Polyatomic Ions / Mole / Molarity / Electrochemistry

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Unit 5

• Polyatomic Ions / Mole / Molarity /
  Electrochemistry

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This unit builds upon previous topics

• Electron Configuration
• Orbital Notation
• Electron Dot Notation
• Octet Rule

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Remember

• Ion-

an atom or group of atoms that has either
lost or gained electron(s) and as a result has an
electric charge.
Cation-
is a positively charged ion.
Anion-
is a negatively charged ion.
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• Recall how we represented atoms with the electron
  dot notation.

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We will represent ions the same way with two
exceptions
1) If the atom has at least 5, but less than 8
valence electrons we will draw as many
additional electrons as needed to acquire a total
of 8 valence electrons. (This will give the ion
an Octet of valence electrons.)
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1st exception cont

• After the "atomic symbol" of the ion, in the
  upper right corner of the symbol, we will write a
  negative sign (or digit, if more than one) for
  each additional electron we added to achieve the
  nearest noble gas core. An octet of valence
  electrons.

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1st exception cont
Example 1
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1st exception cont
Example 2
Bromine
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1st exception cont
Example 3
Oxygen
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We will represent ions the same way with two
exceptions (cont.)
2) If the atom has less than 4 valence electrons
we will remove as many electrons as needed to
move back to the nearest noble gas core. A total
of 8 valence electrons. (This will give the ion
an Octet of valence electrons.)
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2nd exception cont

• After the "atomic symbol" of the ion, in the
  upper right corner of the symbol, we will write a
  positive sign (or digit, if more than one) for
  each additional electron we subtracted to achieve
  the nearest noble gas core. An octet of valence
  electrons.

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2nd exception cont

• Example 1 Sodium

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2nd exception cont

• Example 2 Aluminum

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2nd exception cont

• Example 3 Barium

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Polyatomic ion-

• A type of ion made of more than 2 atoms. These
  will also be classified as either an anion or a
  cation.

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4 steps to drawing polyatomic ions

• Step 1) Determine and write down the "4 key
  numbers".

These are
p e c v
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"4 key numbers (cont.)

• p of protons (add up the atomic number of
  all involved atoms).

e of total electrons (add up the atomic
numbers (which equals the number of total
electrons in a neutral atom) of all atoms in the
ion.
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"4 key numbers (cont.)

• c of core electrons (atomic number minus
  the number of "s" and "p" electrons in the
  highest energy level).

v of valence electrons (determined
by subtracting the of core electrons from the
of total electrons).
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• Step 2)

Draw the "skeleton" of the ion
Step 3)
Give all atoms eight electrons (except Hydrogen,
it only wants two), (remember each bond
(represented by a dash) counts as 2 electrons).
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• Step 4)

Count the number of electrons drawn and compare
this to the calculated number ("Key number 4
the v ).
Adjust as necessary by adding double or triple
bonds to reduce the number of electrons while
still ensuring all atoms still have an octet of
valence electrons.
Draw brackets and valence charge to finish.
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Examples
Ammonium
Carbonate
Cyanide
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The Mole Concept.
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Mole -

• The amount of substance that contains
  Avagadro's number of particles of that substance.

Abbreviated mol. NOT m, M, or M.
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Avagadro's number -

• This is equal to 6.02 1023 .

It is specifically the number of C12 atoms in 12
grams (the chart mass in grams) of this isotope.
Think of this word like the word "Dozen", which
always means twelve. Well this number always
means 6.02 1023 particles.
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Atomic weight -

• The weighted average of the masses of the
  isotopes of an element.

Based on C12 , 1 atom 12 AMUs (atomic mass
units).
AMU -
arbitrary unit equal to 1/12 the mass of a C12
atom. (Atomic Mass Unit)
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AMU continued

• Imagine if we had to determine the "weight"
  of everyone in the classroom, but, we didn't have
  a scale. We could select someone and then assign
  to that individual an arbitrary "weight" unit. We
  might say he/she weight 10 CRUs (Chem. Room
  Units). Everyone in the room will now be weighed
  relative to that student.

The atomic weight of all elements is relative to
the "weight" of carbon 12.
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Gram Atomic Weight -

• The mass in grams of 1 mole of a substance.

