Atomic mass Atoms of different elements have different masses Mass of a single atom is incredibly small so a special unit called atomic mass unit amu is used Atomic Mass Atoms are composed of three particles ID: 755224
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Slide1
Chapter 10
The MoleSlide2
Atomic Mass
Atomic mass
Atoms of different elements have different masses.
Mass of a single atom is incredibly small so a special unit called atomic mass unit (
amu
) is used. Slide3
Atomic Mass
Atoms are composed of three particles:
Electrons
Protons
Neutrons
Electron mass is too small to matter.
Protons + Neutrons = mass of atoms
Units = atomic mass units (
amu
)Slide4
Atomic Mass
Hydrogen – has only one protons
Mass = 1
amu
Oxygen - 8 neutrons and 8 protons
Mass = 16
amu
Carbon – 6 protons and 6 neutrons
Mass = 12
amuSlide5
Atomic Mass
How to find the mass of an entire molecule of a compound:
Example: H
2
O
2 hydrogen atoms and 1 oxygen atom.
Atomic mass of hydrogen = 1.0amu
Atomic mass of oxygen = 16.0amu
2(1.0amu) + 16amu = 18
amu
The sum of the atomic masses of all the atoms in a compound is the
formula mass
of the compound. Slide6
Sample Problems
Find the formula mass of baking soda (NaHCO
3
)Slide7
What is a Mole?
Atomic mass units are not practical for using in the lab.
Easier to work with grams.
In the early 1900s chemists looked for a way to relate the atomic mass of 1 atom of an element in atomic mass units to an amount of that element in grams. Slide8
What is a mole?
A mole is a unit of measurement.
Used to count extremely small particles, atoms, elements, electrons…
A way to translate between grams and atomic mass units (
amu’s
)
Used for measuring the amount of particles in something. Slide9
Understanding the Mole
Units are used to quantify something.
Example:
1 dozen = 12 of something
3 dozen eggs = 36 eggs.
1 mole = 602,200,000,000,000,000,000,000 of something
A mole is just a large number of something.Slide10
The Mole
6.022 x 10
23
=
Avagadro’s
number
Based on the number of atoms in 12 g of Carbon-12.
1 mole of Carbon-12 has
602,200,000,000,000,000,000,000
carbon
atoms.
Just like 1 dozen of eggs has 12 eggs.Slide11
Understanding The Mole
1 mole of anything has 6.022 x 10
23
particles.
1 mole of anything will have the
same
number of particles, but a
different
weight.
Just like a dozen eggs vs. a dozen elephants.
1 mole of an element will equal its atomic mass.
1 mole of carbon has 6.022
x 10
23
atoms and weighs = 12 grams
1 mole of hydrogen
has 6.022 x 10
23
atoms and weighs = 1 gramSlide12
The Mole
The mole established a relationship between the atomic mass unit and the gram.
The mass in grams of 1 mole of a substance is numerically equal to its atomic mass or formula mass in atomic mass units.
Atomic mass of oxygen = 16
amu
1 mole of oxygen = 6.022 x 10
23
atoms
1 mole of oxygen = 16 gramsSlide13
Mole Summary
The number of atoms in 1 mole of an element is always 6.02 x 10
23
The number of molecules (covalently bonded atoms) in a mole of any molecular compound is
6.02 x 10
23
Example: 1 mole of H
2
contains
6.02 x 10
23
H
2
molecules.Slide14
Formula units and moles
Mole is also used to describe number of ions in an ionic compound.
Ionic compounds exist in crystals not as single-bonded compounds.
Ionic compounds are represented by
formula units.
Lowest whole number ratio.
1 mole of an ionic compound contains 6.02 x 10
23
formula units. Slide15
Formula units and moles
Example:
Sodium Chloride
Na
+
Cl
-
Lowest ratio =
NaCl
1 mole of sodium chloride contains
6.02 x 10
23
formula units of
NaCl
Calcium Chloride
Ca
2+
Cl
-
Lowest ratio = CaCl
2
1 mole of calcium chloride contains
6.02 x 10
23
formula units of
CaCl
2Slide16
Molar Mass
*The number of items in a mole of a substance is always the same, but the mass of that mole varies.
The mass in grams of 1 mole of a substance is called the molar mass of the substance.
