1. Chemical Quantities

2. Calculating The Number of Atoms in a Specific Mass
You have a 1.00 g sample of lead. How many atoms of lead are present? -
1.00 g Pb
1 amu
1.66 x 10 24 g
1 atom Pb
207.2 amu =

3. Calculating Mass Example
Calculate the mass (in amu) of 1.0 х 104 carbon atoms
1) Given:
2) Plan: Convert from atoms to amu
3) CF
4) Set Up Problem

4. Formula Mass The sum of atomic masses of all atoms in its formula
Important role in nearly all chemical calculations
Can be calculated for compounds and diatomic elements
The atoms present in one formula unit of a substance
Can be calculated for both ionic and molecular substances

5. Calculating Formula Mass
Calculate the formula mass of calcium chloride
Write the formula from the name given
Ca2+ (from group II) and Cl- (from group VII)
Formula is CaCl2 due to charge balance
Formula mass: Sum of the atomic masses of atoms in the formula (1 Ca atom + 2 Cl atoms)
Write the name of the compound by combining Ca2+ and Cl-
1 amu Ca + 2 amu Cl is the formula mass of CaCl2
= amu
= amu
Formula mass of CaCl2

6. Counting Large Quantities
Many chemical calculations require counting atoms and molecules
It is difficult to do chemical calculations in terms of atoms or formula units
Since atoms are so small, extremely large numbers are needed in calculations
Need to use a special counting unit just as used for other items
A ream of paper
One dozen donuts
A pair of shoes
Because it requires working with large numbers

7. The Mole Mole: A unit that contains 6.022 х 1023 objects
It is used due to the extremely small size of atoms, molecules, and ions
6.022x1023 particles in 1 mole
Called Avogadro’s Number
Periodic Table
The average atomic mass in amu or g/mol (one atom)
The weight of 1 mole of the element in grams
Avogadro’s number provides the connecting relationship between molar masses and atomic masses
Chemist use a special counting units
Objects can be anything
“Objects” can be anything
Particles in the sense of atoms and molecules

8. Calculating the Number of Molecules in a Mole
How many molecules of bromine are present in mole of bromine gas?
Avogadro’s number
Given: mol Br2
Need: molecules of Br2
Equality:
Conversion factors:
Set Up Problem:

10. Subscripts State Moles of Elements
The subscripts in a chemical formula indicate the number of atoms of each element present in a compound
The subscripts in a chemical formula can also indicate the number of moles of atoms of each element present in one mole of a compound
i.e. In one molecule of glucose (C6H12O6) there are 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen
First case is the microscopic level per molecule
Second case is the macroscopic level per mole

11. Calculating the Moles of an Element in a Compound
How many moles of carbon atoms are present in 1.85 moles of glucose?
subscript
Plan: moles of glucose
moles of C atoms
Equality:
(One) mol C6H12O6 = 6 mols C atoms
Conversion Factors:
11.1 mol C atoms
Set Up Problem:

12. Molar Mass The atomic mass of a carbon-12 atom is 12.00 amu
The atomic mass of one mole of carbon-12 atoms g
One mole of any element is the amount of atoms (molecules or ions) that is equal to its atomic mass (in grams)
This mass contains х 1023 particles of that element
Use the periodic table to obtain the molar mass of any element
It was determined experimentally that Avogadro’s number of atoms gives the same numerical value between amu and mol
This is connection between the microscopic and macroscopic values

13. Molar Mass
When the number of grams (weighed out) of a substance equals the formula mass of that substance, Avogadro’s number of molecules of that substance are present

14. Molar Mass of a Compound
Calculate the molar mass of iron (II) sulfate
Formula is FeSO4
Calculate the molar mass of each element
Each element is multiplied by its respective subscript: (number of moles of each element)
The molar mass is calculated by the sum of the molar masses of each element
Formula
Subscript
Moles of
Compound
Moles of Element
in Compound

15. Molar Mass of a Compound
1) Formula is FeSO4: The molar masses of iron, sulfur, and oxygen are
2) Multiply each molar mass by its subscript
3) Find the molar mass of the compound by adding the mass of each element

16. Calculations Using Molar Mass
The three quantities most often calculated
Number of particles
Number of moles
Number of grams
Using molar mass as a conversion factor is one of the most useful in chemistry
Can be used for g to mole and mole to g conversions
Three most commonly calculated quantities for chemical problems
Molar mass can be used to calculate concentrations of solution, to calculate yields of products formed in reactions based on the limiting reagent

