× Didn't find what you were looking for? Ask a question
Top Posters
Since Sunday
a
5
k
5
c
5
B
5
l
5
C
4
s
4
a
4
t
4
i
4
r
4
r
4
Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
New Topic  
nygiantsfan nygiantsfan
wrote...
Posts: 12
Rep: 0 0
11 years ago
Here is a question from a practice exam and I don't quite know where to start. I think it's a henderson-hasselbalch equation but the part that stumps me up is the addition of the two solute in the solution.

Question:
What is the pH of a solution prepared by mixing 15.00 mL of 0.30 M CH3NH2, with 25.00 mL of 0.10 M, CH3NH3+? Assume that the total volume is 40 ml after addition and that Kb = 3.70 × 10-4.
Read 347 times
1 Reply

Related Topics

Replies
wrote...
#1
11 years ago
You are correct, you do use the Henderson-Hasselbach equation, after some preliminary calculations.

First you must compute the new concentrations of the acid and base. Do this by multiplying their respective volumes (in liters) by their molarity and the dividing by the total volume.

[CH3NH2] = (0.01500 L)(0.30 M) / 0.040 L = 0.1125 M

[CH3NH3^+] = (0.02500 L)(0.10 ) / 0.040 L = 0.0625 M

Now compute the pKa

pKa = -log(3.70x10^-4) = 3.427

Finally determine the pH

pH = 3.427 + log(0.1125 / 0.0625) = 3.68
New Topic      
Quick Reply
Explore
Post your homework questions and get free online help from our incredible volunteers
  1120 People Browsing
Start New Topic Take the Tour CoursesNew Study Tools Topics Trending Browse by Textbook
Live Chat
Related Images
  
 405
  
 301
  
 317
Your Opinion
Who will win the 2024 president election?
Votes: 3
Closes: November 4
Previous poll results: What's your favorite funny biology word?