Please complete the whole work sheet and all the questions. Don’t copy anything from internet please.Guided Virtual Lab Activity: The Complex Nickel Salt
Goal of Activity
In the lab you would synthesize a complex nickel salt from the building blocks Nickel, sulfate, water, and
ethylenediamine. Once the complex is formed, you would determine the % mass of ethylendiamine, Nickel,
and sulfate in the synthesized nickel salt, and hence determine the empirical formula. Once you know the
empirical formula of your unknown Nickel salt, you can calculate the % yield. In the worksheet you will
be guided to some of the calculations. You will be provided with data (highlighted in green) to put in your
lab notebook, and you will analyze your salt based on this data. Additionally, you will be asked to answer
conceptual questions along the way. Canvas will provide valuable resources to answer all questions, so
make sure you have access to the content when you work through this activity.
Lab: Synthesis of a Complex Nickel Salt
Please use relevant info from this section to complete your calculations and analysis.
“1.2451 g of NiSO4 · 6H2O was added to a beaker. The 25% volume by volume ethylenediamine in ethanol
was a clear liquid solution with a density of 0.899 g/mL, and 1.03 mL of this solution was added to the
beaker. A lavender precipitate formed when the two products were added. When the solution was stirred
the lavender precipitate disappeared and the solution turned into a blue green color. The cold ethanol was
also a clear liquid and 5.12 mL was added to the solution. This turned the solution cloudy blue. The solution
took 32 minutes to crystalize and 3.56mL of extra ethanol had to be added to speed up the process. The
solution was quantitatively transferred to a Buchner funnel and rinsed using cold ethanol to ensure the
precipitate stayed crystallized. The solution was then vacuum filtered, and only had to be filtered once
because the filtrate was clear. The precipitate was weighed and the actual yield of the nickel salt was 1.2190
g.”
Calculations
1. Calculate the mols of nickel(II) sulfate hexahydrate.
2. Calculate the mols of en used in the synthesis (this can be a bit tricky and might
require a look-up).
Lab: Anlaysis of en Content
Useful Terms:
• Lewis acid – an electron pair acceptor. Note that the substance must have available space to accept the
electron pair. The Ni2+ ion is electron deficient, and can accept an electron pair.
• Lewis base – an electron pair donor. Both water and ethylendiamine contain lone pairs that can be
donated to the metal cation, so both can be classified here as Lewis bases. Since they are attaching to a
metal ion, they are also given the term “ligand”.
o Ligands may be classified by how many attachments they make to a Lewis acid. Monodentate =
one; bidentate = two; polydentate = many.
• Coordination number – number of electron pairs that will attach to the central metal cation.
o Ni2+ has a coordination number (CN) of 6. (What shape will it be?)
• Coordinate covalent bond – sharing of electrons (i.e. covalent bonding) where the shared electrons
originated from one substance (e.g. a Lewis base).
• Octahedral – the central atom will be bonded to 6 atoms, with no lone pair on the central atom. All
bond angles formed are 90 degrees.
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Guided Virtual Lab Activity: The Complex Nickel Salt
1. Sketch the lewis structure of an ethylenediamine molecules and a water molecule.
Water as a ligand.
2. Locate the lone pairs on the water molecule. Form (sketch it on paper) a covalent bond between the
water molecule to the octahedral Ni2+. This is done by converting a lone pair on a water molecule
into a single covalent bond to the Nickel. Attach as many water molecules as possible to an
octahedral Ni2+.
a. How many lone pairs on a given water molecule can simultaneously attach to one octahedral
Ni2+? _____ Describe why this is so.
b. What is the total number of water molecules that can bind to one Ni2+.
c. What type of ligand is water? monodentate
bidentate
polydentate
Ethylendiamine as a ligand.
3. Repeat the process for the ethylenediamine (H2NCH2CH2NH2, or “en” for short).
a. How many lone pairs on a given ethylenediamine molecule can simultaneously attach to one
octahedral Ni2+? _____ Describe why this is so.
b. What is the total number of en molecules that can bind to one Ni2+.
c. What type of ligand is ethylendiamine? monodentate bidentate polydentate
Hexaaquanickel (II)
4. Build a model of what the Ni2+ ion looks like when it is dissolved in water.
a. Draw the 3-D structure here.
