# DETERMINATION OF THE DISSOCIATION CONSTANT OF A WEAK ACID FROM pH MEASUREMENTS

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DETERMINATION OF THE DISSOCIATION CONSTANT OF A WEAK ACID FROM pHMEASUREMENTSAimThe aim of this experiment is to determine the pKafor benzoic acid using pH measurements.Pre-lab questions1. A weak acid is one which doesn't ionise fully when it is dissolved in water. A weak acid is morereluctant to donate protons. And they have a high pKasuch that they release all their hydrogen atomswhen dissolved in water.2. i) Kais a measure of acid strength.Kais defined for the reaction:HA(aq) + H2O(l)H3O+(aq) + A(aq)With an equilibrium constant:A larger equilibrium constant means that the reaction has a greater tendency to drive towards theproducts side (more H3O+and A, less HA).It can be seen that the greater is the extent of dissociation,the greater will be the value of Ka. Or, the stronger is the acid, the greater will be its Ka.Another wayof saying this is that the acid has a stronger tendency to donate its H+, which is the definition of astronger acid.ii)pKais the negative log of the acid dissociation constant (Ka).pKa=−logKaSince, pKa is negative log of Ka, it's values will be greater for weaker acid.The stronger the acid, theweaker will be its conjugate base. (The larger the pKaof the conjugate acid, the stronger the base).3.pKa= −logKapKa= −log(5.6 x 10-4)pKa= 3.254.pH = pKa+ log([A-]/[HA])However, at one half of the equivalence point, half of the weak acid (HA) has been converted into itsconjugate base (A-), so:[HA] = [A-] and log([A-]/[HA]) = 0Therefore,pH = pKa(at half the equivalence point)5. pH = pKa
4.8 = -log10Ka-4.8 = log10KaKa= 10-4.8= 1.58 x 10-56. n (KHP) = cVc (KHP) = 0.1 M, V (KHP) = 100 x 10-3Ln (KHP) = (0.1)( 100 x 10-3) = 0.01 molm (KHP) = n x Mrn (KHP) = 0.01 mol, Mr (KHP) = 204.17 g/molm (KHP) = (0.01)(204.17) = 2.0417 gProcedureAbout 150 mL of distilled water was boiled in a 250 mL beaker and cooled to room temperature. Thebeaker was covered with a petri dish during cooling. 0.1256 g of benzoic acid was accurately weighedinto a clean, dry 250 mL beaker. 40 mL of the boiled, cooled distilled water was added using ameasuring cylinder and stirred to dissolve. (It was necessary to warm the mixture in order to dissolvethe acid. However, the solution was cooled to room temperature before it was transferred into the

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