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EXPERIMENT 3. ACID-BASE TITRATIONS: DETERMINATION OF CARBONATE BY TITRATION WITH HYDROCHLORIC ACID BACKGROUND Carbonate Equilibria In this experiment a solution of hydrochloric acid is prepared, standardized against pure sodium carbonate, and used to determine the percentage of carbonate in a sample. An aqueous solution of hydrochloric acid is almost completely dissociated into hydrated protons and chloride ions. Therefore, in a titration with hydrochloric acid the active titrant species is the hydrated proton. This species is often written H3O+, although the actual form in solution is more correctly (H2O)nH+. For convenience we designate it simply H+. Carbonate in aqueous solution acts as a base; that is, it is able to accept a*…show more content…*

Calculation shows that [H2CO3] at Point A is negligibly small, so our assumption 2is valid. The additional assumption that [CO3 ] is essentially 0.1 M also is confirmed because Equations (9) and (11) show that [HCO3 ] is less than 5% of 2[CO3 ] . Kw Note from this discussion that K = Kb, or Kw = K2Kb. Thus, if Ka for an 2 acid HA is known, Kb for the corresponding base A- can be calculated in aqueous 33 solutions. An acid HA and base A- are called a conjugate acid-base pair; HA is the conjugate acid of A- and A- the conjugate base of HA. pH at Point B. At Point B in Figure 1, 0.5 mole of hydrochloric acid has been added for each mole of carbonate. The solution now contains an equimolar mixture of carbonate and bicarbonate. We can calculate the pH at this point by rearranging Equation (3) to [HCO3] K2 2[CO3 ] [H+] = (14) Since the bicarbonate and carbonate concentrations are equal, the hydrogen ion concentration is equal to K2, and the pH is 10.3. Accurate calculations of concentrations of species during titrations must include the effect of dilution by the titrant, but thus far those caused by the addition of hydrochloric acid have not been considered. To correct calculations of concentrations of the major components for dilution, multiply each calculated concentration by the factor V/(V + v), where V is the volume of the original solution and v is the volume of hydrochloric acid added at any point. Although

Calculation shows that [H2CO3] at Point A is negligibly small, so our assumption 2is valid. The additional assumption that [CO3 ] is essentially 0.1 M also is confirmed because Equations (9) and (11) show that [HCO3 ] is less than 5% of 2[CO3 ] . Kw Note from this discussion that K = Kb, or Kw = K2Kb. Thus, if Ka for an 2 acid HA is known, Kb for the corresponding base A- can be calculated in aqueous 33 solutions. An acid HA and base A- are called a conjugate acid-base pair; HA is the conjugate acid of A- and A- the conjugate base of HA. pH at Point B. At Point B in Figure 1, 0.5 mole of hydrochloric acid has been added for each mole of carbonate. The solution now contains an equimolar mixture of carbonate and bicarbonate. We can calculate the pH at this point by rearranging Equation (3) to [HCO3] K2 2[CO3 ] [H+] = (14) Since the bicarbonate and carbonate concentrations are equal, the hydrogen ion concentration is equal to K2, and the pH is 10.3. Accurate calculations of concentrations of species during titrations must include the effect of dilution by the titrant, but thus far those caused by the addition of hydrochloric acid have not been considered. To correct calculations of concentrations of the major components for dilution, multiply each calculated concentration by the factor V/(V + v), where V is the volume of the original solution and v is the volume of hydrochloric acid added at any point. Although

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