Lab Final

Question 1

If an unknown containing an inorganic ion did not have a definite composition, how could a weighable form of precipitate be produced?

Answer 1

An organic reagent can be used and precipitated with the inorganic ion

Question 2

What is the white powder formed during your precipitation step in gravimetric analysis of an unknown sulfate?

Answer 2

BaSO4

Question 3

What is a gravimetric factor? Find the gravimetric factor (calculate the actual value) of the sulfate in barium sulfate.

Answer 3

A gravimetric factor is mass of analyte divided by mass of precipitate compound.

= 0.4116

Question 4

Why do we need two masses of our precipitate at are within ± 0.001 g of each other?

Answer 4

To make sure the sample is fully dry

Question 5

Find the %SO42- in 1.5803 g of an unknown sulfate that yielded 1.2324 g of white precipitate.

Answer 5

32.1%

Question 6

What is a primary standard? List three of its characteristics.

Answer 6

It is used to standardize other chemicals. It is an accurate reference chemical to measure an unknown concentration of another known chemical. High purity, high stability, and high molar mass.

Question 7

What is a secondary standard? Provide an example.

Answer 7

They are chemicals standardized using a primary standard. Ex: NaOH, its concentration can be determined by titrating with KHP

Question 8

A 1.3363 g sample of a primary standard KHP was titrated with 44.25 mL of NaOH solution to a phenolphthalein end-point. What is the molarity of the NaOH solution?

Answer 8

0.1479 M NaOH

Question 9

Distinguish between accuracy and precision.

Answer 9

Accuracy is how close a value is to the actual value and precision is how close multiple measurements are to each other.

Question 10

The following molarities were calculated from replicate standardization of a solution: 0.09028, 0.09026, 0.09034, 0.09034, 0.09034, and 0.09030. Calculate the 95% confidence interval for this set of data.

Answer 10

0.09031 +/- 3.696 x 10^ -5

Question 11

Assume the procedure above has been followed up to Step 9. The rough titration reached its endpoint with 7.80 mL of titrant. While doing the real titration with the pH meter, at what volume should titrant be added in 2-drop increments?

Answer 11

(7.8 x 4) - 2 = 29.2

Question 12

Assume the procedure above has been followed in its entirety. 11.2224 g of unknown was dissolved in 500.0 mL of DI water. 100.0 mL of that solution was taken and titrated with 0.06301 M NaOH, and the equivalence volume was found to be 25.37 mL. What is the %KHP in the unknown?

Answer 12

14.54%

Question 13

Why is it important to boil off the CO2 in volumetric determination of soda ash?

Answer 13

So the CO2 doesn’t affect the endpoint, ensure accurate and precise results, ensure proper behavior of indicator by changing the pH back

Question 14

Will the unknown solution be pink, colorless, blue, green, or yellow after the phenolphthalein is added?

Answer 14

pink

Question 15

At the start of a titration, you marked your meniscus on the burette as 5.44 mL. You noticed your titrant meniscus approaching the 50 mL mark on the burette midway through the titration, so you paused the titration to refill the burette. When you closed the stopcock, you read your titrant meniscus at 46.60 mL. You refilled your burette, recorded the meniscus at 3.31 mL, and resumed your titration. At the end-point of the titration, your meniscus read 36.83 mL. What is the total volume of titrant used for the titration?

Answer 15

(46.6-5.44) + (36.83-3.31) = 74.68 mL

Question 16

What is the %Na2CO3 (m/m) of a soda ash sample if it took 38.29 mL of 0.09159 M HCl to titrate 0.5997 grams of the sample?

Answer 16

30.98%

Question 17

0.1395 g Fe(NH4)2(SO4)2·6H2O was dissolved in a 500.0-mL volumetric flask to make the Stock Fe solution. Calculate the exact ppm Fe of the stock solution with correct significant figures.

Answer 17

40 ppm

Question 18

Why must you add hydroquinone and ortho-phenanthroline to the solutions?

Answer 18

To create an intensely colored complex

Question 19

Your solution has an absorbance of 0.858. What percentage of the light was transmitted through the sample?

Answer 19

T = 10^-0.858 = 0.138676 x 100 = 13.9%

Question 20

What is the ISA used in potentiometric determination of chloride?

Answer 20

NaNO3

Question 21

Why do we plot mV vs. log[Cl-] instead of mV vs. [Cl-]? What would the mV vs. [Cl-] graph look like?

Answer 21

Because potentiometry measures activity of the species and not the concentration directly; the graph would be negative exponential and not a straight line

Question 22

A solution was made by dissolving 9.1748 g NaCl in a 500.0-mL volumetric flask. What is the ppm Cl- for this solution?

Answer 22

11,132 ppm

Question 23

Explain why an anion exchange resin, which has positively-charged sites, can be used to separate a mixture of cations such as Co(II) and Ni(II).

Answer 23

With the addition of HCl, Co2+ will form anionic complexes while Ni2+ will not, so the cobalt will bind to the resin and the nickel would flow thru. Once the nickel is out of the column, the cobalt can be detached from the resin by the addition of water

Question 24

Distinguish between a complexing agent and a chelating agent.

Answer 24

Complexing: agent that can form a complex when added to other materials in any solution

Chelating: ligands that are able to form 2 or more coordinate covalent bonds w a metal ion

Question 25

Name and write the formula for a species that contains Co(II) (in the experiment you are performing) that imparts the following colors when the species in present in aqueous solutions.
a. blue or blue-green
b. pink

Answer 25

a. CoCl4 2-
b. Co 2+

Question 26

Why is it important to not let your fingers touch the TLC plate and filter paper?

Answer 26

oils from your fingers can contaminate the plate

Question 27

Answer each question based on the procedures above
a. What is the stationary phase for TLC?
b. What is the stationary phase for Paper Chromatography?
c. What is the mobile phase for TLC
d. What are the mobile phases for Paper Chromatography?

Answer 27

a. thin layer of silica gel coated on the plate
b. filter paper
c. ethyl acetate and dichloromethane
d. system 1 & 2

Question 28

If an analyte has a very large Rf value, what does that imply about the interaction between the analyte and the stationary phase relative to the interaction between the analyte and the mobile phase?

Answer 28

It has a stronger affinity for the mobile phase

Question 29

A student spots an unknown sample on a TLC plate. After developing in
hexanes/ethyl acetate 50:50 ratio, they saw a single spot with an Rf of 0.55. Does this indicate that the unknown material is a pure compound? How could the student verify the sample’s purity?

Answer 29

Yes, there’s only one spot. If it had multiple spots it would be contaminated. Try a different mobile phase