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Quiz about Good Practice In the Lab Analytical Chemistry

Good Practice In the Lab (Analytical Chemistry) Quiz

This quiz tests basic laboratory practices and techniques. It will cover a broad range of topics, but will revolve around good practice.

A multiple-choice quiz by dgbayer. Estimated time: 8 mins.

Author
dgbayer

Time
8 mins

Type
Multiple Choice

Quiz #
309,119

Updated
Dec 03 21

# Qns
10

Difficulty
Difficult

Avg Score
4 / 10

Plays
824

One at a Time - Single Page - Timed Game

Question 1 of 10

1. Why shouldn't you touch your beaker or weigh boat with your bare hands when weighing out a sample? Hint

Especially with dry samples, moisture on the hands will attract certain hydrophilic particles and can remove previously "weighed" particles, which introduces systematic error

None of these: my weigh boat is always my hand, especially in cooking.

Risk of contamination from your hands to the sample, which introduces all sorts of error

Your hands will either soak up moisture from or add moisture to the outside of the vessel, which introduces random error.

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Question 2 of 10

2. Is it important to dry your glassware between successive titration trials, if you are using the same flask to run replicates of the same solution and rinsing with DI water? Hint

No, as long as relatively the same amount of water is left in the flask/beaker each time, the error due to dilution should average itself out

No, because titration deals with mol:mol ratios, a fluctuation in concentration does not affect the volume of titrant required.

Yes, the unknown addition of moisture would introduce a systematic error in the initial concentration of the sample

Yes, glassware should always be cleaned and dried as well as possible before running a new trial... no matter what

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Question 3 of 10

3. When performing absorption spectra analysis in the UV/Vis range. Why are high absorbance values (>1) considered to be poor representatives of the true absorbance of the sample? Hint

There is no detector sensitive enough to measure such low values of transmittance

Higher absorbing species require a brighter light source, which increases the amount of stray radiation on the detector

Highly absorbing samples are usually diluted and re-sampled, which introduces a dilution error

Highly absorbing samples allow the error due to stray light to become more and more significant

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Question 4 of 10

4. If I want to prepare 100mL of a mixture using only a 10mL pipet, what would the error of the 100mL mixture be if the error in the pipet is 0.2 mL? Hint

0.2*10

0.2*SQRT(9)

0.4*SQRT(10)

0.2*SQRT(10)

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Question 5 of 10

5. What does a PMT (photo multiplier tube) do? Hint

Increases the intensity of a light signal, usually in the UV/Vis region

Scans many wavelengths at the same time, allowing for simultaneous measurements

Increases the photo output of a laser source

Uses a voltage bias to select the wavelength of light that is being detected

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Question 6 of 10

6. Now for a little biochemistry. What chemical is commonly added to agar (which is intended to grow E. coil) that is crucial in blue/white screening? Hint

lacZ

Beta-Galactose

X-Gal

Ampicilin

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Question 7 of 10

7. You want to separate a mixture of compounds using gel permeation chromatography. What compound, do you predict, will be the first to elute? Hint

Substance P (MW=3900 Da)

Substance G (MW=2.5 AMU/molecule)

Substance X (MW=2.5 kg/mol)

Substance L (MW=90 mg/mmol)

8. Analysis- If a pathway of a particular substrate to product that is crucial for life is studies by creating mutant knockouts for the 3 genes responsible for the enzymes in this pathway. The pathway consists of a starting material, two intermediates, and a final product. (call them A B C and D). For clarity, Each plate is only supplemented with one factor, meaning if it grows on A and B plates, then it grows on a plate supplemented with A alone and a plate supplemented with B alone).

Strain 1 grows on plates containing A, but not on plates containing B C or D)
Strain 2 grows on plates containing C and A but not on plates containing B or D
Strain 3 grows on all 4 plates
Strain 4 grows on A D and C but not on B plates

What is the order of starting material intermediates and final product?

Hint

ACDB

BDCA

BDAC

DBAC

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Question 9 of 10

9. In column chromatography, specifically in TLC (thin-layer chromatography), what is the retention factor? Hint

The distance from the starting line to the sample mark divided by the distance from the starting line to the solvent head

The distance from the sample mark to the solvent line divided by the distance from the starting line to the solvent head

The distance from the starting line to the sample mark divided by the distance from the starting line to the solute head

The distance from the sample mark to the solute line divided by the distance from the starting line to the solute head

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Question 10 of 10

10. If you notice that your data isn't fitting the theoretical model it is supposed to fit, it is best to... (assume you're a student and the lab report is due tomorrow) Hint

Go back and reacquire bad data points

Find a better theory

Remember back to the procedure and try to correlate possible sources of error

Make up "better" data points

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Quiz Answer Key and Fun Facts

1. Why shouldn't you touch your beaker or weigh boat with your bare hands when weighing out a sample?

Answer: Your hands will either soak up moisture from or add moisture to the outside of the vessel, which introduces random error.

