I don't know much about this, but probably the reason why your teacher circled 300k is because of what's in the photo below. If you couldn't tell from the photo, its because 300k isn't negative. Anyway this is my guess, based on my limited knowledge.
Answer: The pH at equivalence point for the given solution is 5.59.
Explanation:
At the equivalence point,
So, first we will calculate the moles of as follows.
= 0.0845 mol
Now, volume of present will be calculated as follows.
Volume =
=
= 0.1891 L
Therefore, the total volume will be the sum of the given volumes as follows.
110 ml + 189.1 ml
= 299.13 ml
or, = 0.2991 L
Now,
= 0.283 M
Chemical equation for this reaction is as follows.
As,
=
=
Now,
=
=
Now, pH will be calculated as follows.
pH =
=
= 5.59
Thus, we can conclude that pH at equivalence point for the given solution is 5.59.