Molar mass =
M
How much does 1 mole of something weigh?
If you have
6.02 x 10
23
of something, how much will it weigh?Slide17
Molar Mass
The molar mass of a substance can be determined from its atomic mass of formula mass.
1 mole
of any element is the amount of that substance that has a mass in grams numerically equal to the
atomic mass or formula mass. Slide18
Molar Mass
What is the molar mass of glucose? (C
6
H
12
O
6
)
Meaning – how much does 1 mole of glucose weigh?
Given: Chemical formula: C
6
H
12
O
6
Need: Molar Mass
Known: molar mass (g) = formula mass (
amu
)
Solve for formula mass:
C = 12.0amu x 6 = 72
amu
H = 1.0
amu
x
12
=
1
2
amu
O = 16.0
amu
x 6 = 96
amu
Formula mass = 180
amu
Molar mass of 1 mole of glucose = 180 g/
molSlide19
Practice Problems:
Find the molar mass of :
Sulfur Dioxide (SO
2
)
Benzene (C
6
H
6
)Slide20
Homework
Pg. 322 1-5Slide21
Mole Conversions
10-2Slide22
Mole Conversion
The mole measures both a mass and a number of particles so it is the central unit in converting the amount of a substance from one type of measurement to another. Slide23
Mole Conversion
Review your Conversions
Examples:
1 m = 100 cm
1 kg = 1000 g
1 dm = 10 cm
1 m = 1000 mm
Remember that 1 dm3 = 1 L = 1OOO cm3. Slide24Slide25
Mole Conversion
The factor-label method treats labels as factors.
These factors divide out.
There are two main rules to solving science problems with the factor-label method:
1. Always carry along your units with any measurement you use.
2. You need to form the appropriate labeled ratios (equalities).Slide26
Mole Conversion
Example Problem:
How many centimeters in 2 meters?
You will see from the metric conversion chart that 1 meter = 100 cm
we turn this into a ratio by writing it like this:Slide27
OrSlide28
Mole Conversion
Once you have the equalities you must pick the one that will cancel out the units leaving the desired units.
Then multiply your starting quantity (2 meters) by the equality that will give you your desired units.Slide29
As a rule of thumb your problem set up should look like this: Slide30
Practice Problems
1. How many wheels on 350 Ford pickups (use the equality 1 pickup = 4 tires) - the starting units are pickups, the ending units need to be wheels. Slide31
Practice Problems
What is the mass of 3.20 moles of zinc nitrate?
Slide32
Practice Problems
What is the mass of 5.50 moles of
NaOH
?
How many moles of PbSO
4
are in 158.1g of the compound?Slide33
SIGNIFICANT DIGITS
In scientific notation, all numbers are expressed as the product of a number between 1 and 10 and a whole-number power of 10.Slide34
SIGNIFICANT DIGITS
In addition and subtraction, the answer may contain only as many decimal places as the measurement having the least number of decimal places.
In multiplication and division, the answer may contain only as many significant digits as the measurement with the least number of significant digits.Slide35
Significant Digit Rules
1) ALL non-zero numbers (1,2,3,4,5,6,7,8,9) are ALWAYS significant.
2) ALL zeroes between non-zero numbers are ALWAYS significant.
3) ALL zeroes which are SIMULTANEOUSLY to the right of the decimal point AND at the end of the number are ALWAYS significant.
4) ALL zeroes which are to the left of a written decimal point and are in a number >= 10 are ALWAYS significant.Slide36
Particles and Moles
Equations are also used to convert between moles and particles.
The identity of the substance is not important because the number of particles in 1 mole of any substance is always the same
Avogadro’s number 6.02 x 10
23Slide37
Particles and Moles
It is important to note that one mole of atoms contains 6.02 x 10E23 atoms.
One
mole of molecules contains 6.02 x 10E23 molecules.
One
mole of formula units contains 6.02 x 10E23 formula units.
One
mole of ions contains 6.02 x 10E23 ions. Slide38
Particles and Moles
How many molecules are in 2 moles of water?
Multiply the number of molecules in 1 mole of of water by 2.
(6.02 x 10
23
) x 2 = 1.20 x 10
24
molecules of water in 2 moles.
How many
formula units are in 3 moles of Zn(NO
3
)
2Slide39
Particles and Moles
If the number of particles in a sample is given the number of moles of that substance can also be calculated.