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Guided Virtual Lab Activity: The Complex Nickel Salt
b. Write the formula for this complex cation. _________________________
Nickel ion after Reaction with Ethylenediamine
5. Ethylenediamine is a stronger ligand than water, so it will preferentially bind to the Ni2+. Assume
that during the synthesis of the complex nickel salt two ethylenediamine molecules bind to the Ni2+,
knocking off waters from hexaaquanickel(II) in the process. (Can more than one structure form?)
6. Draw the 3-D structure(s) here (Can more than one structure form? Look up the term isomer).
a. Write the formula for this complex cation. _________________________
b. Assuming that that the ratio of Ni2+: en : SO42- : H2O in the precipitated salt is 1:2:1:6,
write the formula for the nickel salt.
Ethylenediamine (en) and HCl
7. We’ve established that en is a base, and we know that bases react with acids.
a. Classify HCl: strong acid
weak acid
b. Classify Ni2+:
weak acid
strong acid
c. Which acid will ethylendiamine (en) preferentially react with? HCl
Ni2+
8. Here is the structure of en, shown three ways: complete structure, line structure, and an expanded
formula version. How does the structure of en change when it reacts with HCl? Show the alteration
on these structures (note the en reacts with the H+ of HCl.
H2NCH2CH2NH2
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Guided Virtual Lab Activity: The Complex Nickel Salt
9. Write the net balanced chemical equation to illustrate the reaction of en with HCl. Identify any
spectator ions.
10. What is the stoichiometric ratio of en to HCl? _____________
HCl and NaOH
11. We know that HCl and NaOH react. Write the complete balanced chemical equation for this
reaction.
Putting It All Together
Let’s say we want to know the % mass of en in the synthesized complex nickel salt. We design an
experiment where we add a known, excess quantity of HCl to the nickel salt. This means:
1) We know exactly how much HCl we added; and
2) We know that there is more HCl than is needed to react with the en in the Ni salt, so there is some
unreacted HCl in the flask; and
3) We can titrate the amount of unreacted HCl with NaOH.
Example with food to illustrate the experiment –you can exchange food items to your likings
12. Take a small bowl and place it on a plate. This bowl represents your complex Ni cation, the plate
represents your reaction vessel (such as a beaker or Erlenmeyer flask). Take 3 pretzels and place
them in the bowl. The pretzels will represent en in this example, and since they are in the bowl they
are bonded to Nickel. Now place 12 individual MM’s representing H+ from HCl, and place them
on the plate close to the bowl. (In this activity, we are assuming we added 12 moles of HCl to the
reaction container, which contains one mole of your synthesized nickel salt.)
13. React the ethylendiamine (en) in the complex Ni salt with the HCl (Yes react here means take out
a pretzel from the bowl and combine with the right amount of mm’s and then eat it), according to
your balanced chemical equation.
a. Which runs out first?
Ethylenediamine
H+
b. How much of the excess reagent is left? (count them) ______________
c. Can the excess reagent be titrated with NaOH? Yes
No
d. How many OH- from NaOH will react with this excess? ________________
14. Use this example to come up with the mathematical approach to determining the % mass of en in
the nickel salt, according to the experimental procedure provided on the next page. It might help to
review your notes from the indirect titration examples covered in class.
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Guided Virtual Lab Activity: The Complex Nickel Salt
Sample Student notebook work for the Determination of % Mass Ethylenediamine in the Synthesized
Complex Nickel Salt A by Back-Titration.
Procedure
1) Accurately weigh approximately 0.05 g of
salt A or B and quantitatively transfer it into
an Erlenmeyer flask.
Data & Observations
Salt A was analyzed. It was a pale blue powder.
0.0513 g Salt A.
2) Add 10.00 mL of standardized 0.2 M HCl
(aq) and 2-3 drops of Bromocresol green
indicator to the flask. The solution should be
yellow at this point.
