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Unit 12-2

Unit 12-2. Buffers. BUFFERS. A strong acid or base cannot be buffered! They can be neutralized only. A buffer is prepared by adding a salt of the conjugate ion to the weak acid (base). This results in an equilibrium shift. Buffers.

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Unit 12-2

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  1. Unit 12-2 Buffers

  2. BUFFERS • A strong acid or base cannot be buffered! They can be neutralized only. • A buffer is prepared by adding a salt of the conjugate ion to the weak acid (base). • This results in an equilibrium shift.

  3. Buffers • Highly resistant to changes in pH brought by addition of strong acids (bases) • The pH of the Buffer has to be close to the pKa of the weak acid • Basically it cushions the “shock” that occurs when a strong acid or base is added

  4. Examples of Buffers • Aspirin • Blood plasma • Make-up • Lotions • Perfume • Foods • Shampoos • Soaps • Alcohol production

  5. Buffers • By adding the salt of the conjugate base (acid) to a solution of a weak acid (base), LeChatelier’s principle shows us the shift in the equilibrium expression. • To buffer an acid, use the Na or K salt. • To buffer the base, use nitrate salts.

  6. Le Chatelier’s Principle • An equilibrium will shift to undo a stress. • If you add chemical to the left, the EQ will shift to the right to get rid of it. • If you add chemical to the right, the EQ will shift left to get rid of that. • The salt will add to the conjugate ion on the right. • The shift to the left will result in less H+ or OH-

  7. Henderson-Hasselbach Equation • pH = pKa + log([A-] / [HA]) • pOH = pKb + log([BH+] / [Base]) • [A-] and [BH+] are the salt concentrations • Moles also will work in the brackets

  8. Henderson-Hasselbach Equation • pH = pKa + log {[salt] /[acid]} • pOH = pKb+ log {[salt] /[base]} • You will subtract from 14 somewhere! • A buffer is limited by the solubility of the salt. • The pH is determined by pK

  9. Buffer Example • Calculate the pH when equal amounts of 0.25M HClO and 0.50M NaClO are mixed.

  10. Buffer Example • What is the pH when 1.4 g of NaNO2 is added to 250 mL of 0.40M HNO2?

  11. Buffer Example • What is the pH when 25 mL of 0.40M sodium acetate is added to 75 mL of 0.80 M acetic acid?

  12. Buffer Example • How many grams of the sodium salt is needed to make the following buffers? • 200 mL of a 0.200M HCOOH solution at pH 3 • 500 mL of a 0.050M carbonic acid solution at pH 7.4 (this is the blood buffer system)

  13. Buffer Example • What is the pH when 10 g of sodium bicarbonate is mixed with 10 g of sodium carbonate in 500 mL of water?

  14. Weak AB Titration • When a titration is performed using a strong base and a weak acid, the resulting salt is NOT neutral (this one will be basic). This salt then acts to buffer the acid solution and the H-H equation must be used. • The moles of base added will form the salt and will be subtracted from the moles of acid.

  15. Weak AB Titration • What is the pH of a 1.0 M solution of acetic acid after 10 mL of 0.50M NaOH is added to 25 mL of the acid?

  16. Weak AB Titration • How many mL of the NaOH will be added to reach the equivalence point? (stoichiometry) • What is the pH at the equivalence point?

  17. Titrations of Buffers • Buffers are designed to stabilize the pH even when acids or bases are added to the system. • When an acid is added, the acid will increase as the base decreases (neutralized). • When a base is added, the base will increase as the acid decreases (neutralized)

  18. Buffer Titration • A buffer is made by adding 1 L of 0.20M formic acid (irritant in ants and nettles) to 1 L of 0.15M sodium formate. • (A) What is the pH? • (B) What is the pH when 0.10 molHCl is added? • (C) What is the pH when 0.10 molNaOH is added?

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