Anticipated pH of Buffer Solution ( )

[erjkism]

[SOLVED] Anticipated pH of Buffer Solution (urgent!)

Homework Statement

Can someone verify my process here? i did a lab where i had 100 mL CH3COOH/CH3CO2- buffer solution. I have to find the pH if 100 mL of 0.0983 M HCl is added to it. In the lab i got pH of 1.72

2. Homework Equations

[H3O+][CH3CO2-]
Ka= ---------------------
[CH3COOH]

Ka : 1.8x10-5
initial moles CH3COOH: 0.00996
Initial moles CH3CO2-: 0.0100
moles H+ in 100 mL 0.0983 M HCl: 0.00983

The Attempt at a Solution

It's my understanding that the mole ratio of H+/CH3CO2- is one to one.
Next i think i do this:

moles CH3COOH = 0.00996 + 0.00983 = 0.0198 moles
moles CH3CO2-= 0.0100- 0.00983= 0.00017 moles

concentration CH3COOH= 0.0198 moles/(0.200 L) = 0.099 M
concentration CH3CO2-= 0.00017/(0.200 L) = 0.00085 M

Then by rearranging the equilibrium expression i can solve for [H3O+].
[H3O+]=(1.8x10-5)(0.099M) / (0.00085) = 0.00212 M

pH= -log(0.00212M) = 2.67

is that how am supposed to go about doing this? because my other lab values are very strange. for instance, i keep getting a pH of 4.7 after adding 5, 10, and 20 mL of HCl to it.

 

[erjkism]

k i finally got the answer thanks anyone who looked at it tho

 

[Blueshift5]

Yes, your process for calculating the pH of the buffer solution after adding HCl is correct. However, it is important to note that the pH of a buffer solution is affected by the ratio of the concentrations of the conjugate acid and base, not just the concentrations themselves. In your calculation, you assumed that the concentrations of CH3COOH and CH3CO2- were equal, but this may not have been the case in the actual lab experiment. It is possible that the initial concentrations of the buffer components were not exactly 0.1 M, or that there were other factors that affected the equilibrium. This could explain why your calculated pH values do not match the experimental values.

In order to verify your process and results, it would be helpful to double-check your initial concentrations and the volumes of HCl added in the experiment. It may also be useful to compare your results with those of your classmates or with the expected pH values for a CH3COOH/CH3CO2- buffer solution. Overall, your approach to solving this problem is correct, but it is important to consider all factors that may affect the equilibrium in order to obtain accurate results.