17. Relationship between Moles, Molar Mass and Avogadro’s number
Moles of
substance
Moles of
substance
Moles of
substance
Moles of
substance
Particles of
substance
Particles of
substance
Molar Mass
Moles of
substance
Grams of
substance

18. Converting Mass of a Compound to Moles
International Foods Coffee contains 3 mg of sodium chloride per cup of coffee. How many moles of sodium chloride are in each cup of coffee?
3 mg NaCl
moles of NaCl
= g NaCl
Equality:
1 mol NaCl = g

19. Converting Grams to Particles
Ethylene glycol (antifreeze) has the formula C2H6O2. How many molecules are present in a 3.86 × g sample?
Molar
mass
Avog
Number
Plan: convert g moles molecules of ethylene glycol
Conversion
Factor 1
Equality 1:
Equality 2:
Conversion
Factor 2
375 molecules

20. Percent Composition

21. Percent Composition
Sometimes it’s useful to know the composition of a compound in terms of what percentage of the total is each element
Percent
“Parts per 100”
The number of specific items per a group of 100 items
50% of $100 is $50 (50 items/100 total items)
The number of items of a specified type in a group of 100 items

22. Percent Example
You have 4 oranges and 5 apples. What percent of the total is oranges?
In “parts per 100”

23. Percent Composition
It is the percent by mass of each element in a compound
Can be determined
By its chemical formula
Molar masses of the elements that compose the compound
The percent of each element contributes to the mass of the compound
The formula of carbon dioxide indicates it is composed of one atom of carbon and two atoms of oxygen. Now we will express it in terms of mass percent

24. Calculating Percent Composition Example
What is the percent composition of each element in NH4OH?
Determine the contribution of each element
Molar mass
On a per mol basis

25. Empirical Formulas The simplest ratio of elements in a compound
It uses the smallest possible whole number ratio of atoms present in a formula unit of a compound
If the percent composition is known, an empirical formula can be calculated
Formulas are determined by calculation of data experimentally obtained
Smallest possible set of subscript numbers
The subscript numbers in formulas can be read in terms of the number of moles of each element

26. Empirical Formulas To Determine the empirical formula:
Calculate the moles of each element
Use molar mass (atomic mass)
Calculate the ratios of the elements to each other
Find the lowest whole number ratio
Divide each number of moles by the smallest number of moles present

27. Empirical Formula: Converting Decimal Numbers to Whole Numbers
The subscripts in a formula are expressed as whole numbers, not as decimals
The resulting numbers from a calculation represent each element’s subscript
If the number(s) are NOT whole numbers, multiply each number by the same small integer (2, 3, 4, 5, or 6) until a whole number is obtained
Cannot have 1.5 atoms in a formula

28. Relating Empirical and Molecular Formulas
n represents a whole number multiplier from 1 to as large as necessary
Calculate the empirical formula and the mass of the empirical formula
Divide the given molecular mass by the calculated empirical mass
Answer is a whole number multiplier

29. Relating Empirical and Molecular Formulas
Multiply each subscript in the empirical formula by the whole number multiplier to get the molecular formula

30. Calculate Empirical Formula from Percent Composition
Lactic acid has a molar mass of g and has this percent composition:
40.0% C, 6.71% H, 53.3% O
What is the empirical and molecular formula of lactic acid?
Assume a g sample size
Convert percent numbers to grams
Lactic acid builds up in muscles after aerobic exercise
Since percent represents parts of one element per 100 parts of the total compound

31. Calculate Empirical Formula from Percent Composition
Convert mass of each element to moles
Divide each mole quantity by the smallest number of moles
Empirical
formula is
CH2O
The ratio of C to H to O is 1 to 2 to 1
Empirical formula mass = (1.008) = g/mol

32. Determination of the Molecular Formula
Obtain the value of n (whole number multiplier)
Multiply the empirical formula by the multiplier
The small interger is the name of the small number multiplier
Molecular formula = n х empirical formula
Molecular formula = 3 (CH2O)
C3H6O3

33. Formulas for Compounds
Empirical Formula
Smallest possible set of subscript numbers
Smallest whole number ratio
All ionic compounds are given as empirical formulas
Molecular Formulas
The actual formulas of molecules
It shows all of the atoms present in a molecule
It may be the same as the EF or a whole- number multiple of its EF
Ionic compounds do not exist as molecules
Molecular formulas account for every atom in the structure
Molecular formula = n х Empirical formula