Bellwork

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Bellwork
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Quiz
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Determination of Empirical and Molecular Formulas
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• Modern Chemistry Chapter 7.4 pg. 245-249
• 3221.3.5 Convert percent composition information
  into the empirical or molecular formula of a
  compound.

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Empirical Formulas

• chemists know when they have discovered a new
  cmpd when it no longer has the same
  characteristics as the cmpd they were expecting
• in order to find the formula of this mystery
  cmpd they must run a battery of tests

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Empirical Formula

• Q1 What two things can these tests tell you?
• 1. Elements found in the compound
• 2. amounts of each element in cmpd
• Q2 What was the difference between empirical
  and molecular formulas?
• Empirical-lowest whole ratio
• Molecular-exact formula

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Empirical Formula

• you can interpret the information from these
  gadgets you can determine the empirical formula
  of the mystery cmpd
• Q3 In what cases are empirical formulas, true
  formulas? In what cases are they not?
• all ionic cmpds will be in empirical formulas

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Important concept!

• subscripts in a formula not only tell us the
  relative number of atoms but also the mole ratio.
  How?
• NaCl 11 right?
• Na100Cl100 11 ok?
• NaNClN still 11 but now dealing with molar
  amounts

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Important concept!

• So if we know the molar amounts of each element
  can we not come up with an empirical formula?
• In order to determine the molar amounts we need
  to know the element and the amounts of each
  element

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• Ex. 1 Lab procedures show that a 9.3g sample
  of a compd is 2.9 g N and 6.4 g O. Find the
  empirical formula.
• Convert to moles
• 2.9 g N 1 molN .207 mol N
• 14.0gN
• 6.4g O 1 mol O .400 mol O
• 16.0g O
• answer N.207O.400
• according to Law of Multiple Proportions all
  formulas must be in whole number ratios

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• N.207O.400
• How can you make it into whole s
• Divide all numbers by the smallest decimal and
  round up or down within limits
• .207 mol N 1N .400 mol O 1.9 2 O
• .207 .207
• Answer NO2

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• Ex. 2 What is the empirical formula of the
  cmpd with a percentage composition of 65.2As and
  34.8 O by mass?
• can be easily converted to g. Just imagine
  that the is out of 100g. Ta!Da! 65.265.2g
• 65.2gAs 1 mol As .870mol As
• 74.9 gAs
• 34.8gO 1 mol O 2.175mol O
• 16.0g O

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• .870mol As 1 2.175mol O 2.5
• .870 .870
• Big Problem! It is ok to round up or round
  down if it the number is close but if the decimal
  is between .30 and .70 you cannot round. You
  must multiply both numbers by 2 or maybe even 3
  until you get a set of numbers you can round
• 1 x 2 2 As 2.5 x 2 5 O
• As2O5

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• Ex. 3 A sample of a cmpd of C and H is
  decomposed to produce 0.0134 g of solid C and
  .0500 L at STP of gaseous hydrogen. What is the
  empirical formula of the cmpd?
• No problem with this one! Just convert L to
  mol since it is in STP

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Empirical Formulas

• 1. Convert g or L or whatever to moles
• 2. Divide all the mole amounts by smallest mole
• 3. If any of the numbers computed can be easily
  round up or down then you have your answer
• 4. If the decimal falls between .30 and .70 then
  you must multiply both numbers by 2 or 3 or
  whatever it takes to get it into a number that
  you can round

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Practice

• 1. Decomposition of a substance produced 97.9
  L of O2 gas and 49.0 dm3 of Cl2 gas at STP. What
  is the substance's empirical formula?

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Practice

• 2. What is the empirical formula of the cmpd
  produced from the rxn of 4.80 g of Carbon and
  11.2 L of hydrogen gas at STP?

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Molecular Formulas

• remember that empirical formulas do not give us
  the actual formula of the molecule
• it is easy to determine the molecular formula
  if you have 2 things
• 1. Molecular mass (will always be given to you
  in the problem)
• 2. Empirical formula and its mass

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• Ex. 1 The empirical formula of a nitrogen
  dioxide cmpd is NO2. The molecule has been
  tested to have a molecular mass of 92 g. What is
  its molecular formula?
• NO2 has a mass of 46.0 u
• this is its empirical formula mass 46.0
• its actual mass is 92.0
• in order to determine the MF divide the actual
  molecular mass by the EFM to get the number that
  all the subscripts must be multiplied by
  92.0/46.0 2 N2O4

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• Ex. 2 The compound in Practice 6 above has a
  molecular mass of 52 u. What is its molecular
  formula?

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• Ex. 3 What is the empirical formula of a
  hydrocarbon that consists of 80.0 Carbon and
  20.0 hydrogen? If its molecular mass is 30.0 g,
  what is its molecular formula?

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Molecular Compounds

• Q1 Come up with a set of rules that you can
  use to determine molecular formulas.

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Homework