5.717g/16.0g = 0.3573mol/0.1430mol=2.5
P 1 x 2 = 2 O = 2.5 x 2 = 5 EF= P2O5
Molecular Formula
The actual formula of the compound
relationship between EF and molecular formula
x(empirical formula mass ) = (molecular formula mass)
x is the whole-number multiple indicating the factor by which subscripts in EF are multiplied to obtain molecular formula
Molecular Formula
In the previous problem EF was P2O5. The molar mass of the compound is 283.89g/mol. What is the molecular formula of the compound?
Molecular Formula
x(EF mass) = (Molar Mass)
EF mass is (2x 31.0g + 5 x 16.0g) = 142.0g
x(142.0g) = 283.89g/mol
x = 2
MF = (P2O5)2 = P4O10
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