Atomic Mass and Formula Mass

1
Atomic Mass and Formula Mass

• Atomic mass mass of an atom expressed relative
  to the mass assigned to carbon-12 (carbon-12 is
  used as the standard)
• Formula mass sum of the atomic masses of all
  the atoms in a compound

2
Formula mass calculation

• H2O
• H x 2 1.01 amu x 2 2.02 amu
• O x 1 16.00 amu x 1 16.00 amu
• Sum 18.02 amu
• C6H12O6
• C x 6 12.01 amu x 6 72.06 amu
• H x 12 1.01 amu x 12 12.12 amu
• O x 6 16.00 amu x 6 96.00 amu
• Sum 180.18 amu

3
Measuring Matter

• Counting Units
• 12 dozen
• 2 pair
• 500 ream
• This works for objects we can manipulate with
  bare hands.
• Chemists need a way to accurately count the
  number of atoms, molecules, or formula units in a
  sample.

4
The Mole!!!!!!!

• SI base unit for measuring the amount of a
  substance
• Mole number of carbon atoms in exactly 12 grams
  of pure carbon-12.
• Carbon-12 used as a standard.
• 1 mole anything 6.02 X 1023 representative
  particles
• Representative particles can be
• Atoms
• Molecules (Covalently bonded)
• Formula units (Ionically bonded)

5

• Atoms, molecules, and formula units are so small
  its impossible to count them individually.
• So, chemists need a special counting unit.

6
Avogadros Number

• Avogadros 6.02 X 1023
• 602000000000000000000000
• You dont have to write this number, its just
  for effectjust know its a big number and use
  the scientific notation always

7
So how big is Avogadros ?

• If you gave away 1 million dollars to each person
  on Earth every day, it would take more than 3000
  years to distribute the money.
• If you spread Avogadro's number of unpopped
  popcorn kernels across the USA, the entire
  country would be covered in popcorn to a depth of
  over 9 miles.

8
Molar Mass

• We dont deal with amu balances in Chemistrywe
  deal with balances which measure mass in gramsso
  we need a way to measure the mass of 1 mole of a
  substance
• The molar mass is just like the atomic mass and
  formula massjust a different unit. (g/mol
  instead of amu)

9
Molar mass Calculation example

• Ca2C
• Ca x 2 40.08 g/mol x 2 80.16 g/mol
• C x 1 12.01 g/mol x 1 12.01 g/mol
• Sum 92.17 g/mol
• N2H4
• N x 2 14.01 g/mol x 2 28.02 g/mol
• H x 4 1.01 g/mol x 4 4.04 g/mol
• Sum 32.06 g/mol

10
Watch out for these

• Lets say you have 1 mole of CaCl2
• How many moles of Ca2 do you have?
• How many moles of Cl1- do you have?
• How many formula units of CaCl2 do you have?

11
Mass and the Mole

• 1 mole of Fe has the same number of atoms as 1
  mole of Ne
• 6.02 X 10 23 atoms of each
• 1 mole of Fe does not have the same mass as 1
  mole of Ne
• 1 atom of Fe does not have the same mass as 1
  atom of Ne

12
Mass to Mole Example

• How many moles are present in 4.395 grams of
  sodium chloride?

13
Mole to Mass Example

• Suppose you have a 0.548 mole sample of
  dinitrogen triiodide. What is the mass (in
  grams) of this sample?

14
Moles to Particles Example

• A sample of potassium chloride contains 2.57
  moles. How many formula units of potassium
  chloride are present in this sample?

15
Particles to Moles Example

• A sample of chlorine gas contains 5.48 X
  1024 molecules of chlorine gas. How many moles
  of chlorine gas does this sample contain?

16
Multistep Conversions

• The mole is the key here. It is the thing which
  chemists use the most when dealing with
  calculations involving chemical reactions.
• There are two main types of multipstep
  conversions
• Mass to particle
• Particle to mass

17
Mass to Particle Example

• A baker needs 758 grams of sucrose (C12H22O11) to
  bake a cake. How many sucrose molecules will be
  in the unbaked cake?

18
Particles to Mass Example

• 8.0 X 1020 atoms of calcium are present in a
  multivitamin. How many grams of calcium does a
  person consume when they take the vitamin?

19
Moles and Gases

• At the same temperature and pressure, equal
  volumes of gases contain the same number of gas
  particles.
• STP Standard Temperature and Pressure (0 oC and
  1 atmosphere)
• At STP, it was found experimentally that 1 mole
  of any gas had a volume of 22.4L.
• We call this volume the molar volume

20
Moles to Volume

• A chemical reaction produces 0.82 moles of oxygen
  gas. What volume will that gas occupy assuming
  the chemical reaction occurred at STP?

21
Volume to Moles

• A huge latex balloon with a volume of 3675 L
  contains how many moles of helium at STP?

22
Multistep Conversions using Molar Volume

• How many grams of oxygen gas are present in a
  sample of oxygen gas with a volume of 58.4 L at
  STP?
• A sample of neon gas has a volume of 5.2 L at
  STP. How many grams of neon are present in the
  sample? How many atoms of neon are present in
  the sample?

23
Percent Composition

• The mass of each element in a compound compared
  to the entire mass of the compound, and
  multiplied by 100.
• (Part/Whole) X 100
• This needs to be done for each element!
• Two ways to determine percent composition
• Given a chemical formula
• Given experimental mass data

24
Composition Given Chemical Formula

• Determine the percent composition for the
  following chemical compounds
• C6H12O6
• (NH4)2S

25
Composition Given Mass Data

• If a compound has a total mass of 4.50 grams, and
  contains 1.25 grams of carbon and 3.25 grams of
  oxygen, what is its percent composition?
• If a compound contains only 3.10 grams of iron
  and 1.25 grams of oxygen, what is its percent
  composition?

26
Empirical Formulas

• Formulas that give the simplest-whole number
  ratio of the atoms of the elements in a compound.
• These cannot be reduced any further!
• Example CaCl2 cannot be reduced.
• Example CH2O cannot be reduced.
• Example Ba(OH)2 cannot be reduced.

27
Molecular Formulas

• The formula that gives the actual number of atoms
  of each element in a molecular compound.
• This formula is always a whole-number multiple of
  the empirical formula.
• This formula can be reduced to yield the
  empirical formula.
• Example C6H12O6 can be reduced to CH2O

28
Determining Molecular Formula From Empirical
Formula

• Beta carotene can be broken down to form vitamin
  A. The empirical formula for beta carotene is
  C5H7. The molar mass of beta carotene is 536
  g/mol. What is the molecular formula?
• The molar mass of TNT is 227.0 g/mole. If its
  empirical formula was determined to be C7H5N3O6
  What is its molecular formula?