Divide the number of particles by Avogadro’s number. Slide40
Practice Problems
A piece of marble contains 8.74 x 10
23
formula units of calcium carbonate (CaCO
3
). How many moles of CaCO
3
is that?
Determine the number of atoms that are in 0.58
mol
of Se.Slide41
Multistep Conversions
Converting between mass and particles requires multiple steps.
Example: A sample of
Ca
has 7.2 x 10
22
atoms. What is the mass of this sample?Slide42
Multistep Conversion
Step 1: Convert the number of atoms to moles.
Step 2: Convert the number of moles to mass.Slide43
Practice Problems
2. How many millimeters in 34 hectometers (use the equality 10,000 mm = 1 hectometer)?
Sometimes you will need to multiply by more than one ratio to get to your desired units, you can do this by using linking units. Your setup will look like this:Slide44
Practice Problems
3. How many inches in 1 meter given the equality 1 inch = 2.54 cm and 1 meter = 100 cm? (note the linking unit in this problem is cm) Slide45
Practice Problems
4. How many grams in 150 pounds given the equalities 1 pound = 0.454 kg and 1 kg = 1000 grams? Slide46
Practice Problems
You need 250 grams of sugar (C
12
H
22
O
11
) to bake a cake. How many sugar molecules will be in the cake?Slide47
Moles and Gases
In a gas particles are spread out from one another.
Avogadro stated that at the same temperature and pressure, equal volumes of gases contain the same number of gas particles.
1 mole of a gas occupies the same volume as 1 mole of any other gas at the same temperature and pressure.
Molar volume = 22.4 L
1 mole of any gas at standard temperature and pressure has a volume of 22.4 LSlide48
Practice Problems
How many molecules of carbon dioxide are in a 1.0-L flask?Slide49
Mole Conversions:
MOLES
MASS
PARTICLES
VOLUME
Molar Mass
Number of particles in 1 mole
Molar volume (22.4 L/
mol
)Slide50
Homework
Pg. 331 1-3Slide51
Empirical and Molecular formulas
10-3Slide52
Formulas
The formula for a compound indicates the number and kind of each atom in a particle of the compound.
How is a chemical formula determined?Slide53
Percentage Composition
The percentage composition of a compound is a statement of the relative mass each element contributes to the mass of the compound as a wholeSlide54
Percentage Composition
Salt is composed of two elements, sodium and chlorine.
We
know they are always present in the same ratio by mass.
The
ratio in which they are present is the ratio of their atomic masses.
Therefore
, the percentage of sodium in any sample of sodium chloride would be the atomic mass of the element divided by the formula mass and multiplied by 100. Slide55
Percentage Composition
Often, because we round off answers, we may find that the percentages total one or two-tenths more or less than 100%.Slide56
Percentage Composition
The mass of a compound is made up of smaller amounts of different elements.
We can determine what part of the total mass of a compound is made up by each element in a compound.
The mass of each element in a compound compared to the entire mass of the compound and multiplied by 100 percent is called the percentage composition of the compound.
The
percentage composition
tells you the percent of the mass made up by each element.Slide57
Percentage Composition
The percentage composition can be determined in two ways:
By calculating percentage composition from a given formula.
Example: 1 mole of water – mass equals 18 grams.
1 mole of water is composed of 2 moles of hydrogen and 1 mole of oxygen: H
2
O.
To find percentage composition: determine what part of the total mass is made up of hydrogen atoms and what part is made up of oxygen atoms.
Step 1: find mass of hydrogen atoms: 2.0 grams
Step 2: divide the mass of the hydrogen atoms by the total mass of the compound and multiply by 100 percent.Slide58
Percentage Composition
By experimental analysis:
The mass of the sample is measured.
Sample is decomposed into its components.
The masses of the components are determined and percentage composition is calculated. Slide59
Sample Problems
A sample of 2.45 g of aluminum oxide decomposes into 1.30g of aluminum and 1.15g of oxygen. What is the percentage composition of the compound?
Find the percentage composition of a compound that contains 1.94g of carbon, 0.48g of hydrogen, and 2.58g of sulfur in a 5.00g sample of the compound. Slide60
Empirical Formula
Elements are made of atoms, and compounds are made of elements.
Half an atom does not exist.
