Molarity
(Chemistry 101)
GROUP 4
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What is Molarity?
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Importance of Molarity
• Molarity is the ration used to express the concentration of
the solution. Knowing the molarity of a solution is
meaningful because by knowing it you can not only know if it
is diluted or concentrated, but also the actual concentration.
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Determining Molarity
• A mole of any element always contains 6.02 X 10 23
(Avogadro’s number) atoms.
• A mole of one element weighs a different amount than
another element since some atoms are heavier than others.
• The weight of a mole of a given element is defined to be
equal to its atomic weight in grams, or its gram atomic
weight.
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Determining Molarity
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• The weight of a mole of a given element is defined
to be
equal to its atomic weight in grams, or its gram atomic
weight.
• The atomic weight of an element can be found on a periodic
table of elements. (i.e. one mole of the element carbon
weighs 12.0 grams)
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Determining Molarity
• Compounds are composed of atoms of two or more elements that
are bonded together.
• A mole of a compound contains 6.02 X 10 23 molecules of that
compound.
• The weight in grams of 1 mole of the compound is the gram formula
weight or gram molecular weight (MW) of a compound.
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Determining Molarity
23 weights of
• The MW of a compound is calculated by adding the atomic
the atoms that make up the compound.
• For example, the molecular weight of sodium sulfate (Na 2 SO 4 ) is
142.04 g per mole:
•
•
•
•
2 sodium atoms 2 x 22.99g = 45.98 g
1 sulfur atom 1 x 32.06g = 32.06 g
4 oxygen atoms 4 x 16.00g = 64.00 g
= 142.04 g
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Sample Problem 1:
• What is the molarity of hydroiodic acid if the solution
is 47.0% HI by mass and has a density of 1.50 g/mL?
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Solution:
First calculate the mass of solute in the 47.0% solution using the
density. The 1.50 g/mL is the density of the solution but only 47.0%
of the solution is the solute therefore:
47.0% of 1.50 g/mL = (0.470) (1.50 g/mL) = 0.705 g/mL density of
solute
Since molarity is given in moles per liter and not grams we must
convert the g/mL to mol/mL using the molar mass.
0.705 g/mL (1 mole/ 128 g) = 0.00551 mol/mL
Next convert mL to L:
0.00551 mol/mL (1000 mL/ 1L) = 5.51 mol/L = 5.51 M
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Sample Problem 2:
• What is the molarity of a solution prepared by
dissolving 15.0 g of NaOH in enough water to make a
total of 225 mL of solution?
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Solution:
1 mol of NaOH has a mass of 40.00 g, so
Moles of NaOH = 15.0g NaOH ×
1 mol NaOH
40.00g NaOH
= 0.375 mol NaOH
Litres of solution = 225mL soln ×
1 L soln
1000mL soln
= 0.225 L soln
0.375 mol
0.225 L
= 1.67 mol/L
Molarity = moles of solute
=
litres of solution
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Stoichiometry
(Chemistry 101)
GROUP 4
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Stoichiometry
• Chemical Stoichiometry deals with the calculations
mole/mass/volume of the reactants and products involved in
a balance chemical reaction.
• The word “stoichiometry” is derived from the Greek word
“stoikhein” meaning element, and “metron” meaning
measure.
• The term Stoichiometry was first coined or discovered by a
German chemist named Jeremias Richter.
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Importance of Stoichiometry
• Stoichiometry is an important concept in Chemistry that
helps us use balanced chemical equations to calculate
amounts of reactants and products, during, before, and after
the chemical reaction.
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Things that can be measured are:
• Reactants and products mass
• Molecular weight
• Chemical equations
• Formulas
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Calculating Masses of Reactants
and Products
1. Balance the equation.
2. Convert mass or volume to moles,
if necessary.
3. Set up mole ratios.
4. Use mole ratios to calculate moles
of desired substituent.
5. Convert moles to mass or volume,
if necessary.
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Stoichiometric Coefficient
• The stoichiometric coefficient or stoichiometric
number is the number of molecules that participate in
the reaction.
• Stoichiometric coefficients can be fractions as well as
whole numbers.
• In essence, the coefficients help us to establish the
mole ratio between reactants and products.
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Example of a Stoichiometric Coefficient
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Balanced Reactions and Mole Ratios
• Atoms and molecules are extremely small in size, and their
numbers in a very small amount of a substance are very large.
• Therefore, to represent atoms and molecules in bulk, a mole
concept was
introduced.
One
mole
of
any
substance
contains
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6.022 x 10 numbers of that substance.
• This number is also known as Avogadro’s number, in honor to
Amedeo Avogadro
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Balanced Reactions and Mole Ratios
• The mass of one mole of a substance in grams is called molar
mass. The molar mass of one mole of a substance is numerically
equal to the atomic/molecular formula mass.
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The Mole (abbreviated as mol)
•
•
•
•
•
•
1 dozen cookies = 12 cookies
1 mole of cookies = 6.02 X 1023 cookies
1 dozen cars = 12 cars
1 mole of cars = 6.02 X 1023 cars
1 dozen Al atoms = 12 Al atoms
1 mole of Al atoms = 6.02 X 1023 atoms
• Note that the NUMBER is always the same, but the MASS is very different!
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Sample Problem 1:
What mass of chlorine does the decomposition of 64.0 g of AuCl₃ produce?
Solution:
1. Write the balanced chemical equation.
2AuCl3 → 2Au + 3Cl2
2. Convert grams of AuCl3 to moles of AuCl3.
3. Use the molar ratio to convert moles of AuCl3 to moles of Cl2.
4. Convert moles of Cl2 to grams of Cl2.
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Thank you for listening!
Have a good one!
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