Microscale Stoichiometry
Objectives:
a. To determine the approximate combining ratios for reactions of calcium chloride with
sodium oxalate and sodium phosphate, and for cobalt chloride with sodium
hydroxide.
b. To determine the combining ratios for a monoprotic acid (HCl) and diprotic acid
(H2SO4) with a solution of sodium hydroxide.
Introduction: A balanced chemical equation specifies the mole ratio in which the
reactants in a chemical reaction combine to form the products. These combining ratios
are derived from the coefficients in the balanced equation. This may or may not be a 1:1
ratio.
Procedure I: Calcium Oxalate
1. Place 4 drops of water in each of 9 wells in the 24-well plate.
2. Add 1 drop of 0.10M calcium chloride (CaCl2) to well 1, 2 drops to well 2, 3 drops to
well 3, etc until you add 9 drops to well 9.
3. Add 9 drops of 0.10M sodium oxalate (Na2C2O4) to well 1, 8 drops to well 2, 7 drops
to well 3, etc. until you add 1 drop to well 9.
well
drops H2O
drops CaCl2
drops Na2C2O4
1
4
1
9
2
4
2
8
3
4
3
7
4
4
4
6
5
4
5
5
6
4
6
4
7
4
7
3
8
4
8
2
9
4
9
1
4. Mix the contents of each filled well in the plate by gently shaking, being careful not to
spill any of the contents. Allow about 5 minutes for precipitate to settle.
5. Observe the solids in each of the well and visually determine which well has the most
precipitate. If two wells are difficult to rank, redo those. Recall the drop ratio of CaCl2 to
Na2C2O4 for this well.
6. Since the reactant solutions have the same concentrations (molarities), the volume ratio
of the reactants is the same as the mole ratio. On this drop ratio basis, determine ratio of
the coefficients of the reactants and the subscripts in the precipitate (solid) formed.
7. Use this information to write and balance an equation for the reaction. Be sure to
include the physical states for each compound. Name the products.
Micro-scale Stoichiometry 01-02
1
Procedure II: Calcium Phosphate
1. Place 4 drops of water in each of 9 wells in the 24-well plate.
2. Add 1 drop of 0.10M calcium chloride (CaCl2) to well 1, 2 drops to well 2, 3 drops to
well 3, etc until you add 9 drops to well 9.
3. Add 9 drops of 0.10M sodium phosphate (Na3PO4) to well 1, 8 drops to well 2, 7 drops
to well 3, etc. until you add 1 drop to well 9.
well
drops H2O
drops CaCl2
drops Na3PO4
1
4
1
9
2
4
2
8
3
4
3
7
4
4
4
6
5
4
5
5
6
4
6
4
7
4
7
3
8
4
8
2
9
4
9
1
4. Mix the contents of each filled well in the plate by gently shaking, being careful not to
spill any of the contents. Allow about 5 minutes for precipitate to settle.
5. Observe the solids in each of the well and visually determine which well has the most
precipitate. If two wells are difficult to rank, redo those wells. Recall the drop ratio of
CaCl2 to sodium phosphate (Na3PO4) for this well.
6. Since the reactant solutions have the same concentrations (molarities), the volume ratio
of the reactants is the same as the mole ratio. On this drop ratio basis, determine ratio of
the coefficients of the reactants and the subscripts in the precipitate formed.
7. Use this information to write and balance an equation for the reaction. Be sure to
include the physical states for each compound. Name the products.
Procedure III: Cobalt Hydroxide
1. Obtain a clean 24-well plate.
2. Add 1 drop of 0.10M CoCl2 to well 1, 2 drops to well 2, 3 drops to well 3, etc.until you
have added 8 drops to well 8.
3. Add 8 drops of 0.10M NaOH to well 1, 7 drops to well 2, 6 drops to well 3, etc. until
you have added 1 drop to well 8.
well
drops CoCl2
drops NaOH
1
1
8
2
2
7
3
3
6
4
4
5
5
5
4
6
6
3
7
7
2
8
8
1
3. Swirl the plate gently to mix the solutions, being careful not to spill any of them.
Micro-scale Stoichiometry 01-02
2
4. Observe each well. The darkest blue color will indicate the combination, which has the
greatest amount of precipitate. Recall the drop ratio of CoCl2 to sodium hydroxide
(NaOH) for this well. Repeat the procedure if there is time.
