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Understanding pH Levels: The Complete Guide

Learn about the pH scale, indicators, pH determination methods, and the relationship between pH and pOH. Discover how to calculate pH and understand the significance of pH meters in chemistry.

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Understanding pH Levels: The Complete Guide

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  1. What is pH?

  2. Because you cannot use the concentration to know the strength of the acid, and because you cannot use the strength to know the concentration of the acid, chemists have determined a way to get one all-inclusive number to use. This number is called pH. (pH stands for the “potential of Hydrogen”)

  3. The pH scale goes from 0 to 14. 7 is considered neutral. Anything from 0-7 on the pH scale is an acid and anything from 7-14 is a base. In addition, anything from 0-4 is a strong acid and anything from 4-7 is a weak acid. Anything from 7-10 is a weak base and anything from 10-14 is a strong base.

  4. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Weak Base Weak Acid Strong Acid Strong Base

  5. To determine the pH of a substance requires an indicator. Indicators are substances that turn colors at certain pH’s. Most indicators can only turn one color. For example, phenolphthalein is colorless in acids and pink-purple with bases.

  6. To determine the pH of a substance requires an indicator. Indicators are substances that turn colors at certain pH’s. Most indicators can only turn one color. For example, phenolphthalein is colorless in acids and pink-purple with bases. In order to quickly determine a pH, pH paper has many different indicators soaked into it. This allows pH paper to turn 5 or more colors so 5 or more pHs can be determined.

  7. The best way to determine the pH is with a pH meter or calculation. pH = -log [H+], which means the negative logarithm of the concentration of the H+ ions. If the [H+] is known (in molarity), then a calculator can provide the pH number, but if the [H+] is not known, determining it without the use of technology is a long and grueling process.

  8. pH meters have been created to do these calculations quickly and show the exact pH results almost immediately. Extreme care must be taken with pH meters as they are fragile and will not give correct results if not properly cared for.

  9. Often it is useful with bases to calculate a pOH instead of a pH. For this pOH = - log [OH-]. Also a pOH scale could be created, and is literally the reverse of the pH scale. On the pOH scale, low numbers are strong bases and high numbers are strong acids. The pH and pOH of a particular solution are inversely related, so for any solution, the pH + pOH = 14.

  10. When water ionizes, it forms equal amounts of H+ and OH-. It has been measured that [H+][OH-] = 1 × 10-14 M2. In acidic solutions [H+] > [OH-] and in basic solutions [H+] < [OH-], but for all solutions it still works that: [H+][OH-] = 1 × 10-14M2

  11. pH = -log [H+ ] [H+ ] pH [H+ ] = 10-pH pH + pOH = 14 [H+][OH-] = 1 × 10-14 M2 pOH = -log [OH- ] [OH- ] pOH [OH- ] = 10-pOH

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