Laboratory

How to Use Common Laboratory Glassware

How To Use a Volumetric Flask?

Brief Summary

Volumetric flask is a precise glassware with error of ±0.10-0.50
Volumetric flask is a "to contain (TC)" glassware, the contained quantity of the liquid corresponds to the capacity printed on the flask
Volumetric flask can be used to dilute solution as well with the usage of volumetric or graduated pipette.
Correction in the video: solids especially with obvious dissolution heat are NOT recommended to directly dissolve in the flask
Practice problems in USNCO: N2018-Q12, L2018-Q11

The Volumetric Pipet and Pipetting Technique

Brief Summary

Volumetric pipettes and graduated pipette are both precise glassware with a reading to 0.01 mL, which are more precise than graduated cylinder (to 0.1 mL)
A graduated pipette is used to transfer different measured amounts of liquid material, while a volumetric pipette is used to transfer a specific measured amount of liquid material
the small amount solution remained in the pipette after the draining need NOT to blow out, and mouth is prohibited to use to suck solutions
Correction in the video: gloved finger is recommended to operate the volumetric/graduated pipette with a bulb
Practice problems in USNCO: L2019-Q12, L2018-Q11, L2017-Q12

Analysis of Food Dyes in Beverages

Beer's Law (simulation from PhET)

Brief Summary

The color of a solution shown is the complement to the light it absorbs, refer to the color wheel
Absorbance quantifies how much light is absorbed at a certain wavelength, is the common logarithm of the ratio of incident (I_0) to transmitted (It) light intensity through a material (I_0/I_t)
In a certain concentration range, absorbance is proportional to the length of light path (b = 1 cm for standard cuvette) and molarity (c) of the colored species, Abs = abc, where a is a constant called molar absorptivity (Beer's Law)
Another great interactive platform about the Beer's Law can be found here (shiny-app written by R)
Practice problems from USNCO: N2020-Q8, N2015-Q6, N2009-Q8, N2000-Q6

Determination of a Dye Concentration in a Sports Drink

Briefly Summary

the idea of using Beer's Law to quantify or semi-quantify color species in solutions is called colorimetry
theoretically, absorbance at any wavelength should follow the Beer's Law, but the wavelength with a maximum absorbance (λ_max) is commonly used for measurement
A calibration curve (λ_max vs molarity) is needed for any spectrophotometric determination to get the molarity of target sample by plotting its absorption
when holding cuvette, fingerprints should NOT be put on the more transparent side (light path), otherwise, the absorption measured is higher than actual value
Another good video to refer to is here
Practice problems from USNCO: L2019-Q10, N2017-Q9, L2015-Q29, N2010-Q5, N2009-Q4, L2009-Q5, N1999-Q4

Measurement of Cu% in Brass Using Spectrometry

Dissolution of Cu in nitric acid

Brief Summary

Cu-containing alloys such as penny (Zn/Cu) and brass (Zu/Cu/Fe) reacts with concentrated nitric acid to produce the metal cations (Zn->Zn^2+, Cu->Cu^2+, Fe->^Fe3+) and a reddish brown gas NO2
though Cu is a less reactive post-H metal, it can be oxidized by the strong oxidizing agent, nitric acid, please be aware there is no H2(g) produced
if the nitric acid gets diluted, there will be more colorless NO(g) produced when reacting with Cu
Cu2+(aq) is supposed to be blue, but here is a mix color of blue and yellow (dissolved NO2), which is green
Zn2+(aq) is colorless due to fulfilled d sub-shell (no d-d transition), but Fe3+(aq) has color (yellow), which might affect the determination of Cu2+(aq) using spectrometry.
Practice problems from USNCO: L2011-Q3, N2018-Q10

Spectrophotometric determination of Cu% in brass

Brief Summary

The determination is based on Beer's Law with a calibration curve
The wavelength (λ) should be carefully chosen to ensure the sensitive and minimize the interference of other ions with color such as Fe^3+(aq)
The stock solution made by brass reacting with conc. nitric acid may need to be diluted to fit in the molarity range of Cu^2+(aq) for calibration curve
Another video to refer to is here
Alloy is different from the component metal in terms of hardness (typically harder) and melting point (typically lower)
Extension: another method to quantify the copper in brass is redox titration, iodometry (N2018-Q10, N2017-P2-Q2)
Practice problems from USNCO: N2018-Q10, N2005-Q5

