Working with Formulas, Moles and Masses

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Working with Formulas, Moles and Masses
loosely based on Chap 3 Sec 2 3of Brady
Senese

• Dr. C. Yau
• Fall 2009

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Chemical Formula at the particulate and
macroscopic levels

• When we see the formula H2S
• at the particulate level we should be thinking of
  one molecule of H2S with
• 2 H atoms and 1 S atom,
• at the macroscopic level we should be thinking of
  one mol H2S with
• 2 mol H and 1 mol S.

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• Example 3.5 p.92
• Calcium phosphate is widely found in nature in
  the form of natural minerals. It is also found
  in bones and some kidney stones.
• In many instances if we determine one element in
  a compound we can find out how much of the
  compound is present.
• In one case a sample is found to contain 0.864
  mole of phosphorus. How many moles of Ca3(PO4)2
  will that represent.
• Hint Do we need Avogadro's number?
• Do Practice Exercises 5 6 on p.92

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• Example 3.6 p.93
• Chlorophyll a, the green pigment in leaves, has
  the formula C55H72MgN4O5.
• If 0.0011 g of Mg is available to a plant cell
  for chlorophyll a synthesis, how many grams of
  carbon will be required to completely use up the
  magnesium?
• Do Practice Exercises 7, 8, 9 on p.94

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Percent Composition

• LEARN THIS!
• "Percent composition" refers to the of each
  element in a compound by mass.
• For example, the composition of CO2 is 27.29
  C and 72.71 O.
• How do we come up with these numbers?
• (Unless specified otherwise, please always give
  answer to 4 sig. fig.)

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• Example 3.7 p. 94
• A sample of a liquid with a mass of 8.657 g was
  decomposed into its elements and gave 5.217 g of
  carbon, 0.9620 g of hydrogen and 2.478 g of
  oxygen. What is the percentage composition of
  this compound?
• Remember To find percent, always think..
• Do Practice Exercises 10 11 p.95

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• Example 3.8 p. 95
• Do the mass percentages of 25.94 N and 74.06 O
  match the formula N2O5?
• Do Practice Exercises 12 13 p.96

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Empirical Formulas

• The word "empirical" means "experimental".
• "Empirical formula" refers to a formula based on
  experimental evidence (rather than theory).
• If we base a formula on experimental evidence, we
  can only come up with the elements in the lowest
  ratio.
• This is why students commonly mistake the meaning
  of "empirical" to mean "simplest ratio."

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Empirical Formulas

• When we write a formula, whether it is an
  empirical formula or molecular formula, remember
  that the subscripts refer to the number atoms (or
  moles of atoms).
• THEY DO NOT REFER TO GRAMS!
• So when we see H2S, we are not thinking of 2
  grams of H and 1 g of S, but
• 2 atoms S and 1 atom S
• 2 mol H and 1 mol S.

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Empirical Formula

• Example 3.9 p.96
• A 2.57 g sample of a compound composed of only
  tin and chlorine was found to contain 1.17 g of
  tin. what is the compound's empirical formula?
• Hint We need a ratio of moles not grams!
• Do practice exercises 14 15 p.97.

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Empirical Formula

• Example 3.10 p.97
• One of the compounds of iron and oxygen, "black
  iron oxide," occurs naturally in the mineral
  magnetite.
• When a 2.448 g sample was analyzed it was found
  to have 1.771 g of Fe. Calculate the empirical
  formula of this compound.
• Do Practice Exercises 16 17 P.99

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Empirical Formula

• Example 3.11 p.99
• A white powder used in paints, enamels, and
  ceramics has the following percentage
  composition Ba, 69.6 C, 6.09 and O, 24.3.
• What is its empirical formula? What is the name
  of this compound?
• I STRONGLY recommend you set up the problem the
  way I am going to show you.
• You will be penalized if you have any "run-on"
  statements in your setup! My setup will help you
  avoid such "run-on" statements.
• Do practice exercises 18 19 p.100.

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Empirical Formula

• Example 3.12 p.101
• A 0.5438 g sample of a liquid consisting of only
  C, H, and O was burned in pure oxygen, and 1.039
  g of CO2 and 0.6369 g of H2O were obtained. What
  is the empirical formula of the compound.
• Do Practice Exercises 20 21 p.102.

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From Empirical Formulato Molecular Formula

• Remember that ionic compounds do NOT have
  "molecular" formula! So, we are dealing with only
  molecular compounds here.
• Ionic compounds have ONLY empirical formulas
  (subscripts are in the lowest ratio).
• To determine the molecular formula, you must be
  given one extra piece of information
• the molecular weight weight of one molecule or
  weight of one mole of molecules.

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Difference between Empirical Formula and
Molecular Formula

• Hydrazine used in rocket fuel has the molecular
  formula of N2H4.
• It tells us that each molecule has 2 N atoms and
  4 H atoms. It is NOT just a ratio.

Its MW is 2(14.01) 4(1.008) 32.05
amu/molecule or 32.05g/mol
Its empirical formula is N1H2, usually written
simply as NH2. Its empirical weight (EW) is 16.03
amu/f.u. or 16.03 g/mol
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Empirical Formula vs Molecular Formula

• molecular formula of N2H4
• MW 32.05 amu/molecule or 32.05g/mol
• empirical formula is NH2
• (EW) 16.03 amu/f.u. or 16.03 g/mol

Note The MW is a multiple of its EW. Divide MW
by EW to determine which multiple it is and
multiply the empirical formula by this multiple
to get the molecular formula.
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• With hydrazine, if we knew ONLY the empirical
  formula to be NH2, and its molecular weight is
  32.05 amu/molecule, we can figure out its
  molecular formula
• 1) Calculate EW from its empirical formula 14.01
  2(1.008) 16.03 amu/f.u.
• 2)
• 3) Multiple is 2 (must be very close to a whole
  number).
• Molecular formula 2x(empirical formula)
• 2x(NH2)
• ANS. Molecular formula is N2H4

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Molecular Formula

• Example 3.13 p.102
• Styrene, the raw material for polystyrene foam
  plastics, has an empirical formula of CH. Its
  molecular mass is 104 g mol-1. What is its
  molecular formula.
• Do Practice Exercises 22 23 p.103.