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Titration Term Review

Titration Term Review. Titrant in buret = solution of known concentration Equivalence point = stoichiometric point = moles of acid equal moles of base Endpoint = color changes due to pH and depends on indicator used

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Titration Term Review

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  1. Titration Term Review • Titrant in buret = solution of known concentration • Equivalence point = stoichiometric point = moles of acid equal moles of base • Endpoint = color changes due to pH and depends on indicator used • pH curve = titration curve is a plot of the pH of the solution being analyzed as a function of the amount of titrant added

  2. Titrations and pH Curves • Strong Acid - Strong Base • Weak Acid with Strong Base • Weak Base with Strong Acid • Mole is too large of a unit when working with mililiters, so generally a millimole (mmol) is used…1000 mmol = 1 mol and mmol/mL = M

  3. Strong Acid - Strong Base Titration • pH changes gradually until the titration is close to the equilvalence point where a dramatic change occurs • pH = 7.00 at equivalence point • Curve points right/left based on beginning solution • Polyprotic acids have multiple curves

  4. Weak Acid with Strong Base Titration • Essentially a set of buffer problems • **Even though it is a weak acid, it reacts essentially to completion with the strong base’s hydroxide ion • Stoichiometry Problem: OH- reacts with weak acid. [ ] of acid remaining and conjugate base formed are determined • Equilibrium Problem: Position of the weak acid equilibrium is determined, and pH can be calculated

  5. Example • Hydrogen cyanide gas (HCN), a powerful respiratory inhibior, is highly toxic. It is a very weak acid (Ka = 6.2 X 10-10) when dissolved in water. If a 50.0 mL sample of 0.100 M HCN is titrated with 0.100 M NaOH, calculate the pH of the solution • After 8.00 mL of 0.100 M NaOH has been added. • At the halfway point of the titration. • At the equivalence point of the titration.

  6. Important Notes • pH at the equivalence point of a titration of a weak acid with a strong base is always greater than 7.00 (basic) • pH is determined by the amount of excess OH- present • Curve looks different before and the same after the equivalence point • The AMOUNT, not strength of acid determines the equivalence point. The STRENGTH affects the pH at the equivalence point, however. This pH affects the titration curve.

  7. Titrations vs. Ka • Titration curves can be used to determine equilibrium constant values • EX: 2.00 mmol of a monoprotic weak acid is dissolved in 100.0 mL of water and is titrated with 0.0500 M NaOH. After 20.0 mL NaOH has been added, the pH is 6.00. What is the Ka value for the acid? • Answer: 1.0 X 10-6

  8. Weak Base - Strong Acid Titration • **Always think about the major species in solution, then use stoichiometry, then choose the dominant equilibrium and find pH • The pH at the equivalence point will be less than 7.00 (acid)

  9. Example • Calculate the pH at each of the following points in the titration of 50.00 mL of a 0.01000 M sodium phenolate (NaOC6H5) solution with 1.000 M HCl solution (Ka for HOC6H5 = 1.05 X 10-10)… • Initially -Midpoint - Equivalence Point • Answers: 10.99 9.99 5.99

  10. Determining the Equivalence Point • pH meter can be used and a plot of the titration curve can be made. • Acid-base indicator can be used to see the endpoint (NOT SAME AS EQUIVALENCE POINT, however various indicators can be used so this error won’t be a big deal).

  11. Acid-Base Indicators • Indicators are represented by HIn. As the H+ ions react with OH- ions from the basic titrant (are removed from the HIn), In- ions remain. These In- ions cause the color to change based on the indicator present. • Indicators can be chosen where the endpoint and equivalence point are very close. • Determine the pH at the equivalence point • Use pH range chart on page 732

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