Quantitative Composition of Compounds

1
Quantitative Composition of Compounds

• Define the MOLE
• Determine molar mass of compounds
• Calculate percent composition of compounds
• Distinguish the differences between (and be able
  to calculate) empirical formulas and molecular
  formulas

2
Composition of Compounds

• Ratios of ingredients determines end result
• Ratios in chemical compounds
• Determine identity of the substance
• Changing changes the entire compound
• Example

3
Chemical Measurements

• How can we measure masses of atoms?
• Counting by weighing

4
Chemical Measurements

• Atomic mass
• Mole (mol)

5
Chemical Measurements

• Avogadros number
• Molar mass

6
The Mole

• Mole unit for counting atoms

7
The Mole

• Molar Mass
• Grams of element (or compound) that contain
  Avogadros number of atoms

8
The Mole

• The atomic mass expressed in grams is the molar
  mass of an element
• One mole of any element contains Avagadros
  number of atoms

9
The Mole

• Can now convert between
• 25.0 g of iron is how many moles?

mass
moles
molecules

10
The Mole

• How many grams of Na are in 2.55 moles?
• How many atoms of Carbon are in 2.35 grams?

11
Molar Mass of Compounds

• One mole of a compound contains 6.022 x 10-23
  formula units of that compound

12
Molar Mass of Compounds

• Molar mass add molar masses of all atoms
• What is the molar mass of H2O?
• What is the molar mass of Ca(OH)2?

13
Molar Mass of Compounds

• How many molecules are there in a 3.46 g sample
  of HCl?

5.71 x 1022 molecules
14
Molar Mass of Compounds

• If you burned 6.10 x 1024 molecules of ethane
  (C2H6) what mass of ethane did you burn?

304.7 g C2H6
15
Moles and Gases

• At the same temperature and pressure, equal
  volumes of gases contain the same number of gas
  particles
• STP Standard Temperature (00 C) and Pressure (1
  atmosphere)

16
Moles and Gases

• A student fills a 1.0 L flask with CO2 at STP.
  How many molecules of gas are in the flask?

17
Moles and Gases

• A container with a volume of 893 L contains how
  many moles of air at STP?
• A chemical reaction produces 0.37 moles of N2
  gas. What volume will that gas occupy at STP?

18
Percentage Composition of Compounds

• Mass of one element in a compound compared to the
  total mass of the whole compound
• Two ways to determine

19
Percentage Composition of Compounds from
Formula

• Calculate the molar mass
• Divide the total mass of each element in the
  formula by the molar mass and multiply by 100.
• Total mass of element x 100 of element
• molar mass

20
Percentage Composition of Compounds from Formula

• What is the percent composition of each element
  in table salt (NaCl)?

Should equal 100
21
Percentage Composition of Compounds from
Experimental Data

• Calculate the mass of the compound formed
• Divide the mass of each element by the total mass
  of the compound and multiply by 100

22
Percentage Composition of Compounds

• A sample of 2.45 g of aluminum oxide decomposes
  into 1.30 g of aluminum and 1.15 g of oxygen.
  What is the percent composition of each element?

23
Percentage Composition of Compounds

• Find the percentage composition of a compound
  that contains 1.94 g of carbon, 0.480 g of
  hydrogen, and 2.58 g of sulfur in a 5.00 g sample
  of that compound.

24
Empirical Formula

• A formula that gives the SIMPLEST whole-number
  ratio of elements in that compound

Ratio of atoms (formula)
Ratio of masses (percent composition)
25
Empirical Formula

• What is the empirical formula of a compound
  containing 40 carbon, 53.3 oxygen, and 6.7
  hydrogen?

1
Divide by smallest number
1
2
26
Empirical Formula

• Determine the empirical formula of a compound
  containing 5.75 g Na, 3.5 g N, and 12.0 g O

27
Empirical Formula

• Determine the empirical formula of a compound
  containing 2.644g of gold and 0.476 g of chlorine.

28
Molecular Formula

• Formula that gives the actual number of atoms in
  a given compound
• Some compounds may have same empirical formula
  but actually be very different
• Sometimes empirical formula and molecular formula
  are the same
• Compare molar mass with empirical formula mass

29
Molecular Formula

• Ribose has a molar mass of 150 g/mol and a
  chemical composition of 40.0 carbon, 6.67
  hydrogen, and 53.3 oxygen. What is the
  molecular formula for ribose?

30
Molecular Formula

• Find the molecular formula of a compound that
  contains 42.56 g of palladium and 0.80 g of
  hydrogen. The molar mass of the compound is
  216.8 g/mol.
• Octane, a compound of hydrogen and carbon, has a
  molar mass of 114.26 g/mol. If the compound
  contains 18.17 g/mol hydrogen, what is its
  molecular formula?

31
Homework

• Questions 1, 6-10
• Paired Exercises 11-31 odd (a, b, c only) 33
  37-41 odd
• Additional Exercises 43, 47, 50 51
• NO CLASS TUESDAY Study help???
• Morning, afternoon, evening?
• TEST THURSDAY Chapters 11, 6 7