Chem 7L Final (Tim Lam)

experiment 2: what are you given in and what are you asked to find?

given: colorless unknown solution (soluble analyte)
find: Al ion concentration in solution (insoluble product precipitate)

What ligand is chosen to be used to bind to Al3+, and how does pH affect its properties?

hq, 3 ligands: 1 Al
Hhq+: low pH, soluble but not binding

hq: minimally soluble, binding

hq-: soluble and binding

Why not use hq- if soluble and binding?

at a high pH Al starts to form side products such as Al(OH), leading to impurities

what pH gives what ideal ratio of ligand, hq and why?

A pH~ 4 gives optimal binding to solubility, 10 Hhq+ : 1 hq, as the hq binds to the Al, the reaction favors that production and more hq is formed, more binds (after initial precipitation)

give an overview of the procedure for experiment 2:

1. Warmed Al sol. up

2. added HCl

3. buffer little at once to slowly bring to 10:1 ratio

4. Let sit overnight for slow precipitation

5. filtration and measurement

why warm up the Al sol. ?

reaction proceeds faster and adds to the solubility, no premature precipitation

why add HCl (acid)?

turn the hq all to Hhq+ initially then slowly add buffer to bring it to the desired 10:1 ratio

what are the assumptions of experiment 2?

Assumption 1: adequate drying time; Effect: measured mass will be an artificially high conc if water left over
Assumption 2: chemical equilibrium between Al3+ —> Al(hq)3 has been reached; Effect: in reality, equilibrium means that there is still some reactants, not just products, so little Al 3+ left over will lead to an artificially low measured mass
Assumption 3: washing with water will remove impurity but not solid; Effect: water could create insoluble ligand and will stay with collected products, artificially high measured mass

what are assumptions in general?

things in the procedure you DID NOT CHECK

what could too much excess hq do?

precipitates and mass will be artificially high

what buffer is used in exp 2?

ammonium acetate

why use the desiccator ?

removes excess weight so you don’t have artificially high mass

what is your limiting reagent?

Al3+

experiment 3: what are you asked to find?

identity of an unknown acid through volumetric analysis, MW and pKa

what does the flask beneath the titration contain? what does the buret contain?

analyte: acid
buret: titrant (NaOH)

why standardize NaOH?

Because NaOH cannot be made into a solution of exactly known concentration directly:

• It absorbs CO₂ from air → reacts with NaOH → changes concentration

• It absorbs water from air (hygroscopic) → can never be weighed purely

what acid did you standardize NaOH with, and why?

KHP, KHP has a 1:1 molar ratio with NaOH, it is available in pure form to weigh with precision

why use phenophyline (indicator) for NaOH ~ KHP titration?

phenophaliyne was the indicator that changed color at the equivalence pt. pH of KHP~NaOH (1:1 mols of base to acid, equivalence point)

why use pheno. for indicator of unknown acid?

there is a big pH change in a small volume range, allowing for approximation of equivalence point

why does the analyte turn pink then back to clear as pheno. is added?

Because CO2 is actively dissolving into the solution making it more acidic

what is the overall procedure for exp 3:

1. Standardize NaOH into a known [conc]

2. Titrate acid with NaOH to find # of moles in acid —> molarity—> MW

3. Find the equivalence point using a precise titration curve by measuring out (.050; specific) molarity of acid (need MW to be able to do this)

what is shows the equivalence point in a precise titration?

peak, or maximum first derivative

how did you find the 1/3, 1/2, and 2/3 eq. points?

interpolation of the slope

how did you determine your acid’s unknown identity after the exp?

lowest sum percent error

What are 6 assumptions of exp 3?

1. Assumption: CO2’s ONLY effect is during titration of acid to determine MW; Effect: you need a higher volume of base to complete titration because your solution additional acid, larger mols calc—> Artificially low MW

2. Assumption: CO2’s ONLY effect is during pKa titration; Effect: higher volumes for each eq. point —> Artificially high pKa

3. Assumption: CO2 only effects NaOH standardization; Effect: you need a higher volume of base to complete standardization, determined NaOH conc lower than should be, artificially high MW

4. Assumption: Full drying of KHP; Effect: mass of KHP will be higher, molarity will be higher for NaOH, lower MW

5. Assumption: pheno. is the right indicator of choice; Effect: either the acid has a slightly lower eq. than assumed: artificially high MW or acid has slightly higher eq. than assumed: artificially low MW

6. Assumption: the full range of pH is valid (even though only calibrated from 4~7); Effect: reading consistently high values: eq lower (pKa) reading consistently low values: eq higher (pKa)

exp 4: what are you given? what are you asked to analyze?

given: (NH4)2Fe²(SO4)2 asked to find: Empirical Formula Kw[Fex(C2O4)y]zH2O

what is the oxidizing agent and what is oxidized?