Although this effect may not be so significant when measuring large quantities of mass, being careful about this can be the difference between 99% yield and 65% yield. The error truly is random. I personally usually remove mass from a beaker by touching it, while other people add mass. My reasoning is because my hands are dry, they may suck up any moisture already present on the outside surface.

2. Is it important to dry your glassware between successive titration trials, if you are using the same flask to run replicates of the same solution and rinsing with DI water?

Answer: No, because titration deals with mol:mol ratios, a fluctuation in concentration does not affect the volume of titrant required.

Titration is based on mol:mol ratios and is determined by a standardized titrant of known concentration. The indicator in the sample solution will only change colors when all of the initial moles of acid or base in the sample are neutralized by the titrant. Knowing when washing is necessary is crucial in being speedy in the lab. Addition of DI water will not affect the starting moles of acid or base.

3. When performing absorption spectra analysis in the UV/Vis range. Why are high absorbance values (>1) considered to be poor representatives of the true absorbance of the sample?

Answer: Highly absorbing samples allow the error due to stray light to become more and more significant

Actually the optimum range is about 0.3-0.6 absorbance units. Imagine placing a block in the sample holder and taking a reading of the "dark current." As your sample gets more and more concentrated, the signal recognized at the detector approaches the value of the room or stray light.

4. If I want to prepare 100mL of a mixture using only a 10mL pipet, what would the error of the 100mL mixture be if the error in the pipet is 0.2 mL?

Answer: 0.2*SQRT(10)

If each 10mL aliquot is made with the same 10mL pipet, then the error in each delivery is thought of as independent from one another and equal to the error in the pipet. the general formula for the error in any function resulting in the simple addition of the individual components:

error = SQRT(nx^2) or SQRT(n)x, where n is the number of trials. in our case n=10 and x=0.2.

5. What does a PMT (photo multiplier tube) do?

Answer: Increases the intensity of a light signal, usually in the UV/Vis region

PMTs are highly sensitive detectors, theoretically they can amplify 1 photon into 10^6 photons. PMTs can greatly increase the signal:noise ratio.

6. Now for a little biochemistry. What chemical is commonly added to agar (which is intended to grow E. coil) that is crucial in blue/white screening?

Answer: X-Gal

Blue/white screening is a useful technique used in cloning. Plasmids are engineered such that they contain many restriction sites within a lacZ gene. The lacZ gene is responsible for production of Beta-Galactosidease. So bacteria that have his enzyme will break down X-Gal into a blue colored product, thus signifying a failed attempt at cloning. On the other hand, "white" colonies cannot break down X-Gal but still can live (due to antibiotic resistance also within the engineered plasmid).

7. You want to separate a mixture of compounds using gel permeation chromatography. What compound, do you predict, will be the first to elute?

Answer: Substance P (MW=3900 Da)

Gel permeation separates compounds based on size. Molecular weight, MW, can be used as a good approximation for relative size. A gel permeation column,GPC, consists of many micro beads that contain pores. Small particles can traverse these crevices, but large particles just flow along the outside, which is a more direct route to the bottom.

8. Analysis- If a pathway of a particular substrate to product that is crucial for life is studies by creating mutant knockouts for the 3 genes responsible for the enzymes in this pathway. The pathway consists of a starting material, two intermediates, and a final product. (call them A B C and D). For clarity, Each plate is only supplemented with one factor, meaning if it grows on A and B plates, then it grows on a plate supplemented with A alone and a plate supplemented with B alone). Strain 1 grows on plates containing A, but not on plates containing B C or D) Strain 2 grows on plates containing C and A but not on plates containing B or D Strain 3 grows on all 4 plates Strain 4 grows on A D and C but not on B plates What is the order of starting material intermediates and final product?

Answer: BDCA

Strain 1 can only live when A is supplemented meaning that the enzyme responsible for A's formation is blocked. Strain 2 is similar, growth on both C and A is possible because the C is able to be converted to A which is essential for survival. The rest just falls into place from here.

9. In column chromatography, specifically in TLC (thin-layer chromatography), what is the retention factor?

Answer: The distance from the starting line to the sample mark divided by the distance from the starting line to the solvent head

Rf is defined as the fraction of the plate where the sample can be found- i.e. an Rf of 0.7 means that the sample is found at 70% of the total migration distance done by the solvent (which experiences minimal interactions with the stationary phase.)

10. If you notice that your data isn't fitting the theoretical model it is supposed to fit, it is best to... (assume you're a student and the lab report is due tomorrow)

Answer: Remember back to the procedure and try to correlate possible sources of error

The best thing to do at this point is to assess where these sources of error came from. Evaluate partial derivatives with respect to each experimental value and its error and determine where the greatest contributor of error comes from. Although it would probably be a good idea to reacquire more data points, this sometimes changes the reliability that the new data points will fit right in with the old ones. for example making up a new stock solution would affect the accuracy of the previously determined values based off this stock with the values determined from the original stock. Making up better data points is always a bad idea, and on the professional level is illegal. Finding a bettor theory... not likely.

Source: Author dgbayer

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