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Acids and Bases

Investigation 2 – Concept Day

 

 

 

 

 

 

 

Acids and Bases: Investigation 2

Concept Day

SLIDE ACIDS-2-1

  • In this Investigation, we wish to focus on the concentration of hydrogen ions in solutions of varying pHs.

Note: Since you are probably familiar with concentration as the mass of a compound in a given volume, we will begin with a quick review of this topic. You will also be introduced to the logarithmic nature of pH concentrations. You probably have had very little previous exposure to logarithms at this point. Finally, in this Investigation, we will also introduce you to the concept and methods of serial dilutions.

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SLIDE ACIDS-2-2

  • This slide reviews “traditional” concentrations.
  • Concentration = Mass/Volume is reviewed.
Note: You are likely familiar with this concept of concentration and have prepared many solutions in LabLearner using this strategy. This is the correct way to think of concentration under most circumstances in chemistry.  We can now move into a discussion of logarithmic concentrations and pH.

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SLIDE ACIDS-2-4

  • The logarithmic nature of the pH scale is discussed on this slide.
  • Unfortunately, the pH scale may look a lot like a simple number line.
  • It may be initially thought that going from pH 1 to pH 2 would result in a solution half as acidic, as would going from pH 2 to pH 4. However, since the pH scale is logarithmic, going from pH 1 to pH 2 actually results in a solution that is ten-times less acidic. Going from pH 2 to pH 4 would be 100-times less acidic.
Note: You and your lab group or teacher may wish to go over several examples like those described above to further familiarize yourself with the concept of logarithms and pH values. Additional examples are presented beneath the graphic in this slide. 

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SLIDE ACIDS-2-5

  • This slide introduces the concept of serial dilution. A serial dilution is a stepwise dilution of a substance in solution and allows for the accurate preparation of extremely dilute solutions.
  • Serial dilutions are also used to determine the number of bacterial cells in a sample by serially diluting the sample until a reasonable number of cells can be counted from a specific volume of a diluted sample.
  • This “counted” number (of bacteria in the case above) can then be multiplied by the “dilution factor” to arrive at a remarkably accurate approximation of the number of bacterial in the original, undiluted sample.

Note: The most common point of confusion regarding serial dilution is that the addition of 1 part (ml in this case) of a sample to 9 parts (mls in this case) solvent, results in a ten-fold dilution (or 1/10 concentration), NOT a 1/9 dilution or concentration.

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SLIDE ACIDS-2-6

  • This slide gives an illustration of another system in which the value is increased from one step to the next by a factor of ten.
  • Notice that there are no five dollar or twenty dollar bills in this series. They would have disrupted the progressive, geometric increase in value between the copper penny and the 1,000 dollar bill.

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SLIDE ACIDS-2-7

  • Remember to use gloves, goggles, and lab coats when working in the lab with strong acids and bases.