Therefore
, we can state that the elements in a compound combine in simple whole number ratios, such as
1 to 1, 1 to 2, 2 to 3
, and so on.
If
the atoms of the elements are present in simple ratios, then the moles of atoms for each element in the substance will be in small whole number ratios. Slide61Slide62
Empirical Formula
Consider the following example:
We find a 2.5-gram sample of a certain substance contains 0.9 gram of calcium and 1.6 grams of chlorine.
The substance is composed of only two elements. Slide63
Empirical Formula
We can calculate the number of moles of calcium and the number of moles of chlorine in the compound.
Then
, we can find the ratio of the number of moles of calcium atoms to the number of moles of chlorine atoms.
From
this ratio, we can find the empirical formula, which is the simplest ratio of atoms in a compound. Slide64Slide65
Determining Empirical Formula
By knowing the percentage composition the empirical formula of a compound can be determined.
Empirical formula
– a formula that gives the simplest whole-number ratio of the atoms of the element.
Example: Hydrogen Peroxide – HO
Shows that a molecule of hydrogen peroxide always has equal numbers of hydrogen and oxygen atoms.
Does not mean that a molecule of hydrogen peroxide has only 1 of each atom.
Shows ratio: HO – 1:1Slide66
Empirical Formula
Determining empirical formula:
Example: A compound has the following percentage compositions: 80.0 percent carbon, 20.0 percent hydrogen.
Assume you are dealing with 100.0 grams.
A 100.0 gram sample contains 80.0grams carbon and 20.0grams hydrogen.
The empirical formula is found by converting the mass of each element in the sample to the number of moles of atoms of that element.
Conversion factor is
molar mass.Slide67
Empirical Formula
100.0 grams of the compound contains 6.67 moles of carbon atoms and 19.8 moles of hydrogen atoms.
Smallest
whole number ratio
is found by dividing each mole value by the smaller of the two values.
Empirical formula: Slide68
Sample Problems
Determine the empirical formula of a compound containing 5.75g Na, 3.5g N, and 12.0g O.
Determine the empirical formula of a compound containing 2.644g of gold, and 0.476g of chlorine.Slide69
Empirical Formula
Sometimes by dividing by the smallest number the moles does not yield a ratio close to a whole number.
Examples:
2.33 to 1 = 7 to 3
1.33 to 1 = 4 to 3
1.67 to 1 = 5 to 3Slide70
Molecular Formula
In order to calculate a molecular formula, we need one additional piece of data, the
molecular mass
. Slide71
Determining Molecular Formula
The empirical formula for a compound indicates the simplest ratio of the atoms in the compound.
Does not tell you the actual numbers of atoms in each molecule of the compound.
Hydrogen peroxide:
empirical formula: HO
molecular formula: H
2
O
2
The ratio is 1:1, the number of atoms of each element is not necessarily 1 and 1.
The exact number of atoms in each molecule of a compound is very important because it determines the properties, or characteristics, of that compound. Slide72
Molecular Formula
The formula that gives the actual numbers of atoms of each element in a molecular compound is called the molecular formula.
The molecular formula is always a whole-number multiple of the empirical formula.
Molecular formula can be determined by comparing the molar mass of the unknown compound with the molar mass of the empirical formula. Slide73
Molecular Formula
In one of the examples in the previous section, the empirical formula calculated was CH2O.
If we know that the molecular mass of the compound is 180 g/mol, how can we find the molecular formula? Slide74
Molecular Formula
The molecular formula shows the number of atoms of each element in a molecule.
Knowing
that the elements will always be present in the ratio 1:2:1, we can calculate the mass of the empirical formula. Slide75
Molecular Formula
Then we can find the number of these empirical units present in one molecular formula. In the substance CH20, the empirical unit has a mass of
12 + 2(1) + 16, or 30 Slide76
Molecular Formula
It will, therefore, take six of these units to equal 180 or one molecular formula.
Thus, the molecular formula is C6H1206. Slide77
Practice Problems
Find the molecular formula of a compound that contains 45.56g of palladium and 0.80g of hydrogen. The molar mass of the compound is 216.8g/mol.
Find the molecular formula for a compound that contains 4.90g N and 11.2 g O. The molar mass of the compound is 92.0 g/mol.Slide78
Homework
Pg.
339 1-3