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pH Indicators

pH Indicators. The One Only Adrian Arias. What is pH Indicator. A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually. How does a pH Indicator work (Litmus).

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pH Indicators

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  1. pH Indicators The One Only Adrian Arias

  2. What is pH Indicator • A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually.

  3. How does a pH Indicator work (Litmus) • Litmus is a weak acid. It has a seriously complicated molecule which we will simplify to HLit. The "H" is the proton which can be given away to something else. The "Lit" is the rest of the weak acid molecule. • There will be an equilibrium established when this acid dissolves in water. Taking the simplified version of this equilibrium: • Hlit(aq) <> H+[aq] + Lit-[aq]

  4. LE CHATELIER'S PRINCIPLE • Only works if one molecule is more Ex A + 2B <> C + D • If a change in concentration equilibrium moves to the right if you increase concentration of A. Moves left if decrease concentration of A • If using this Le Chatelier’s principle for changing this can only be applied if the equation is dealing with gases Ex A(g) + 2B(g)<> C(g) + D(g). If equilibrium moves to the right if you increase A’s pressure. Moves left if you decreases A’s pressure. • Dealing with changes for temperature if is increased it moves equilibrium to the left. If decrease temperature it moves equilibrium to the right.

  5. Bring back thosefond memories of Ka & Kb • pKa = - log (Ka) • pKb =  - log (Kb) • Kb = 10-pKb or Kb = antilog ( - pKb)

  6. Revenge of the pH Buffers OH MY!! • As explain by the buffer group a buffer keeps a pH around a constant point. By keeping at a near constant pH to expand the window for a titration lab.

  7. Sample problem #1 • 0.00026 moles of acetic acid are added to 2.5 L of water. What is the pH of the solution? • Answer: M=n/L : Macetic acid= 0.00026/2.5 =1.04E-4 mol/L • pH= -log[1.04E-4]= 3.98

  8. Sample Problem #2 • Let's say the concentration of Hydronium ions in an aqueous solution is 0.033 mol/L. What is the corresponding pH of this solution, and based on your answer identify whether the solution is acidic, basic or neutral. • Answer: Using the formula • pH= -log[0.033]= 1.48

  9. Sample Problem# 3 • Calculate the value of the ionization constant, Kb, for aniline if the pKb is 9.38. • 9.38 = - log (Kb) • -9.38 = log (Kb) • Kb = 10-9.38 = 4.17 x 10-10

  10. AP Question • Based upon the following values of Ka, place the acids in order from weakest to strongest: • HCN (Ka = 6.2 x 10-10) • HCOOH (Ka = 1.78 x 10-4) • HC2H3O2 (Ka = 1.8 x 10-5) • HNO2 (Ka = 4.6 x 10-4) • A HCN < HC2H3O2 < HCOOH < HNO2 * • B HC2H3O2 < HCN → HNO2 < HCOOH • C HCN < HNO2 < HC2H3O2 < HCOOH • D HNO2 < HCOOH < HC2H3O2 < HCN • E HCOOH < HNO2 < HCN < HC2H3O2

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