BUFFERS:

Biochemical experiments frequently utilize a buffer system based on TRIS/THAM (HOCH2)3CNH2.
The pKa of the conj. acid of TRIS (HOCH2)3CNH3+ is pKa=8.075. What mole ratio ("X") of base-to-acid is required to prepare a buffer at the same pH as human blood, pH=7.4???

Question

BUFFERS:

Biochemical experiments frequently utilize a buffer system based on TRIS/THAM (HOCH2)3CNH2.
The pKa of the conj. acid of TRIS (HOCH2)3CNH3+ is pKa=8.075. What mole ratio ("X") of base-to-acid is required to prepare a buffer at the same pH as human blood, pH=7.4???

xishem · Answer

Try using the Henderson-Hasselbalch equation...$$pH=pKa+\log(\frac{n_{A^-}}{n_{HA}})$$$$7.4=8.075+\log (\frac{n_{A^-}}{n_{HA}})$$$$-0.675=\log(\frac{n_{A^-}}{n_{HA}})$$$$10^{-0.675}=\ 10^{log(\frac{n_{A^-}}{n_{HA}})}$$$$0.2113=\frac{n_{A^-}}{n_{HA}}$$$$0.211:1.00$$So the ratio of base-to-acid required is...
$$1.00:4.73$$