HCl is clear, colorless. [HCl] = 0.2108 M
Used volumetric pipet to transfer the HCl.
The salt dissolved in the HCl, and the resulting
solution was an extremely pale green-blue color.
Solution turns yellow when indicator is added.
3) Slowly titrate the solution with standardized
0.1 M NaOH (aq) until you observe the first
permanent color change. (You are forming a
new complex cation in the titration, which
requires a little time – slower titrations tend
to give better results on this experiment.)
4) Repeat steps 1 – 3 until you have 3 good
trials.
NaOH is clear and colorless. [NaOH] = 0.1008 M
Trial
1
2
3
Initial
Volume
NaOH
2.61 mL
20.78 mL
0.33 mL
Final
Volume
NaOH
20.78 mL
39.16 mL
18.56 mL
Endpoint
Observations
Pale green
Pale green-blue
Pale green
Mass of salt A used in trial 2 = 0.0489 g
Mass of salt A used in trial 3 = 0.0508 g
Titration data is in data table above.
Assume someone performed this experiment on the salt you synthesized. Use this data to calculate the
percent by mass of en in the synthesized salt.
15. Report the mass % of en and report the ASD/RSD/CI (99%). Comment on the precision of the
experiment.
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Guided Virtual Lab Activity: The Complex Nickel Salt
Lab: Beer’s Law & Absorption Spectroscopy
We are interested in determining the mass % of Nickel in a sample of our synthesized salt. One commonly
used technique to determine Nickel content in a homogenous solution is spectroscopy. Let’s do a basic
review on spectroscopy and see how this helps us determine the amount of Nickel in our synthesized salt.
Spectroscopy – color of a solution and λmax
Spectroscopy is the study of the interaction of light and matter. A solution that has a specific color typically
absorbs at a specific wavelength in the visible region from ~375-750nm. A student dissolved a sample of
Nickel sulfate hexahydrate in water and analyzed it with a spectrophotometer.
1. What is the formula of the complex cation?
Absorbance
2. Inspect the absorbance curve below, where a student did a wavelength scan from 375-750nm and
recorded the absorbance at each wavelength, determine the approximate wavelength of maximum
absorbance?
Wavelength in nm
3. Sketch the curve of absorbance versus wavelength into your lab notebook and record the
maximum absorbance.
Spectroscopy – Beer’s Law
Now that we know the maximum wavelength for Nickel in water, we can ask ourselves if it is possible to
determine the amount of Nickel in our unknown salt that we synthesized. In order to do so, a sample of the
unknown salt is dissolved in sulfuric acid.
4. What is the purpose of the sulfuric acid (recall what you learned in the titration section that sulfuric
acid is a strong acid, Nickel is a weaker acid, and en is a base)?
5. What will be the cation in solution? Write the chemical formula!
6. What wavelength can we use to study the Nickel in our Nickel salt?
Beer’s law allows us to quantitatively determine the amount of a selected component in a homogenous
mixture through absorption of light. If you have a homogenous sample, such as our Nickel salt A or B
dissolved in water, you can determine its amount by setting the spectrophotometer to the correct wavelength
and recording the absorbance.
7. Write down the equation for Beer’s law and define all the variables used in it!
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Guided Virtual Lab Activity: The Complex Nickel Salt
The power of Beer’s law is that we can determine the concentration of a sample from its absorbance
(simply substitute your absorbance reading into the equation of Beer’s law and calculate Molarity). In
general, we can directly use Beer’s law to calculate Molarity from Absorbance. However, as a scientist
we take advantage of the linear relationship of Molarity and Absorbance, and we prefer to use a standard
curve of Nickel to determine the Nickel content of a solution.
Spectroscopy – Standard Curve
To create a standard curve for Nickel, we need to have both concentration and absorbance values for a
number of standards.