Oxidized: Fe2+ —> Fe3+
Oxidizing Agent: H2O2

what is the reducing agent and what is reduced?

Reduced: Fe3+ —> Fe2+

Reducing Agent: Ascorbic Acid

What is Beers Law?

A = εbc

A= absorbance

ε=molar absorptivity

b= pathlength

c= concentration

overall procedure of exp 4?

1. Crystal Synthesis

   1. Fe2+ salt & oxalic acid

   2. yellow precipitate forms

   3. H2O2 used to oxidize Fe2+ —> Fe3+

   4. excess oxalic acid+potassium creates oxalate ligands which displace H2O2

   5. brown dissolves—> green/yellow substance

2. Primary Stock/Secondary Stock

   1. make primary stock with KNOWN Fe conc (weighed out)

   2. make secondary stock that is simply primary, diluted

   3. Ascorbic Acid is used to reduce Fe3+ to Fe2+ in order to bind to bpy

   4. make 5 dilutions—> calibration curve with known conc of Fe

3. Unknown Stock

   1. take crystals and make unknown stock

   2. release iron from oxalate by adding Ca2+ (CaC2O4 precipitates out)

   3. centrifuge

   4. add ascorbic acid to match conc in secondary stock

   5. make 5 dilutions and plot for absorbance

Why use a secondary stock not just primary stock?

A secondary stock will be more dilute, the primary stock is too concentrated giving lower absorbance values that 10% making it imprecise for use

why add ascorbic acid to unknown if not needed to reduce anything?

because you want to keep your unknown concentration contents same to secondary stock in order to correctly use the calibration curve

why 10% to 90% transmittance?

anything below: small conc differences give big changes in A (imprecise)
anything above: too little absorption, signal too weak (imprecise)

how does A vs c plot?

linearly, so that any unknown point on that same plot should tell the conc at that absorbance

why use 520 nm for Fe T%?

520 nm is the wavelength of maximum absorbance

how do you find the molar ratio of Fe to C2O4?

find:
mols of Fe = mass%Fe/ MW Fe
mols of C2O4 = mass%C2O4/MW
divide both by the small # = molar ratio

why convert crystals to Fe(bpy)3² for analysis?

Fe is not a strong absorber of light, Low molar absorptivity = low sensitivity = poor precision
bpy ligand is a much stronger absorber at 520 nm

what is T

fraction of light that passes through

how to find absorbance?

take T —> T% —> -log(.xxx)= A

what are constants at a fixed wavelength in the Beers-Lambert law?

ε and b

is it Fe3+ or Fe2+ in the final product that forms?

Fe3+

what are the assumptions of exp 4:

experiment 5: overall goal?

find the identity of your unknown salts via spectrometry

why is Balmer the only visible light?

Lyman: energy gap between nf=1; so large that its a very high energy photon : UV light

Paschen: energy gap between nf=3; so small that its very low energy photon : infrared light

what is the relationship between energy gap, wavelength, and energy?

bigger the gap between ni and nf, higher energy, shorter wavelength

why does red have the lowest energy?

ni =3 nf = 2; smallest gap —> lowest energy photon —> longest wavelength —> red

Why create Hg calibration curve?

mercury gives known emission lines, lets us calibrate our spectroscope to have uniformity (i.e. corrected wavelength)

Why does He have more distinct emission lines

second electron creates shielding effect, reducing effective nuclear charge, shifting the energy levels breaking degeneracy and now 2 s and 2 p have differing energy levels leading to more distinct emission lines

why do He and H emission wavelengths differ?

shielding: helium’s 2nd electron reduces effective nuclear charge, shifting all energy levels relative to H
singlet vs triplet states: once in the 2p orbital, there is further energy transition differences breaking the degeneracy, singlet states have opposite spinning electrons and are at a higher energy level; triplet states have parallel spinning electrons, with a lower energy level

what is degeneracy?

orbitals in the same energy levels have the exact same energy
ex: 2s, 2p = identical energy or 3s, 3p, 3d = identical energy

why does hydrogen emit 4 lines?

4 different options of excited electrons to fall to:
n=6 —> n= 2 (shortest wavelength/most energy)

n=5 —> n=2
n=4 —> n=2
n=3 —> n=2 (longest wavelength/least energy