patino_chm2046_chapter16

B buffers are used as indicators in acid base

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Unformatted text preview: ffering range. pH – pKa = 0.51, it is in the rank pH - pKa = ± 1 27 All of the following statements concerning buffers are true EXCEPT a) buffers resist pH changes upon addition of small quantities of strong acids or bases. b) buffers are used as indicators in acid-base titrations. c) the pH of a buffer is close to the pKa of the weak acid from which it is made. d) buffers contain appreciable quantities of a weak acid and its conjugate base. e) buffers are resistant to changes in pH when diluted with water. Answer: b Preparing a Buffer Preparing What mass of NH4Cl should be added to 2.55 L of a 0.155 M NH3 to make a buffer having a pH of 9.55? Kb of NH3 = 1.8× 10-5 pKb = -log 1.8× 10−5 = 4.74 Plan: Kb, pOH and [NH3] ⇒[NH4Cl] ⇒mass NH4Cl Cl] pOH =14 –pH =4.45 [OH-]=10−4.45 =3.55x10−5M NH3 +H2O ⇄ NH4+ +OH− The fundamental eq. 0.155 M ? 0.0000355 M Kb=[NH4+][OH−]/[NH3] [NH4+] can be calculated [NH4+] [NH4+]= Kb [NH3]/[OH−] [NH [NH +] = 1.8× 10−5× 0.155 / 3.55× 10−5 = 0.0786 M Preparing a Preparing What mass of NH Cl should be added to Buffer Buffer 4 2.55 L of 0.155 M NH3 to make a buffer having a pH of 9.55? Kb = 1.8 × 10-5 Plan: Ka, pH and [NH3] ⇒ [NH4Cl] mass NH4Cl Cl moles of NH4Cl = moles of NH4+ = moles = 0.0786 mol/L x 2.55 L = 0.201 mol Molar mass of NH4Cl = 53.59 g/mol mass NH4Cl = 0.201 mol x 53.49 g/mol= 10.6 g You cannot add isolated NH4+ ions, but a salt such You add as NH4Cl (this example) or NH4NO3. These salts are as These Acid-Base Reactions Acid-Base • Strong acid + strong base HCl + NaOH → NaCl + NaCl • Strong acid + weak base HCl + NH3 → NH4Cl HCl NH • Weak acid + strong base HOAc + NaOH → NaOAc + NaOAc • Weak acid + weak base HOAc + NH3 → NH4OAc HOAc What is the relative What pH before, during, before during & after an acid-base after reaction? reaction? H3CCOOH = HC2H3O2 = HOAc Titrations & titration curves Titrations & titration curves pH pH pH SA-SB, pH = 7 WA-SB, pH > 7 SA-WB, pH &...
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