Example 1 Hydrogen.
1 atom weighs ?
1.0079 amu
1 mole of hydrogen atoms weighs ?
1.0079 g.
However what is special about Hydrogen ?
Its diatomic
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• What is the formula for a molecule of Hydrogen?

H2
1 molecule of Hydrogen weighs ?
2.0158 amu
1 mole of Hydrogen gas weighs ?
2.0158 g.
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• Example 2 CO2

1 molecule of Carbon dioxide weighs ?
44.0098 amu
1 mole of Carbon dioxide weighs ?
44.0098 g.
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• Make sure you are aware of what you are working
  with

Atoms or Molecules
AMUs or Grams
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Molarity -

• Is a concentration unit in
• moles / liter. (The liter is usually water)

Note that the numerator is of moles. We will
learn how to convert this to grams in the next
topic.
Abbreviated M NOT mol, M, m, or mol.
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DIMO -

• An acronym that means
• Divide In Multiply Out.

It is a drawing that you have to be able to
reproduce and use.
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of grams
of particles
Avagadros
Chart mass
of moles
22.4
of liters of a gas
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4 Steps to use the DIMO chart.

• 1 ) Determine the chart mass of
  the substance you're working with.

2) Deal with the concentration of that
substance.
3) Deal with the volume of the solution
you're working with.
4) Use "DIMO" to solve.
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How many grams of NaOH are in 1 liter of a 1M
solution of the NaOH?
Example 1
STEP 1
Determine the chart mass
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Example 1 (cont.)
STEP 1
40 g/mol
STEP 2
Deal with the concentration
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Example 1 (cont.)
STEP 1
40 g/mol
STEP 2
1 mol/L
STEP 3
Deal with the volume
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Example 1 (cont.)
STEP 1
40 g/mol
STEP 2
1 mol/L
STEP 3
1 mol of NaOH in sample
STEP 4
Use "DIMO" to solve
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Solution to Example 1
How many grams of NaOH are in 1 liter of a 1M
solution of the NaOH?
There are 40 g of NaOH in 1 liter of a 1M
solution of NaOH.
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0.75 liters of a 0.5M solution contains how many
grams of CaCl2?
Example 2
STEP 1
Determine the chart mass
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Example 2 (cont.)
STEP 1
111 g/mol
STEP 2
Deal with the concentration
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Example 2 (cont.)
STEP 1
111 g/mol
STEP 2
0.5 mol/L
STEP 3
Deal with the volume
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Example 2 (cont.)
STEP 1
111 g/mol
STEP 2
0.5 mol/L
STEP 3
0.375 mol of CaCl2 in sample
STEP 4
Use "DIMO" to solve
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Solution to Example 2
How many grams of CaCl2 are in 0.75 liter of a
0.5M solution of the CaCl2?
There are 41.625 g of CaCl2 in 0.75 liter of a
0.5M solution of CaCl2.
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• Notice in the previous examples we
  calculated the number of grams of a substance.
• In the next example, we will determine how
  many moles of a substance are in a given volume
  of a solution.

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What is the Molarity of a solution created by
dissolving 150 grams of NaI into 250 mL of
distilled water?
Example 3
STEP 1
Determine the chart mass
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Example 3 (cont.)
STEP 1
150 g/mol
STEP 2
Deal with the concentration
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Example 3 (cont.)
STEP 1
150 g/mol
STEP 2
150 g / 0.25 L
STEP 3
Deal with the volume
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Example 3 (cont.)
STEP 1
150 g/mol
STEP 2
150 g / 0.25 L
STEP 3
600 g / 1 L
STEP 4
Use "DIMO" to solve
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Solution to Example 3
What is the Molarity of a solution created by
dissolving 150 grams of NaI into 250 mL of
distilled water?
600 g of NaI is 4 moles
There are 4 moles of NaI in 1 liter of the
solution of NaI.
Therefore the molarity is
____
4M
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• Finding the number of particles (atoms or
  molecules) is done the same way except you would
  divide or multiply in or out by "Avagadro's
  number".
• You will get practice doing this on your study
  guide.