8. Define standard (as it is used to describe a solution in chemistry) or a standard solution.
The general equation of a straight line will be of the form = + , where in this experiment y is
absorbance and x is concentration of Nickel II at λmax. A typical standard curve generated by a student
looks as follows:
9. What is the purpose of the R2 and what are good R2 values ?
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Guided Virtual Lab Activity: The Complex Nickel Salt
Sample Student notebook work for the Determination of a standard curve for Nickel.
Procedure
Accurately weigh approximately 1 g of Nickel II
sulfate hexahydrate and quantitatively transfer it
into a 50mL volumetric flask .
Use 1M sulfuric acid as solvent to make the
nickel standard.
Data & Observations
The blue nickel II sulfate hexahdrate salt was
weighed on analytical balance #17.
Tare mass 0.0000g
Mass of weighing boat 1.5413g
Mass of weighing boat and salt 2.5444g.
Sulfuric acid is clear, colorless, and has a
concentration of 1.108 M. The volumetric flask
was filled 2/3, all solid was dissolved, and then
the flask was filled with sulfuric acid so that the
meniscus is on the graduation mark. The flask was
then capped and inverted 30 times.
This solution was labeled stock solution.
Prepare 5:25, 10:25, 15:25, and 20:25 solutions
For the 5:25 dilution, the 5mL of the stock
solution was transferred with a 5mL volumetric
pipet into a 25mL volumetric flask. The flask was
then filled properly with the sulfuric acid as
solvent. All other dilutions were also made from
the stock solution using a 10mL, 15mL, and 20mL
volumetric flask.
Measure the absorbance of all standards
The spectrophotometer was set to 395nm and the
lamp was allowed to warm up for 15 min. The
instrument was blanked with sulfuric acid and
read 0.000 as absorbance. The absorbance for
each solution was measured in the order least to
most concentrated (see table below)
Solution Abs
5:25
0.121
10:25
0.198
15:25
0.243
20:25
0.304
Stock
0.348
10. Use this data to create a standard curve. Sketch the standard curve below. Label axes
appropriately. Report the equation of the straight line and its R2 and use this equation for future
calculations.
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Guided Virtual Lab Activity: The Complex Nickel Salt
Lab: Analysis of Nickel Content
The mass percent of Nickel can be calculated using the following equation:
× 100
% =
In order to calculate the mass % we need two pieces of information, the mass of the sample we analyze,
and the mass of the Nickel in our unknown salt. The mass of the sample is straightforward, just simply
weigh out a sample of your unknown salt using the analytical balance. To get the mass of Nickel is a bit
complicated and involves some steps. First things first, the procedure asks to prepare a 25 mL solution of
your salt by using approximately 0.25 g of salt and 1 M sulfuric acid as the solvent.
11. What is the purpose of the sulfuric acid as solvent? What is the formula of the complex cation
you will observe in solution?
Once you dissolve your salt, we can analyze the Nickel content using the spectrophotometer. The
spectrophotometer gives us an absorbance reading, and this absorbance reading needs to be converted into
the mass of Nickel. Let’s think about the flow chart for this calculation.
Absorbance
Nickel
concentration
Ni
mol Ni
mass Ni
12. Use the flow chart to come up with the mathematical approach to determining the % mass of
Nickel in the nickel salt. Ask yourself for every arrow indicated, how you would perform this
step, for example to go from “Absorbance Nickel” to “concentration Nickel” what information
and equations might be needed to do so? (hints: use the equation for molarity, the equation for the
conversion from grams to mol, and the standard curve we determined earlier)
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Guided Virtual Lab Activity: The Complex Nickel Salt
Sample Student notebook work for the Determination of the mass % of Nickel.
Procedure
Prepare a 25 mL solution of ~0.25g of salt and
1M sulfuric acid as the solvent.
Measure the absorbance of all solution
Data & Observations
The nickel salt was weighed on analytical balance
#17.
Tare mass 0.0000g
Mass of weighing boat 1.5665g
Mass of weighing boat and salt 1.8163g.
The salt was transferred to a 25mL volumetric
flask. Sulfuric acid is clear, colorless, and has a
concentration of 1.108 M. The volumetric flask
was filled 2/3, all solid was dissolved, and then
the flask was filled with sulfuric acid so that the
meniscus is on the graduation mark. The flask was
then capped and inverted 30 times.
