Transcript math_in_chemistry

Math in Chemistry
Percent Composition
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Purpose:
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Can be used to figure out chemical formulas.
I. Percent Composition
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Two different types of problems:
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1) Masses are given
2) No Masses are given
Masses are Given
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Steps to solve problem:
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1) Add given masses to get total mass for one
compound
2) Divide mass of each element by the total mass
3) Multiply by 100 to get the percent
Masses are Given
Examples
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1) A sample of Silver sulfide was found to
contain 29.0 g of Ag and 4.30 g of S.
Calculate the percent composition.
Masses are Given
Examples
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2) 222.6 g of Na combines completely with
77.4 g of O. Calculate the percent
composition.
No Masses Given
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Steps to solve problem:
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1) Assume you have 1 mole of the compound and
calculate its molar mass
2) Determine the molar masses of each element
in the compound
3) Divide the molar mass for the element by the
molar mass of the compound
4) Multiply by 100 to get the percent
No Masses Given
Examples
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1) Calculate the percent composition of C
and H in ethane, C2H6.
No Masses Given
Examples
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2) Calculate the percent composition of
sodium hydrogen sulfate.
II. Calculate the Mass of an
Element in a Compound
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Steps to solve the problem:
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1) Find the molar mass of the compound
2) Find the percent composition of the element
3) Set up a conversion factor problem
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Given mass (g compound) x percent composition/100
(g compound)
Calculate the Mass of an Element
in a Compound
Examples
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1) Calculate the mass of hydrogen in the
following:
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A) 350 g C3H8
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B) 20.2 g NaHCO3
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C) 378 g HCN
III. Empirical Formulas
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Empirical formulas are the lowest whole
number ratio of the atoms of the elements in
a compound.
Molecular Formula
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C2O4
Na2O2
Empirical Formula
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CO2
NaO
B. Empirical Formula
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Smallest whole number ratio of atoms in
a compound
C2H6
reduce subscripts
CH3
B. Empirical Formula
1. Find mass (or %) of each element.
2. Find moles of each element. (divide
given mass by molar mass)
3. Divide answers by the smallest # to
find subscripts.
4. When necessary, multiply subscripts
by 2, 3, or 4 to get whole #’s.
B. Empirical Formula
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Find the empirical formula for a
sample of 25.9% N and 74.1% O.
25.9 g 1 mol
= 1.85 mol N
14.01 g
1.85 mol
74.1 g 1 mol
16.00 g
= 4.63 mol O
1.85 mol
=1N
= 2.5 O
B. Empirical Formula
N1O2.5
Need to make the subscripts whole
numbers  multiply by 2
N2O5
Empirical Formulas
Examples
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2) What is the empirical formula for a
compound that is 79.8% C and 20.2% H?
Empirical Formulas
Examples
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3) What is the empirical formula of a
compound that is 67.6% Hg, 10.8% S, and
21.6% O?
IV. Molecular Formulas
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Usually the empirical formula is the molecular
formula for a compound. When it is not, the
molecular formula is defined as the elements
and number of atoms that are contained in a
compound.
IV. Molecular Formulas
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Molecular formulas are always multiples of
empirical formulas.
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CH3
C 2H 6
IV. Molecular Formulas
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Steps to solve the problem:
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1) Determine the empirical formula
2) Divide the molecular mass by the empirical
formula mass to get a ratio.
3) Multiply the elements’ subscripts by the number
you get in step 2.
Molecular Formulas
Examples
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1) Calculate the molecular formulas of the
following compounds:
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Molecular Mass
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60 g
78 g
181.5 g
Empirical Formula
CH4N
NaO
C2HCl
Molecular Formulas
Examples
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2) The compound methyl butanoate smells
like apples. Its percent composition is 58.8%
C, 9.8% H, and 31.4% O. If the molecular
mass is 102 g/mol, what is the molecular
formula?
Molecular Formulas
Examples
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3) You find 7.36 g of a compound has
decomposed to give 6.93 g of oxygen. The
rest is hydrogen. If the molecular mass is
34.0 g/mol, what is the molecular formula?
STP
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STP = Standard temperature and pressure
Where,
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Temperature = 0°C = 273 K
Pressure = 101.3 kPa = 1 atmosphere (atm)
For ANY gas at STP,
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1 mol = volume of 22.4 L (molar volume) = 6.02 x
1023 particles of that gas
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Unit of density of a gas = g/L
Can be used to calculate the gram formula
mass of that gas
Example 1:
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Determine the volume of 0.600 mol of sulfur
dioxide gas at STP
More Examples
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Determine the # of moles in 52.5 L of He gas
at STP
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Find the Number of Particles in 30 L of sulfur
gas at STP
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Find the mass of 43 L of nitrogen gas at STP