Stoichiometry Problem

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Stoichiometry Problem

• An Example

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Sodium phosphate barium nitrate

• Its a double replacement reaction, since we have
  2 ionic compounds.
• Try to come up with the formulas before you go to
  the next slide

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Na3PO4 Ba(NO3)2

• Those are the reactants, now try and predict the
  products before going to the next slide.
• Remember AX BY ? AY BX

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Na3PO4 Ba(NO3)2 ? Ba3(PO4)2 NaNO3

• Ok, there is the reaction.
• Now before we can do any kind of stoichiometric
  problem, we need a balanced chemical equation.
• Is it balanced?

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• Now, its balanced.
• OK, here is your problem
• 3.50 g of sodium phosphate are reacted with 6.40
  g of barium nitrate. What mass of barium
  phosphate can be produced?

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• Things to remember
• 1. Everything must be calculated in moles.
• 2. They gave us mass therefore we must convert
  to moles.
• 3. To figure out moles of sodium phosphate and
  barium nitrate, I need to know the molar mass of
  each.
• Calculate those now, before you go to the next
  slide.

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• No seriouslycalculate it

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• Na3PO4 Molar mass 164 g/mole
• Ba(NO3)2 Molar mass 261.3 g/mole
• Now find moles of each

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• Na3PO4 3.50 g / 164 g/mole
• We have 0.021 moles of sodium phosphate.
• Ba(NO3)2 6.40 g / 261.3 g/mole
• We have 0.024 moles of barium nitrate.

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• Now what?
• Well, now you look at your equation.
• For every 2 moles of sodium phosphate, we need 3
  moles of barium nitrate to react with it.
  Correct?

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• We have 0.021 moles of sodium phosphate and 0.024
  moles of barium nitrate. Which one are we going
  to run out of first?
• Think about it before you go to the next slide.

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• Yes, of course, were going to run out of barium
  nitrate first.
• To calculate that, lets look at moles of sodium
  phosphate and figure out how much barium nitrate
  we need to react with all of it.

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• 0.021 moles Na3PO4 x (3 moles Ba(NO3)2 / 2 moles
  Na3PO4 ) 0.032 moles barium nitrate.
• But we dont have that much barium nitrate.
  Therefore, were going to run out of barium
  nitrate and have some sodium phosphate left over,
  after the reaction is done. So, barium nitrate
  is the LIMITING REAGENT.

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• Using the moles of barium nitrate, we next need
  to calculate how much barium phosphate we can
  produce.
• 0.024 moles Ba(NO3)2 x (1 mole Ba3(PO4)2 / 3
  molesBa(NO3)2 ) 0.008 moles barium phosphate.

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• The equation tells me that for every 3 moles of
  barium nitrate you will produce 1 mole of barium
  phosphate. Thats why I picked the factor I did.
• So, I will produce 0.008 moles of barium
  phosphate. But the problem asked me for MASS,
  not moles.

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2 Na3PO4 3 Ba(NO3)2 ? Ba3(PO4)2 6 NaNO3

• 0.008 moles of barium phosphate x the molar mass
  of barium phosphate should give me the answer.
• Did you get 601.9 g/mole for the molar mass? Did
  you get 4.82 g of Ba3(PO4)2 for the answer?

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STOICHIOMETRY STEPS

• Thats how you do it.
• Remember, if they give you grams, convert to
  moles. If they give you volume of a gas in
  liter, convert to moles. If they give you
  particles, convert to moles. THATS STEP 1.

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STOICHIOMETRY STEPS

• Step 2 is to use a balanced chemical equation to
  determine the stoichiometric relationship. The
  balanced equation tells you how many moles of
  each substance participate in the reaction.
• USE A FACTOR!! Its easy! And now you have your
  answer in moles. If they want moles, you are
  done. If not, go to step 3.

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STOICHIOMETRY STEPS

• Step 3!
• If they want the answer in grams, convert to
  grams. If they want the answer in liters,
  convert to liters. If they want the answer in
  particles, convert to particles. THATS CHAPTER
  7!! (Remember the triangular chart I gave you?)

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The End

• Now practice, practice, practice and then if you
  think you know it,
• Practice some more!!