Calculating pH Help

[Chu]

The pH of a mixture of 15 mL of 0.1 M NaOH with 25 mL 0.1 M HBZ is:

K_a=6.28x10^-5

The correct answer is 4.38, but I keep coming up with 4.42.

If someone could should me how to get to 4.38 that would be greatly appreciated.

 

Thanks!

 

 

[hypervalent_iodine]

Can you show how you get your answer? Honestly, it's not a huge difference. Are you solving using a specific formula or something else?

[hypervalent_iodine]

Just to add, I performed the calculation myself and got the correct answer. If you could show your working, that would help to figure out where the issue is.

[Chu]

I used the ka to find the pka and got 4.202. 

15 mL * 0.1 M = 1.5

25 mL * 0.1 M = 2.5

 

I then used the equation

pH=pka+log([base]/[acid])

=4.202+log(2.5/1.5)

=4.42

[hypervalent_iodine]

Right. So your problem is two fold. The first problem is that you put your acid concentration on the numerator side of the fraction. The second problem is that you have misunderstood what is meant by base in the equation. It is actually conjugate base - ie the deprotonated form of the acid. Not NaOH. The NaOH is there to react with your acid. Once it has reacted, you will have a certain concentration of base left, and then because there is less base than acid, you will have some excess acid. Those concentrations are what you use.