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Empirical Formula & Molecular
Formula
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Empirical Formulas
•Best way to identify an unknown compound is to determine its chemical formula.
•Empirical Formula: the smallest whole number ratio of a compound. (Ionic compounds are already in its empirical formulas)
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Empirical Vs. Molecular Formulas
•Molecular Formula
• a formula that shows the element symbols & exact number of each type of atom in a molecular compound
• Empirical Formula
• a formula that shows the simplest whole-number ratio of elements in compounds
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CH4 Vs. Vs. C18H72C6H24
Lowest ratioIn empirical
& molecular formula
can be reduced to lowest ratio6 can go into 6 & 24--> CH4
same empirical formula. But not the
same molecular formula
can be reduced to lowest ratio18 can go into
18 & 72--> CH4
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•If you know the chemical formula you can find percent composition.
CH4 C6H24
M= 12.01g/mol + (4 x 1.01g/mol) = 16.00 g/mol
MC= 12.01g/mol
MH= 1.01g/mol
(12.00g/mol )x 1mol
16.00g
x100% = 75%
(4.01g/mol)x 1mol
16.00g
x 100% = 25%
MC6H24= 96 g/mol
MC= 72 g/mol
(72 g/mol ) 1molx96.00g
x100%= 75%
- same empirical formula- should be ratios of each other-helps determine percent composition
CH4 C6H24&
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Determining Empirical Formula• Find the empirical formula of a compound
with percentage composition 35.4% Na and the remainder nitrogen
Step 1: Assume your conveniently working with a mass of 100.0 g.
100.0g - 35.4g = 64.6 g
Na= 35.4 g
N= 64.6 gStep 2: Calculate the amount of each element.
nNa = (35.4 g) (1 mol)22.99g
nN = (64.5 g) (1 mol)14.01g
= 1.540 mol = 4.6110 mol
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Determining Empirical Formula• Find the empirical formula of a compound
with percentage composition 35.4% Na and the remainder nitrogen
Step 3: To determine the simplest ratio of the elements in the compound, divide the amount of each element by the smallest amount
nNa
= 1
nN
1.540 mol4.6110 mol
nNa= 1.540 mol
nNa= 1.540 mol
= 2.98
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Determining Empirical Formula• Find the empirical formula of a compound
with percentage composition 35.4% Na and the remainder nitrogen
Step 2: Calculate the amount of each element.
nNa
= 1nN 1.540 mol
4.6110 molnNa
= 1.540 mol nNa
= 1.540 mol
= 2.98The ratio for Na & N is1:2.98. Since we cannot have a fraction of an element in a compound, the value of N is rounded off to the nearest whole number. As a result, the simplest whole-number ratio is 1:3........NaN3
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•If a number is within 0.05 of a whole number, you can round it up or down to the nearest whole number.
•But what do you do if one of the values is not close to a whole number? You multiply all the subscripts by the fraction denominator gives whole numbers
•C1 x 3 H1.333 x 3 = C3H4
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Practice• Determine the empirical formula of a compound listed that
contains 52.2% carbon, 6.15% hydrogen and 41.7% oxygen
Step 1: Assume your conveniently working with a mass of 100.0 g.
nc =(52.2 g) (1 mol)12.01g
= 4.3464 mol
nH =(6.15g) (1 mol)1.01g
= 6.0891mol
n0 =(41.7g) (1 mol)16.00g
= 2.6063mol
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Practice• Determine the empirical formula of a compound listed that
contains 52.2% carbon, 6.15% hydrogen and 41.7% oxygen
Step 2: Divide the amount of each element by the smallest amount.
nc =
= 1
nH =
= 1.67
n0 =2.6063moln0
n0
n0
2.6063mol
2.6063mol
2.6063mol6.0891mol
4.3464 mol
= 2.29
Step 3: These calculations give an empirical formula of C1.67H2.29O1
Multiplying each of the subscripts by 3 give C5H7O3
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Practice• Write the empirical formula of compounds with the
following percentage
• 20.2% Al; 79.8% Cl
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Practice• Write the empirical formula of compounds with the
following percentage
• 50.85% carbon; 8.47% hydrogen; and 40.68% oxygen
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Molecular Formula•gives the exact number of atoms of
each element present
•“chemical formula” = molecular formula
•A compound’s molar mass is always a whole number multiple of the molar mass of the empirical formula. You ca find this multiple x, by dividing the molar mass of the compound by the molar mass of the empirical formula
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Molecular Formula
x = molar mass of compoundmolar mass of empirical
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MCH2 = 1( 12.01 g/mol) + 1(1.01g/mol)
Find a Molecular Formula Given its Empirical formula
Step 1: Find the empirical molar mass by adding the molar mass of each element
Compound with empirical formula CH2 & the molar mass of 84.18 g/mol
= 14.03 g/mol
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x = 84.18 g/mol14.03 g/mol
Find a Molecular Formula Given its Empirical formula
Step 2: Solve for x,
Compound with empirical formula CH2 & the molar mass of 84.18g/mol
x = 6
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CH2= C6H12
Find a Molecular Formula Given its Empirical formula
Step 3: therefore, the molar mass of the compound is 6 times the molar mass of the
empirical formula
Compound with empirical formula CH2 & the molar mass of 84.18g/mol
molecular formula
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Practice• Determine the molecular formulas
• KSO4; 270.32 g/mol
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Using Percentage Composition & Molar
Mass Data•Determine the molecular formula of vitamin C
(ascorbic acid). This compound contains 40.5% Carbon, 4.6% hydrogen, 54.5% oxygen. Its molar mass is 176.14g/mol