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Decoding of Chemistry Olympiad

Decoding of Chemistry OlympiadDecoding of Chemistry OlympiadDecoding of Chemistry Olympiad

Decoding of Chemistry Olympiad

Decoding of Chemistry OlympiadDecoding of Chemistry OlympiadDecoding of Chemistry Olympiad
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    • About
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      • FAQs
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      • Annotated Solutions
      • Misconception
      • Laboratory
      • Study Camp
    • Classes
      • Introduction
      • Registration
      • Placement
      • Testimony
      • National Preps
      • Open Classes
      • Lecture Notes
    • Resources
      • Textbooks
      • IChO
      • CChO
      • Mendeleev
      • VChO
      • WUCT&BCT
      • Other Sites
    • Consultation
      • Newsletters
      • Email
      • Discord
      • WeChat 微信
  • Home
  • About
  • USNCO
    • General Overview
    • FAQs
    • Learning Objectives
    • DCC
    • Annotated Solutions
    • Misconception
    • Laboratory
    • Study Camp
  • Classes
    • Introduction
    • Registration
    • Placement
    • Testimony
    • National Preps
    • Open Classes
    • Lecture Notes
  • Resources
    • Textbooks
    • IChO
    • CChO
    • Mendeleev
    • VChO
    • WUCT&BCT
    • Other Sites
  • Consultation
    • Newsletters
    • Email
    • Discord
    • WeChat 微信

Laboratory in National Exam

USNCO Webinar

Laboratory Guidance

Past PIII Summary

Preparing for the National Exam Part III for Students, by ACS

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Past PIII Summary

Laboratory Guidance

Past PIII Summary

A detailed summary for all of National Part III (Lab Practical) from 1999 to 2017 by DavidQ.

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Laboratory Guidance

Laboratory Guidance

Laboratory Guidance

Written by Allen Ding, one of the IChO alternates for the USA 2019 IChO team.

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Doing chemistry requires both understanding ideas and remembering key information!


From Chemistry by Zumdahls

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

Gas Law

Measurement of Ideal Gas Constant Using a Barometer

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Measurement of the Molar Mass of a Volatile Liquid

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Organic Laboratory Techniques

Acid-Base Extraction

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Recrystallization of Benzoic Acid

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Organic Reactions

Synthesis of Aspirin

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Silver Mirror Test

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Other Analytical Methods

Redox Titrations

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Gravimetric Analysis

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Precipitation Titrations

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Cu% in Brass Using Iodometry

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Complexation Titrations

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