Achieve Week 2 Question 10

[905749989]

A weak base, B, is ionized forming its conjugate acid ( BH+ ) according to the reaction:

B(aq)+H2O(l)↽−−⇀BH+(aq)+OH−(aq)


The conjugate acid BH+ has a pKa=8.78.

For this weak base, the predominant species present at pH 6.04 is:

neutral.

unknown.

charged.

[Julie Tran 3C]

Hello,

Since pH < pKa, the acid BH+ will not donate a proton/act as an acid and stay in its protonated BH+ form. Thus, BH+ is the dominant species and so the answer is "charged."

[Ginny Ghang 1B]

When pH is equal to pKa, there are equal amounts of the conjugate acid and the weak base in the solution. When pH is less than pKa, there will be more charged, protonated BH+ in the solution. When pH is above pKa, there will be more neutral, deprotonated B in the solution.

[Abu Zhang 2D]

Hi! I also have been struggling with understanding the relationship between pKa and pH, and how these indicate whether there is a predominant species in the solution. Why does pH<pKa indicate that the charged species will become more predominant in the solution?

[Holland Smith 3C]

comparing the pKa and the pH show which component is favored at that state. When pH > pKa, then we see that the reactants are favored. If the pH < pKa, then we know that the product is favored. in terms of this question, the answer would be charged because the pH is lower than the pKa AND the favored side (product) has a charge (BH+). hope this helps!