You are given 10.00 mL of a solution of an unknown acid. The pH of this solution is exactly 4.08. You determine that the concentration of the unknown acid was 0.3250 M. You also determined that the acid was monoprotic (HA). What is the pKa of your unknown acid

Explanation:

pH = 4.08

[ H⁺ ] = 10⁻⁴°⁰⁸

= 8.3 x 10⁻⁵

concentration of acid = .3250 M

           HA      ⇆      H⁺     +     A⁻

      .325                 8.3 x 10⁻⁵     8.3 x 10⁻⁵

Ka = (8.3 x 10⁻⁵ )² / .325

= 68.9 x 10⁻¹⁰ / .325

= 212 x 10⁻¹⁰ M .

pKa = - log ( 212 x 10⁻¹⁰ )

= 10 - log 212

= 10 - 2.32

= 7.68 .

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.