The spectrophotometer was set to 395nm and the
lamp was allowed to warm up for 15 min. The
instrument was blanked with sulfuric acid and
read 0.000 as absorbance. The absorbance for the
nickel salt read 0.174.
13. Assume someone performed this experiment on the salt you synthesized. Use the data to
calculate the percent by mass of Nickel in the synthesized salt.
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Guided Virtual Lab Activity: The Complex Nickel Salt
Once you have calculated the mass % Ni, you can also determine the mass % of sulfate. Since in our formula
the ratio of Nickel and sulfate has to be 1:1, we know that the mol:mol ratio of Nickel and sulfate has to be
1:1.
14. Calculate the percent by mass of sulfate in the synthesized salt.
Determine the empirical formula
Now that we have the mass % of en, Nickel, and sulfate, we can determine the empirical formula. Review
your lecture notes on how to solve empirical formula problems. Before we start, notice that the sum of all
mass % has to equal 100%
15. Calculate the mass % of water
16. Fill out the following table. Assume 100g when converting mass % in grams. Note that the mass
% and mass will have the same numerical value. The mol fraction here means divide each mol by
the mol of Nickel (we can only have one Nickel in our formula), this will make all numbers 1 or
greater than 1.
Atom/
Building block
Mass in g assuming
Mass %
Mol
Mol fraction
100g
Ni2+
En
H2O
SO42Show your calculations from grams to mol here:
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Guided Virtual Lab Activity: The Complex Nickel Salt
17. What is the molar mass of your empirical formula?
Now that you have the empirical formula (EF) we can balance the generic chemical equation.
4 ∙ 6 2 ( ) + ( ) �⎯⎯�
Note that is ok to add waters on either the reactant or the product side to balance the chemical equation.
18. Write down the balanced chemical equation for this reaction, and copy it in your notebook.
Determine the % yield for the synthesis reaction
We will refer back to the synthesis section on page 1 a lot in this section, so make sure you have all this
information available to answer this section. By definition, the % yield can be calculated using the
following equation:
% =
× 100
ℎ
The actual yield is the grams of product you made and can be found in the section “Synthesis lab and
Separation of Nickel salt”
19. Record in your lab notebook “Actual yield = xg” where x is the actual number.
The theoretical yield requires you to determine the grams of product from the mols of product. We obtain
the mols of product through a stoichiometry calculation, and then convert the mols to grams using the
molar mass of product. As always, in stoichiometry you need the mols of the reactant(s). Let’s see what
information we need to calculate the mols.
20. What info is given that allows you to calculate the mols of NiSO4.6H2O. Calculate the mols of
NiSO4.6H2O.
The mols of en are a little more complicated. We are given a volume of a 25%v/v (volume by volume
percent) en in ethanol, we are given the density of en, and the molar mass of en.
21. What is the molar mass of en?
22. What is the density of en solution according to canvas?
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Guided Virtual Lab Activity: The Complex Nickel Salt
23. If I add 100mL of 25%v/v en in ethanol, I in essence add 25mL of en. Given this fact, and the
volume of en in ethanol in the synthesis section, what is the volume, grams, and mols of en used
in the reaction?
24. Now that we have calculated the mols of en and the mols of NiSO4.6H2O, we can do
stoichiometry. Classify the type of stoichiometry problem where you are given the mols of the
two reactants, and you need to find the mols of product? This is a very special type of
stoichiometry problem that you do need to know how to solve.
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Guided Virtual Lab Activity: The Complex Nickel Salt
25. Based on your mols of reactants, and the balanced chemical equation, determine the limiting
reactant.
26. Calculate the mols of product, and using the molar mass, calculate the grams of product. Copy
this information as theoretical yield into your lab notebook.
27. Calculate the percent yield and comment on it.
In this worksheet you are supplied with data of the Nickel lab experiments that help you determine the
mass %, empirical formula, and % yield. Make sure to copy all calculations into your lab notebook.
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