The barbituate pentobarbitone, which is used as an anesthetic in vertebrates, has a single ionizable group with a pka of 8.0. what percentage of protonated pentobarbitone will be present in a solution that has a ph of 8.7?

Answer : The percentage of protonated pentobarbitone is 20%

Explanation :

Let's say B represents pentobarbitone and BH⁺ represents protonated pentobarbitone.

The protonation reaction can be written as

When the above reaction reaches equilibrium, we get a mixture of B and BH⁺.

This mixture contains a weak base pentobarbitone and its protonated form which is a conjugate acid.  Therefore the mixture acts as a buffer.

The pH of a buffer is calculated using Henderson formula which is written below.

We have acid =  BH⁺

base = B

pH = 8.7

pka = 8.0

Let us plug in the above values in Henderson equation.

We want to find fraction of BH⁺ , which is written as  BH⁺/B.

Therefore we will inverse the above equation.

The percentage of protonated pentobarbitone is 0.200 x 100 = 20%

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