Chemistry - desperately with buffer calculation.

In summary, to prepare a buffer at pH=9.5 with Ka of NH4+ of 5.6x10^-10, a 100.0ml of a 0.100M NH3 solution mixed with a 0.200M solution of NH4Cl must be used.
  • #1
emilj
7
0
1. A buffer is prepared by mixing a 100.0ml of a 0.100M NH3 solution with a 0.200M solution of NH4Cl and making the total volume up to 1.000L of water. What is the volume of the NH4Cl solution required to achieve a buffer at pH=9.5
Ka of NH4+ = 5.6x10^-10

The textbook tell me that the answer is 28.1mL, can anyone please help me to figure this out? I've been trying for the past few hours and just about ready to give up!

I'm almost certain you need to use hendersen hasselbach but i cannot get the correct answer.
Thank you :)
 
Physics news on Phys.org
  • #2
HH equation will give a ratio of concentrations of NH3 and NH4+. You are given amount of ammonia and the final volume, so you can easily calculate ammonia concentration in the final solution. Use this number and the ratio to calculate concentration of NH4+ in the final solution, then calculate how much NH4+ in the form of 0.200M solution is needed.

Actually dilution calculations can be ignored, but I am afraid explaining why will just confuse you more.
 
  • #3
oh ok, thank you for your feedback. is there any chance you would mind showing me the working for the question?
im just a bit confused about which ratio you're talking about, I've completed the question and gotten an answer of 0.2823L which i know isn't right.
 
Last edited:
  • #4
Show what you did and how.
 
  • #5
okay, so i began by finding the moles of ammonia in 1L,
first found moles in 100mL
c=n/v so n=cV
n=0.100M/0.1
= 0.01

then using henderson hasselbach,

pH=pKa+ log (NH4+/NH3)
9.5=9.25+log(NH4+/0.01)
0.25=log(NH4/0.01)
10^-0.25= (NH4/0.01)
NH4= 0.564 x 0.01
n of NH4 = 0.00565

then from moles find volume of NH4 required,
c=n/v
v=n/c
v= 0.00564/0.02M
v= 0.2823L
v= 282.3mL

i know my working isn't right but I'm at a loss about what to do. i have an exam on this on friday so i really appreciate your help.
 
  • #6
emilj said:
v= 0.00564/0.02M

Why 0.02M?
 
  • #7
i've just had a look at the question again,

i think I've calcualted the number of moles incorrectly,

moles of ammonia should actually be,
n= 0.1M/1.0L
n=0.1

is that correct?
 
  • #8
oh that should be 0.2M not 0.02. my mistake
 
  • #9
so then if i sub that in, so..

0.00564 M/0.2M
=0.0282L, is that then correct?
 
  • #10
Looks OK to me, and fits the book answer, doesn't it?
 
  • #11
emilj said:
i've just had a look at the question again,

i think I've calcualted the number of moles incorrectly,

moles of ammonia should actually be,
n= 0.1M/1.0L
n=0.1

is that correct?

No.

First, it is C*V, not C/V (actually you wrote it wrong in the original solution as well, but then you calculated right).

Second, in the final solution concentration of ammonia is not 0.1M, it was diluted tenfold, to 0.01M.

Your original calculation (0.1M*0.1L=0.01 moles) was OK.
 
  • #12
oh okay thank you very much for your help :) i appreciate it!
 

FAQ: Chemistry - desperately with buffer calculation.

What is a buffer solution?

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added. It is composed of a weak acid and its conjugate base or a weak base and its conjugate acid.

Why is buffer calculation important?

Buffer calculation is important because it allows us to determine the amount of acid and base needed to create a buffer solution with a desired pH. This is crucial in many laboratory experiments and industrial processes.

What factors affect buffer capacity?

The factors that affect buffer capacity are the concentration of the buffer components, the pKa of the weak acid or base, and the volume of the solution. A higher concentration, closer pKa, and larger volume will result in a higher buffer capacity.

How do you calculate the pH of a buffer solution?

The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([conjugate base]/[weak acid]). Alternatively, a pH meter can be used to directly measure the pH of the solution.

What is the purpose of a buffer solution in chemistry?

The purpose of a buffer solution in chemistry is to maintain a relatively constant pH level in a solution, even when small amounts of acid or base are added. This is important in many biochemical and industrial processes, as well as in maintaining the proper pH balance in our bodies.

Similar threads

Back
Top