you are asked to prepare 100. ml of 0.100 m (buffer ... Question: You are asked

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you are asked to prepare 100. ml of 0.100 m (buffer ... Question: You are asked to prepare 100. mL of 0.100 M (buffe... You are asked to prepare 100. mL of 0.100 M (buffer concentration), pH 8.00 Tris buffer. This will be achieved by titrating a solution of tris base using 1.00 M HCl. The pKa of tris is 8.30 at 20 oC. pKa of tris changes according to the equation DeltapKa = -0.031/oC. The temperature of the solution was found to be 23.5 oC. Therefore, the newly adjusted pKa is 8.19. Calculate the theoretical volume of HCl needed to adjust the Tris buffer to pH 8.00 at 23.5 oC (pKa 8.19), starting with the henderson-hasselbach equation. Equations to remember: pH = pKa + log10([conj. Base]/[wk. acid]) Conc.buff = Conc.wk.acid + Conc.conj.base VHCl x CHCl = Vbuff x Conc.wk.acid

Explanation / Answer

Henderson-Hasselbalch Equation:

pH = pKa + log([base] / [acid])

Given, pH = 8.00 and pKa = 8.19

8 = 8.19 + log(0.1 / [acid])

8 - 8.19 = log(0.1) - log([acid])

-0.19 + 1 = - log([acid])

[acid] = 0.15 M

Now using the formula, VHCl x CHCl = Vbuff x Conc.wk.acid

VHCl = 100 x 0.1 / 0.15

        = 66.6 ml

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