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CHEMISTRY 59-320 ANALYTICAL CHEMISTRY Fall - 2012

CHEMISTRY 59-320 ANALYTICAL CHEMISTRY Fall - 2012. Lecture 2. Scales of Measurements. The possible scale of measurements in analytical chemistry is astounding, ranging from the atomic level to the size of galaxies!.

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CHEMISTRY 59-320 ANALYTICAL CHEMISTRY Fall - 2012

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  1. CHEMISTRY 59-320ANALYTICAL CHEMISTRYFall - 2012 Lecture 2

  2. Scales of Measurements The possible scale of measurements in analytical chemistry is astounding, ranging from the atomic level to the size of galaxies! • (left) Carbon-fiber electrode with a 100-nanometer-diameter (100 × 10−9 meter) tip extending from glass capillary. The marker bar is 200 micrometers (200 × 10−6 meter). [From W.-H. Huang, D.-W. Pang, H. Tong, Z.-L. Wang, and J.-K. Cheng, Anal. Chem. 2001, 73, 1048.] (middle) Electrode positioned adjacent to a cell detects release of the neurotransmitter, dopamine, from the cell. A nearby, larger counterelectrode is not shown. (right) Bursts of electric current detected when dopamine is released. Insets are enlargements. [From W.-Z. Wu, W.-H. Huang, W. Wang, Z.-L. Wang, J.-K. Cheng, T. Xu, R.-Y. Zhang, Y. Chen, and J. Liu, J. Am. Chem. Soc. 2005, 127, 8914.]

  3. Chapter 1: Measurements 1-1 SI Units

  4. Using prefixes as multipliers We customarily use prefixes for every third power of ten, e.g. 10-9. 10-6, 10-3 …

  5. Converting between units Example: Express the energy 20 Calories in terms of ? kilojoules (kJ)

  6. Why should we be concerned of units?

  7. 1-2 Chemical concentrations A few concepts • Solution: homogeneous mixture of two or more substances. • Solute: a minor species in a solution. • Solvent: the major species in a solution. • Concentration: how much solute is contained in a given volume or mass of solution or solvent. • Strong & weak electrolytes.

  8. 1-2 Chemical concentrations(Continued) Molarity and Molality: Molarity (M) is the number of moles of a substance per liter of solution. Molality (m) is concentration expressed as moles of a substance per kilogram of solvent. Problem 1-14. What is the formal concentration (expressed as mol/L = M) of NaCl when 32.0 g are dissolved in water and diluted to 0.500 L? Answer: The molecular mass of NaCl is 58.44 g/mol The moles of sale in 32.0 g are 32.0 g /58.44(g/mol) = ……

  9. 1.3 Preparing solutions • Preparing a solution with a desired molarity • Dilution Problem 1-32: A bottle of concentrated aqueous sulfuric acid labeled 98.0wt % H2SO4, has a concentration of 18.0 M. How Many millilitres of reagent should be diluted to 1.000 L to give 1.00 M H2SO4? Solution:

  10. 1.4 Stoichiometry Calculations for gravimetric Analysis One must make sure that the reaction equation is balanced!!

  11. Gravimetric calculation (2) • Problem 1-34: How many millilitres of 3.00 M H2SO4 are required to react with 4.35 g of solid containing 23.2% Ba(NO3)2 if the reaction is Ba2+ + SO42-→BaSO4(s)? • Answer: (in class demon.)

  12. 1-5 Introduction to Titrations • A titration is a form of volumetric analysis in which increments of reagent solution—the titrant—are added to analyte until their reaction is complete. • The equivalence point occurs when the quantity of added titrant is the exact amount necessary for stoichiometric reaction with the analyte. • The equivalence point is the ideal (theoretical) result we seek in a titration. What we actually measure is the end point. • The difference between the end point and the equivalence point is an inescapable titration error.

  13. More terminologies associated to titration • Primary standard • Standardization • Standard solution • Direct titration • Back titration • Gravimetric titration: • Volumetric titration:

  14. 1-6 Titration calculations Exercise 1-E. A solution of NaOH was standardized by titration of a known quantity of the primary standard, potassium hydrogen phthalate (page 223): The NaOH was then used to find the concentration of an unknown solution of H2SO4: (a) Titration of 0.824 g of potassium hydrogen phthalate required 38.314 g of NaOH solution to reach the end point detected by phenolphthalein indicator. Find the concentration of NaOH (mol NaOH/kg solution). (b) A 10.00-mL aliquot of H2SO4 solution required 57.911 g of NaOH solution to reach the phenolphthalein end point. Find the molarity of H2SO4.

  15. Chapter 2: Tools of the Trade • 2.1 Safe, ethical handling of chemicals and waste -- Chemical experimentation creates hazards! -- The primary safety rule is to familiarize yourself with the hazards and then to do NOTHING that you consider to be dangerous. -- Minimize waste production and RESPONSIBLY dispose of waste. -- Recycling chemicals. -- Clean spills immediately. -- Label all vessels to indicate what they contain.

  16. What safety information can be obtained from the following label?

  17. 2.2 The lab notebook • State what you did and what you observed in a way that it can be understood by a stranger. • Record the names of computer files where programs and data are stored. • Copy important data into your notebook. (Computers may crash and become garbage at any time).

  18. 2.3 Analytical balance • Tare: the mass of an empty vessel that is used to receive the substance to be weighted . • Sensitivity: the smallest increment of mass that can be measured. • Chemicals should NEVER be placed directly on the weighting pan • Handle the vessel you are weighting with a paper towel so that the tare value does not change.

  19. Principle of operation (1)

  20. Principle of operation (2)

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