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Mr. Chapman Chemistry 30

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We are ready to use what we know about acids and bases to calculate the pH of various solutions. Before we do this, however, we need to make sure that we are able to use our calculators properly when it comes to logarithms. In math you learned about logarithms, or logs for short. We need to be able to use them for pH calculations.

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Find the log of each of the following numbers on your calculator. Make sure you get the answers that are pictured here:

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pH is another method of expressing the concentration of H + ions in solution. pH is defined as the negative log of hydrogen ion concentration, and mathematically it looks like this: pH = -log[H + ]

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1. [H + ] = 1 x 10 -3 2. [H + ] = 2.5 x 10 -11 3. [H + ] = 4.7 x 10 -9 4. [H + ] = 5.8 x 10 -4 5. [H + ] = 1 x 10 -7 1. 3.0 2. 10.6 3. 8.3 4. 3.2 5. 7.0

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You’ll notice from the last example that the final question gave you the hydrogen ion concentration of water. This seemed like a good time to remind you that water has a neutral pH of 7, and that the reason for that is the fact that both the [H + ] and [OH - ] concentrations in water are equal at 1.0 x 10 -7.

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We can determine by looking at the concentration of [H + ] whether a substance is an acid or base. Any substance with an [H + ] lower than at 1.0 x 10 -7 is a base. Higher than that, and you have an acid. This is obviously directly related to pH. pH 7 means you have an acid.

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1. Calculate the pH of a 0.01M HNO 3 solution. 2. Find the pH of a 0.01 M solution of ammonia. Ammonia is a weak base with K b = 1.8 × 10 -5. After you complete #2, it’s important that you know life doesn’t have to be so hard, because we have...

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Can you guess what pOH refers to? Remember that pH refers to the concentration of hydrogen ions in solution... pOH is defined as the negative log of the hydroxide ion concentration: pOH = -log[OH - ]

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pOH can make the previous question a lot easier. Instead of using K w to find [H + ], we can use a different method. We can make use of the following easy-to- memorize relationship: pH + pOH = 14 Does the number 14 ring a bell? Remember K w = 1.0 × 10 -14. The negative log of 1.0 × 10 -14 = 14

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Once you find pOH, it is easy to find pH: pH + pOH = 14 Similarly, it is also easy to find pOH if you are given pH. With one, you can find the other!

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To convert pH into [H + ] involves taking the antilog of the negative value of pH [H + ] = antilog (-pH) Example: We have a solution with pH 8.3. What is the concentration of hydrogen ions? You should get the answer 5.0 x 10 -9 M

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Think very hard about what these questions are asking you before you try to answer them... 1. Find the hydronium ion concentration in a solution with a pH of 12.6. Is this solution an acid or a base? How do you know? 2. A 0.24M solution of the weak acid, H 2 CO 3, has a pH of 3.49. Determine K a for H 2 CO 3 (carbonic acid). Hint: start with a balanced equation.

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Here’s another example calculation question involving these quantities [H 3 O + ], [OH – ], pH, and pOH Calculations Example 2.

Here’s another example calculation question involving these quantities [H 3 O + ], [OH – ], pH, and pOH Calculations Example 2.

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