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The pH Scale

 

Overview:

 

pH is a measure of the acidity of an aqueous solution. Another way of looking at pH is to think of it as a measure of the concentration of hydrogen, or H+, ions in a solution; the higher the H+ concentration, the more acidic a solution is.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

pH scale:

 

The pH scale generally goes from 0 to 14, but there are cases for which the pH of a solution is negative or higher than 14.

 

  • A solution with a pH of 7 is neutral

  • Ex: pure water

 

  • A solution with a pH < 7 is acidic

  • The lower the pH of a solution is, the more acidic the solution

  • Ex: lemon juice

 

  • A solution with a pH > 7 is basic or alkaline

  • The higher the pH of a solution is, the more alkaline the solution

  • Ex: bleach

                 

*For more information on the pH scale, check out this website:

http://www.epa.gov/acidrain/education/site_students/phscale.html

 

 

 

Calculating pH:

 

To calculate pH, find the H+ concentration of the solution and take the negative log of the value. Essentially, plug H+ into the equation pH = -log[H+] (note that the notation [H+] stands for the concentration of H+ ions).

 

If you are given the OH- concentration instead, you have two options. The [H+] multiplied by the [OH-] of a solution will always be equal to 10^-14, so you can find [H+] by dividing 10^-14 by [OH-].

 

Alternatively, you could use the pOH to calculate the pH of a solution. To find the pOH of a solution, you use a similar equation to the one used to find pH but using [OH-] instead

pOH = -log[OH-]. For any solution, pH + pOH = 14, so after you find pOH, you can find pH by simply subtracting the pOH from 14.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Calculating [H+] and [OH-]:

 

On the flipside, to calculate [H+] from pH, you simply take 10^-pH to find the concentration of H+. To calculate [OH-], you can either take 10^-pOH or 10^-(14-pH), depending on the information you are given.

 

*For more information, check out this website: http://www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Calculating_pHandpOH.htm

 

 

The pH scale is logarithmic, meaning that each whole number value on the scale represents a shift by a power of 10. For example, a solution with a pH of 3 is 10 times more acidic than a solution with a pH of 4; a solution with a pH of 8 is 100 times more alkaline than a solution with a pH of 6. This also corresponds to the concentration of H+ and OH- ions in the solution. A solution with a pH of 3 has 10 times the concentration of H+ ions as one with a pH of 4.

 

For an example of what a logrithmic scale actually looks like, go to:

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_AcidsBasespHScale.shtml

 

 

 

 

Buffers:

 

A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.

 

The role of a buffer is to resist changes in pH. So for example, a neutral solution would usually become acidic if you added an acidic solution to it, but if you have the proper buffer in the solution, then adding the acidic solution will not change the pH of the neutral solution by a significant amount.

 

Buffers accomplish this by regulating H+ ions. When pH decreases, the buffer accepts H+ so that the H+ ions do not affect the solution, and when pH increases, the buffer donates H+ ions.

 

An example of a buffer is the blood circulating throughout your body.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

http://www.wiley.com/college/pratt/0471393878/student/review/acid_base/6_hh_equation.html

 

*For more information on buffers, try this website:

http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

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