Question

In: Chemistry

(a) For the reaction 2 A(aq) ⇋ 2 B(g) + C(g), the equilibrium constant is 4.59...

(a) For the reaction 2 A(aq) ⇋ 2 B(g) + C(g), the equilibrium constant is 4.59 at 25.0oC. If the concentrations of B(aq) and C(aq) are each 0.311 M, what concentration of A(aq) is required to have a ΔG value of -6.66 kJ/mol? The temperature is 25.0oC.

(b) We have a buffer solution that was produced by adding 11.9 g of NaOH to 2.000 L of a 0.666 M solution of HA(aq). The pH of the buffer solution is 3.25 . What is the value of Ka for HA(aq)? The temperature is 25.0oC.

Expert Solution

Answer B:

As mole of NaOH = 11.9/40 (Molecular weight of NaOH is 40 g/ mole)

= 0.2975

So total mole of acid in the weak acid is

= 0.666*2 = 1.332 moles.

HA +NaOH ------>Na+ + A- + H2O

So out of 1.332 moles of HA , 0.2975 moles will be neutralized by the NaOH and form the acid ion of 0.2975 and reaminig moles of acid is (1.332 -0.2975 ) 1.0345 moles.

So concentraiton of acid = 1.0345 /2 = 0.51275 M of acid

cocnentraio of acid ion = 0.2975 /2 = 0.14875 M of acid ion

as per the Henderson-Hasselbalch equation:

pH = pKa + log [A-]/[HA]

3.25 =pKa + log[0.14875] /[0.51275]

pKa = 3.25 +0.5412

pKa = 3.7912

So

pKa = -log(Ka)

3.7912 = -log(Ka)

log(Ka) = -3.7912

Ka = 10^(-3.7912)

Ka =1.6 *10^(-4)

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