Download 1. 2.3 moles 2. 88.1 moles 3. 11.9 grams of Ca(OH)2 (HINT: TWO

Percent Calc, EF and MF
February 02, 2015
WARM UP: Feb. 2
Convert the number of moles or grams to the number of particles using
Avogadro’s number:
1. 2.3 moles
2. 88.1 moles
3. 11.9 grams of Ca(OH)2 (HINT: TWO STEPS)
MM Ca(OH)2
Ca: 40 g/M
O: 16 x 2 g/M
H: 1 x 2 g/M
MM: 74 g/M
Convert to Moles first:
11.9 grams
74 grams/mole
= 0.16 moles
Convert to Molecules second:
0.16 moles x (6.02 x 1023 mol./moles)
= 9.6 x 1022 molecules
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4. Find the Molar Mass of Al(NO3)3?
Al:
N:
O:
February 02, 2015
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February 02, 2015
Calculating Percent Composition,
Empirical Formula and Molecular
Formulas
February 2, 2015
1
Percent Calc, EF and MF
February 02, 2015
Percent Composition
• Sometimes it’s useful to know the composition
of a compound in terms of what percentage
of the total is each element
• Percent
>
>
>
“Parts per 100”
The number of specific items per a group of 100
items
50% of $100 is $50 (50 items/100 total items)
2
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February 02, 2015
Percent Composition
• Each compound has a different percent
composition
3
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February 02, 2015
Percent Example
• You have 4 oranges and 5 apples. What
percent of the total is oranges?
• In “parts per 100”
4
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February 02, 2015
Percent Composition
• It is the percent by mass of each element in a
compound
• Can be determined
>
>
By its chemical formula
Molar masses of the elements that compose the
compound
• The percent of each element contributes to
the mass of the compound
5
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February 02, 2015
Calculating Percent Composition Example #1
• What is the percent composition of each element in
NH4OH?
Determine the contribution of each element
CALCULATE
MOLAR MASS
Molar OF
massNH OH
4
6
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February 02, 2015
Calculating Percent Composition Example # 2
• Calculate the percent composition of CuBr2?
7
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February 02, 2015
Empirical Formulas
• The simplest ratio of elements in a
compound
• It uses the smallest possible whole number
ratio of atoms present in a formula unit of a
compound
• If the percent composition is known, an
empirical formula can be calculated
7
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February 02, 2015
Empirical Formulas
• To Determine the empirical formula:
• Calculate the moles of each element
> Use molar mass (sum of atomic masses)
• Calculate the ratios of the elements to each
other
• Find the lowest whole number ratio
> Divide each number of moles by the smallest
number of moles present
8
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February 02, 2015
Empirical Formula: Converting Decimal
Numbers to Whole Numbers
• The subscripts in a formula are expressed as
whole numbers, not as decimals
• The resulting numbers from a calculation
represent each element’s subscript
• If the number(s) are NOT whole numbers,
multiply each number by the same small integer
(2, 3, 4, 5, or 6) until a whole number is
obtained
9
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February 02, 2015
Relating Empirical and
Molecular Formulas
• n
represents a whole number multiplier from 1 to as large as
necessary
• Calculate the empirical formula and the mass of the
empirical formula
• Divide the given molecular mass by the calculated
empirical mass
> Answer is a whole number multiplier
10
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February 02, 2015
Relating Empirical and
Molecular Formulas
• Multiply each subscript in the empirical
formula by the whole number multiplier
to get the molecular formula
11
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February 02, 2015
Calculate Empirical Formula from Percent
Composition
• Lactic acid has a molar mass of 90.08 g and has
this percent composition:
40.0% C, 6.71% H, 53.3% O
• What are the empirical and molecular formulas of
lactic acid?
• Assume a 100.0 g sample size
> Convert percent numbers to grams
> Ex: 40.0% C is 40.0 g
12
Percent Calc, EF and MF
February 02, 2015
Calculate Empirical Formula from
Percent Composition
• Convert mass of each element to moles
• Divide each mole quantity by the smallest number of moles
Empirical
formula is
13
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February 02, 2015
Determination of the
Molecular Formula for Lactic Acid
• Obtain the value of n (whole number multiplier)
• Multiply the empirical formula CH2O by the
multiplier
Molecular formula = n х empirical formula
Molecular formula = 3 (CH2O) =
14
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February 02, 2015
Formulas for Compounds
• Empirical Formula (EF)
> Smallest possible set of subscript numbers
> Smallest whole number ratio
> All ionic compounds are given as empirical formulas
• Molecular Formulas (MF)
> The actual formulas of molecules
> It shows all of the atoms present in a molecule
> It may be the same as the Empirical Formula or a
whole-number multiple of its EF
Molecular formula = n х Empirical formula
15
Percent Calc, EF and MF
February 02, 2015
A 5.438 g sample of ludlamite, a greyish-blue mineral
used in ceramics was found to contain 2.549 g of iron,
1.947 g of oxygen and 0.9424 g of phosphorous. What is
the EF?
Mole Ratio:
Convert to moles:
Percent Calc, EF and MF
February 02, 2015