Download 1 Packet #3 Mass Relationships in Chemical Reactions How is

Packet #3
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Mass Relationships in Chemical Reactions
How is mass measured?
Can a single atom be weighed?
How is this done experimentally?
Step 1
 Assign a value to the mass of one atom of a given element and use it as a standard for comparison
Step 2
 Define the standard

One atomic mass unit is defined as a mass equal to 1/12 the mass of one carbon-12 atom.

Atomic mass (atomic weight) is the mass of the atom in atomic mass units (amu).

1 amu = 1 Dalton

Now that we have set the atomic mass of carbon-12 to 12 amu we can compare other elements to
carbon-12 to determine their atomic weights.

Example: Experiments have shown that the mass of a hydrogen atom is about 8.400% that of a
carbon-12 atom. Therefore, we can calculate the atomic mass of hydrogen via 0.084 x 12.00 amu =
1.008 amu.
Average Atomic Mass
 Unfortunately, Carbon does not exist solely as Carbon-12. Carbon-13 also exists.

When you look at a periodic table, Carbon will not have an atomic mass of 12.00 amu.

When atomic masses are calculated, an average is taken of the known abundances.

98.90% of all carbon exists as carbon-12

1.10% of all carbon exists as carbon-13

So, (0.9890)(12.00000 amu) + (0.0110)(13.00335 amu) = 12.01 amu
Practice Exercise 3.1 (page 77)
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Avogadro’s Number (NA) and Molar Mass of an Element

Mole (mol) = SI unit of Amount of a substance
 Specifically, it is the amount of a substance that contains as many elementary entities
(atoms, molecules, or other particles) as there are atoms in exactly 12 g of the carbon-12
isotope.
 NA = 6.0221367 x 1023 particles
Molecular Mass (molecular weight)
The sum of the atomic masses (in amu) in a substance
Molar Mass
Molar mass (in grams) is numerically equal to its molecular mass in (amu)
Practice
1. Convert 3.52 g Fe to moles.
2. Convert 0.253 g H2O to moles.
3. How many molecules of O2 are in 31.5 g O2? How many atoms of oxygen?
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Mass Spectrometer
Percent Composition
 Defined as % by mass of each element in a compound.
 = (# moles of an element in 1 mol of the substance x molar mass of the element/ molar mass)100
Practice
4. Calculate the % composition by mass of hydrogen, oxygen, and sulfur in sulfuric acid (H2SO4)
5. Determine the empirical formula of Barium carbonate given that a sample contains 69.58% Ba, 6.090%
C and 24.32% O.
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Empirical Formulas from Combustion Analyses
 Works great for substances that burn completely!
 For example substances containing C, H and O
Practice
6. A sample of a liquid consisting of only C, H, and O and having a mass of 0.5438 g was burned in pure
oxygen. The following products were obtained: 1.039 g CO2 and 0.6369 g H2O. What is the empirical
formula of the compound?
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Molecular Formulas
1. Styrene has an empirical formula of CH. Its formula weight is 104 g/ mol. What is its molecular
formula?
2. A compound with 87.42% N and 12.58% H has a formula weight of 32.1 g/mol. What is its molecular
formula?
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Stoichiometry 화학량론

Stoichiometry refers to the number of reactants and products counted in moles as indicated by the
coefficients in a balanced chemical equation.
Consider the meaning of the following reaction: 2Na + Cl2  2NaCl
1. Two atoms of sodium react with one molecule of chlorine to form two formula units of sodium chloride.
2. Two dozen sodium atoms react with one dozen chlorine molecules to form two dozen formula units of
sodium chloride.
3. Two moles of sodium atoms react with one mole of chlorine molecules to form two moles of sodium
chloride.

나트륨 원자의 2마리의 두더지는 염화 나트륨의 모양 2 두더지에 염소 분자의 1마리의
두더지로 반작용한다
4. 2(6.022 x 1023) atoms of sodium react with 6.022 x 1023 molecules of chlorine to form 2(6.022 x 1023)
formula units of sodium chloride.
Balancing Equations
Be organized with the bookkeeping! Make a list when needed.
Examples
1. Mg +
O2

MgO
2.
C2H6 + O2  CO2 + H2O
3.
Al(OH)3
+ H2SO4

Al2(SO4)3
+ H2O
Mole – Mole Relationships
You need a balanced chemical equation!
Example 2K + Cl2  2KCl

How many moles of potassium are needed to produce 0.2136 moles potassium chloride?
Mass – Mole Relationships
You need a balanced chemical equation!
Example 2K + Cl2  2KCl

How many grams of potassium chloride are produced from 0.0235 g of potassium?
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Limiting reactants, Theoretical yield, & Percentage yield
1. Carbon tetrachloride can be made by reacting chloroform with chlorine according to the following reaction:
CHCl3 + Cl2  CCl4 + HCl
If 29.7 g of CCl4 were produced when 25 grams of CHCl3 reacted with 25 g of Cl2, what was the % yield?
2. Aluminum chloride, Al2Cl6, can be made according to the following reaction:
2Al + 3Cl2  Al2Cl6
When 20.0 g of aluminum was allowed to react with 30.0 g chlorine, 35.5 g of aluminum chloride was
recovered. What is the percent yield for this reaction?
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