Determination of Rate Law

Kinetics of crystal violet fading

Brief Summary

another video with great content but a bit noisy is here
CV+(purple) + OH- = CVOH(colorless), rate = k[CV+]^a[OH-]^b
pseudo kinetics used here since [OH-] >> [CV+], then rate = k[CV+]^a[OH-]^b = k'[CV+]^a, k' = k[OH-]^b since [OH-] does not change with time
plot [CV+], ln[CV+], 1/[CV+] vs t to see which one is linear to figure out the order to [CV+], [CV+] is proportional to Abs(λ)
then double [OH-], compare the slope of the linear curve above with the previous case with a lower [OH-], slope_2/slope_1 = k'_2/k'_1 = ([OH-]_2/[OH]_1)^b
Practice problems from USNCO: *N2020-Q29, L2019-Q29, N2019-Q30, L2018-Q27, N2018-Q28, N2016-Q27

Reaction rate of thiosulfate and acid

Brief Summary

the lab handout used for this lab could be found here
S2O3^2- + 2H^+ = S(ppt.) + SO2(aq) + H2O, a disproportionation with white or pale yellow precipitate
the time t for the precipitate to block the "X" underneath the beaker is monitored with the molarity of thiosulfate when [H^+] is in much excess
1/t (rate) is plot with [S2O3^2-] to get a linear curve, which means the reaction is first order with respect to S2O3^2-
another similar and interesting laboratory is the determination of the rate law and the activation energy for iodination of acetone (to be updated)
Practice problems from USNCO: N2016-PIII-L2

Acid-Base Titrations and Titration Curve

Setting up and Performing a Titration

Brief Summary

titration is used to measure the accurate molarity of an acid or a base based on stoichiometry: c1V1 = c2V2
typically, the ~20.00 mL analyte is transferred using a volumetric pipette to a Erlenmeyer flask, the titrant (with known molarity) is added through a buret dropwise
titrant may need to be standardized prior to use, such as HCl(aq), NaOH(aq), etc., which might change molarity after standing up
dionized water is used to rinse the buret tip and the internal wall of the flask, which wont' affect the titration
phenolphthalein, methyl red, methyl orange are common used indicator to show the end point with a sharp color change in 1-2 drops
both volumetric pipette and buret need to be rinsed using the corresponding solution prior to use

Different Titration Curves

Brief Summary

a few concepts to know: equivalence point vs end point, half-equivalence point, pH window, transition pH range of indicator, etc.
pH can change 3-6 units (pH window) with 1-2 drops of titrant is added when approaching to the equivalence point
end point is determined by the indicator, so choosing an appropriate indicator is critical for titrations, transition range of indicator needs to fit in the pH window in the titration curve
when strong base titrating weak acid, pH at half equivalence point equal to the pKa of the weak acid, and the titration curve after the equivalence point overlaps with that of strong base titrating strong acid
when strong base titrating weak acid, the pH changes slowly at half-equivalence point, which is called buffer region
not all equivalence points have significant pH windows for polyprotic acids, such as the 3rd eq point of H3PO4
Problem problems from USNCO: a bundle, N2016-PIII-L2

Laboratory in National Exam

USNCO Webinar

Laboratory Guidance

Past PIII Summary

Past PIII Summary

Laboratory Guidance

Past PIII Summary

Laboratory Guidance

Laboratory Guidance

Laboratory Guidance

Doing chemistry requires both understanding ideas and remembering key information!

How to Use Common Laboratory Glassware

How To Use a Volumetric Flask?

The Volumetric Pipet and Pipetting Technique

Analysis of Food Dyes in Beverages

Beer's Law (simulation from PhET)

Determination of a Dye Concentration in a Sports Drink

Measurement of Cu% in Brass Using Spectrometry

Dissolution of Cu in nitric acid

Spectrophotometric determination of Cu% in brass

Determination of Rate Law

Kinetics of crystal violet fading

Reaction rate of thiosulfate and acid

Acid-Base Titrations and Titration Curve

Setting up and Performing a Titration

Different Titration Curves

Gas Law

Measurement of Ideal Gas Constant Using a Barometer

Measurement of the Molar Mass of a Volatile Liquid

Organic Laboratory Techniques

Acid-Base Extraction

Recrystallization of Benzoic Acid

Organic Reactions

Synthesis of Aspirin

Silver Mirror Test

Other Analytical Methods

Redox Titrations

Gravimetric Analysis

Precipitation Titrations

Cu% in Brass Using Iodometry

Complexation Titrations

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