Transcript The Mole - South Shore International College Preparatory High School

Forensic Chemistry-Mrs. Terry-McCants
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Measuring matter deals with how you can
convert a count, mass and mass of
something.
Knowing how the count, mass and volume of
an item relate to a common unit allows you to
convert among these units.
For example:
1 dozen apples/12 apples
1 kg Apples/ 2.2 lbs. apples
A mole of a substance is 6.02 x 10^23
particles
Of a substance.
 A mole is the SI unit for measuring the
amount of a substance.
 Avogadro’s number is the number of
representative particles in a mole:
6.02 x 10^23
 The mole allows chemists to count the
number of representative particles in a
substance.
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Magnesium is a light metal used in the
manufacture of aircraft, automobile wheels, and
tools. How many moles of magnesium is 1.25 x
10^23 atoms of Mg?
Steps:
1. Analyze: List the knowns and the unknowns
2. Calculate: Solve for the unknowns.
3. Evaluate: Does this make sense?
4. Refer to pg. 309 in your textbk.
Analysis: Known= 1.25 x 10^23 atoms of Mg
Unknown= moles= ? Mol Mg
 Calculate: Solve for the unknown and set up
as a unit conversion.
 Equivalent:1 mol Mg = 6.02 x 10^23 atoms
Mg
 1.25 x 10^23 atoms Mg x 1 mole Mg
6.02 x 10^23 atoms of
Mg
Ans: 0.208 mol Mg
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Problem 1
How many moles is 2.80 x 10^24 atoms of
silicon?
Problem #2
How many moles is 2.17 x 10^23
representative particles of bromine?
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How do you convert the mass of a substance
to the number of moles of the substance?
Ans: Use the molar mass of an element or
compound to convert between the mass of a
substance and the moles of the substance.
Example: Molar mass
1 Mole
and
1 mole
Molar mass
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Known: # moles= 9.45 mol Al2O3
Unknown: mass= ? g Al2O3
Steps:
1. Determine the mass of 1 mole of Al2O3
2. Identify the conversion factor relating
moles of Al2O3 to grams of Al2O3.
3. Multiply the given number of moles by
the conversion factor.
Refer to p.318 in text sample problem 10.5.
Find the mass in grams of 2.50 moles of
iron II hydroxide.
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The empirical formula of a compound gives
the lowest whole-number ratio of the atoms
or moles of the elements in a compound.
The percent composition of a compound can
be used to calculate the empirical formula of
that compound.
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Steps:
1. Analyze by listing your knowns &
unknowns.
2. Convert the % composition of each element
to the moles of each element.
3. Divide each molar quantity by the smaller
number of moles to get 1mol for the element
with the smaller number of moles.
4. Multiply each part of the ratio by the
smallest whole number that will convert both
subscripts to whole numbers.
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Calculate the empirical formula of each of the
following:
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A. 94.1% O, 5.9% H
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B. 67.6% Hg, 10.8% S, 21.6% O
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Steps:
1. Calculate the empirical formula mass.
2. Divide the molar mass by the empirical
formula mass.
3. Multiply the formula subscripts by this
value.
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Calculate the molecular formula of a
compound whose molar mass is 60 g/mol
and empirical formula is CH4N.
Try #41 & #42 p.333 for additional practice.