Henderson Hasselbalch equation Where did I go wrong?

  • Thread starter shenzys
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In summary, the question asks for the volume of 0.1M HCl needed to be added to 200ml of 0.15M sodium acetate solution to obtain a buffer of pH 4.2. The equation pH = pKa + log [salt]/[acid] is used to calculate the ratio of salt to acid, which is found to be 0.275. However, the attempt to solve for the volume of acid needed results in an incorrect value. The error may be due to incorrect accounting for the protonation reaction, where the amount of acetic acid increases and the amount of acetate decreases when strong acid is added.
  • #1
shenzys
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0

Homework Statement



Q) Calculate the volume of 0.1M HCl you would need to add to 200ml of 0.15M sodium acetate solution to obtain a buffer of pH 4.2.
Ka = 1.74 * 10^-5 moldm-3


Homework Equations



pH = pKa + log [salt]/[acid]


The Attempt at a Solution



Log[salt]/[acid] = pH - pKa

[salt]/[acid] = 0.275

Equation: [salt] = 0.275 [acid]

[salt] = 200*0.15/200+v

[acid] = (0.1*V / 200+v) - (200*0.15/200+v)

Substituting into above equation:

200*0.15/200+v = 0.275(0.1*V/200+v) - 0.275(200*0.15/200+v)

30 = 0.0275v - 8.25

38.25= 0.0275v

V = 38.25/0.0275v

V = 1390 cm^3

This is obviously wrong! Where did I go wrong?
 
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  • #2
Moles of salt + moles of acid should be constant.
 
  • #3
Borek said:
Moles of salt + moles of acid should be constant.

I still don't understand... could you explain a bit more please - sorry to bother you, I am just really desperate!
 
  • #4
Take a look at the protonation reaction. When you add strong acid to acetate amount of acetic acid grows, but amount of acetate goes down.

Could be you tried to account for that, but you did it wrong way.
 

What is the Henderson Hasselbalch equation?

The Henderson Hasselbalch equation is a formula commonly used in biochemistry and chemistry to calculate the pH of a buffer solution. It relates the ratio of the concentrations of a weak acid and its conjugate base to the dissociation constant (pKa) of the acid.

How is the Henderson Hasselbalch equation derived?

The Henderson Hasselbalch equation is derived from the equilibrium constant expression for the dissociation of a weak acid. By taking the negative logarithm of both sides and rearranging the equation, the Henderson Hasselbalch equation is obtained.

What are the assumptions made in the Henderson Hasselbalch equation?

The Henderson Hasselbalch equation assumes that the acid and its conjugate base are the only species present in the solution, and that the concentrations of these two species remain constant during the reaction. It also assumes that the acid is a weak acid and that the dissociation constant is known.

How is the Henderson Hasselbalch equation used in laboratory experiments?

The Henderson Hasselbalch equation is often used in laboratory experiments to calculate the pH of a buffer solution. It can also be used to determine the pKa value of an unknown weak acid by measuring the pH of a solution with known concentrations of the acid and its conjugate base.

What are common mistakes made when using the Henderson Hasselbalch equation?

One common mistake when using the Henderson Hasselbalch equation is not using the correct units for the concentrations. The concentrations must be in molarity (M) for the equation to work. Another mistake is using the wrong value for the pKa, as this can significantly affect the calculated pH. It is also important to make sure the acid and its conjugate base are the only species present in the solution, as any other species can throw off the calculation.

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