5. Since the reactant solutions have the same concentrations (molarities), the volume ratio
of the reactants is the same as the mole ratio. On this drop ratio basis, determine ratio of
the coefficients of the reactants and the subscripts in the precipitate formed.
6. Use this information to write and balance an equation for the reaction. Be sure to
include the physical states for each compound. Name the products.
Procedure IV: Acid-Base Titration
Introduction: A titration is a reaction between an acid and a base carried out in such a
manner that one is added to the other to reach the equivalence point. At this point, the
number of moles of base added is exactly the number required to react with the moles of
acid present. Neither the acid nor the base is in excess. Since the products are often water
soluble and colorless and there is no visual indication of when the equivalence point has
been reached, an acid-base indicator is used to detect the change in the acidity of the
solution. A base titrated with an acid will become more acidic as the titration progresses
or the reverse will happen if base is added to acid. An indicator which is one color in
water solution which is acidic and another color in basic conditions may be used to detect
an endpoint or the visual indication of an equivalence point. The indicator,
phenolphthalein, is colorless in acidic solution and pink in basic conditions.
1. Place the 24 well plate on a piece of white cardstock.
2. Place 10 drops of 0.10M HCl in each of three wells.
3. Place 10 drops of 0.10M H2SO4 in each of three wells.
4. Add one drop of phenolphthalein to each of the wells. Swirl gently to mix.
5. Add NaOH dropwise with swirling to each of the 6 wells being careful not to spill
any solution until the solution turns slightly pink and remains that color.
6. Record the number of drops required for each acid. Average the results for each acid.
7. From the ratio of drops of 0.10M HCl/0.10M NaOH, determine the coefficients in the
balanced equation. Write and balance the equation including physical states and
check for consistency between experimental results and your balanced equation.
8. From the ratio of drops of 0.10M H2SO4/0.10M NaOH, determine the coefficients in
the balanced equation. Write and balance the equation including physical states and
check for consistency between experimental results and your balanced equation.
Micro-scale Stoichiometry 01-02
3
Microscale Stoichiometry
NAME:______________________________________ PERIOD:_________
LAB PARTNER:_______________________________ DATE:___________
Data Table
Procedure I: Calcium Oxalate
Tube
1
2
3
4
Drops
0
1
2
3
CaCl2
Drops
10
9
8
7
Na2C2O4
Height
of solid
(mm)
Balanced Reaction Equation:
Procedure II: Calcium Phosphate
Tube
1
2
3
4
Drops
0
1
2
3
CaCl2
Drops
10
9
8
7
Na3PO4
Height
of solid
(mm)
Balanced Reaction Equation:
Procedure III: Cobalt Hydroxide
Tube
1
2
3
4
Drops
0
1
2
3
CoCl2
Drops
10
9
8
7
NaOH
Height
of solid
(mm)
5
4
6
5
7
6
8
7
9
8
10
9
11
10
6
5
4
3
2
1
0
5
4
6
5
7
6
8
7
9
8
10
9
11
10
6
5
4
3
2
1
0
5
4
6
5
7
6
8
7
9
8
10
9
11
10
6
5
4
3
2
1
0
Balanced Reaction Equation:
Micro-scale Stoichiometry 01-02
4
Procedure IV: Acid-Base Titration
Trial 1
Drops HCl
Drops NaOH
10
Trial 2
Drops HCl
Drops NaOH
10
Trial 1
Drops H2SO4
Drops NaOH
10
Balanced Reaction Equations:
Trial 3
Drops HCl
Drops NaOH
10
Trial 2
Drops H2SO4
Drops NaOH
10
Micro-scale Stoichiometry 01-02
Trial 3
Drops H2SO4
Drops NaOH
10
5