a solution of aqueous ammonia was titrated with HCL using phenolphthalein as the
ID: 744048 • Letter: A
Question
a solution of aqueous ammonia was titrated with HCL using phenolphthalein as the indicator a) is phenolphthalein an appropriate indicator for this titration? why or why not? b) choose another indicator for thetitration of nh3 by hclExplanation / Answer
Arrhenius Definition of Acids and Bases · Arrhenius Acids = increase the concentration of in water · Arrhenius Bases = increase the concentration of in water · (definition restricted to aqueous solutions) Bronsted- Lowry Acids and Bases · Acid = any substance that can transfer a proton to another substance · Base = any substance that can accept a proton A substance can only function as an acid only if another substance simultaneously behaves as a base Bronsted- Lowry Conjugate Acid Base Pairs HCl (g) + H2O(l) ‡ H3O + + Cl - NH3(aq) + H2O(l) ?> NH4 + + OH - Strength of Conjugate Acid-Base Pairs If a conjugate acid is strong, then its conjugate base is If a conjugate acid is weak, then its conjugate base is2 Amphiprotic (Amphoteric) Substances Substances that can act as both an acid or a base H3O + ‹ gain H + ‹ H2O ? loss of H + ?OH - H2SO4 ‹ gain H + ‹ HSO4 - ? loss of H + ? SO4 -2 Strong Acids –Careful! This does not indicate concentration!! Strong electrolytes – ionize completely in solution HNO3 (aq) + H2O (l) ‡ H3O + + NO3 - Generally speaking, in strong acids the H is bonded to either a very electronegative element (eg HBr and HI) or to an oxygen bonded to a non metal (eg H2SO4). In these oxyacids the strength of the acid increases with the electronegativity of the non metal (H2SO4 is a strong acid, H3PO4 is a weak acid) and the number of oxygens present (HNO3 is a strong acid, HNO2 a weak acid). Memorize: HI, HBr, HCl, HClO3,HClO4, HNO3, H2SO4 The strong acid is the only source of H + in solution. Therefore, the pH of a solution is calculated directly from the initial molarity of the acid [H + ] in 0.01M HCl(aq) =? [H + ] in 0.5M H2SO4(aq) =? Weak Acids- Careful! This does not indicate concentration!! Weak Acids only partially ionize in aqueous solutions HA (aq) + H2O (l) ¤ H3O + + A - ~99% ~1% Almost all organic acids are weak acids (ethanoic acid = acetic acid ‡ CH3COOH) Aqueous carbon dioxide can act as a weak acid ‡ biological function CO2 (aq) + H2O(l) ¤ H + + HCO3 - HA (aq) + H2O (l) ¤ H3O + + A - ~99% ~1% There is an equilibrium between the ions and the unionized acid in solution We can therefore write an equilibrium expression for the dissociation:
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