Several of you tried to work problem number 25 in chapter 2 of our text book abo

Question

Several of you tried to work problem number 25 in chapter 2 of our text book about Tris buffer (see left picture for the acid and base forms of Tris buffer) by using the equation shown below right: The problem with trying to do it this way is that K. just like At. is dependent on temperature ano some or you aid not consider that. I lowevcr. if we assume that Delta H degree and Delta S degree are independent of temperature between 25 degree C and 4 delta C_cnn fiourc out the value of pK. at 4degreeC. Let's use the values your book gives: pK, of Tris at 25 degree C is 8.08 and All is 50 kJ/mol and independent of temperature in this range. Also, the pH was 7.50 at 25 degree C and 8.10 at 4 degree What is the value of pK* at 4 degree C? What is the ratio of (Base form J/l Acid form] at 25 degree C?

Explanation / Answer

a. using the formula

pKa = (delta H / (2.303 RT)) - (delta S / (2.303 R))

we know that pKa at 25deg C = 8.08

delta H = 50 KJ /mol = 50000 J/ mol

R = 8.314 J/molK

T = 298 K

therefore the value of delta S = 13.0763 J/mol

since the values of delta H and delta S are independent of temperature for the given temperature range ,

thus the value of pKa at 4deg C can be found using the formula

pKa = (delta H / (2.303 RT)) - (delta S / (2.303 R))

therefore pKa = 8.74

b. using the equation given in the question

pH = pKa + log { [base form]/[acid form] }

at 25 deg C pH =7.50 & pKa = 8.08

therefore , [base form]/[acid form] = 0.2630

c.

using the equation given in the question

pH = pKa + log { [base form]/[acid form] }

at 4 deg C pH =8.10 & pKa = 8.74

therefore , [base form]/[acid form] = 0.2290

d. the direction of equilibrium lies towards left (acid side)

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