Tris(hydroxymethyl)aminomethane, commonly called TRIS or Trizma, is often used a
ID: 486499 • Letter: T
Question
Tris(hydroxymethyl)aminomethane, commonly called TRIS or Trizma, is often used as a buffer in biochemical studies. Its buffering range is pH 7 to 9, and Kb is 1.19 10-6 for the following aqueous reaction.
(a) What is the optimal pH for TRIS buffers?
(b) Calculate the ratio [TRIS] / [TRISH +] at pH = 6.56 and at pH = 9.31.
pH = 6.56
pH = 9.31
(c) A buffer is prepared by diluting 52.7 g TRIS base and 68.7 g TRIS hydrochloride (written as TRISHCl) to a total volume of 2.0 L.
What is the pH of this buffer?
What is the pH after 0.63 mL of 12 M HCl is added to a 200.0 mL portion of the buffer?
I got a and b I need both parts to C
(HOCH2)3CNH2 + H2O (HOCH2)3CNH3+ + OH - TRIS TRISH +Explanation / Answer
Moles of tris = 52.7 g/121.14 g/mol = 0.44 mol
Moles of tris hydrochloride = 68.7 g/157.59 g/mol =0.44
pOH = pKb + Log [trisH+/tris]
or, pOH = pKb = -log (1.19 10-6) = 5.92
pH of the solution = 14-pOH = 14-5.92 = 8.08
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Moles of tris present in 200mL = 0.44 mol *200mL/2000mL = 0.044 mol
Moles of TrisH+ present in 200mL = 0.044 mol
When HCl is added , it will convert some of the tris to trisH+. As a result amount of trisH+ will increase and concentration of tris will decrease in the solution.
Moles of HCl added = 12 M * 0.00063 L =0.00756 moles
Moles of tris present after adding HCl = 0.044 - 0.00756 = 0.0364 moles
Moles of trisH+ present after adding HCl = 0.044 + 0.00756 = 0.0516 moles
pOH = pKb + Log[trisH+/tris]
= 5.92 + log [0.0516/0.0364] = 6.07
pH of the solution is 14-6.07 = 7.93
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