If the same volume (130-mL) of the buffer were 0.265 M in NH3 and 0.400 M in NH4Br, what mass of HCl could be handled before the pH fell below 9.00?
I used the henderson-hasselbalch equation to solve this. i set it up as 9=9.24551+log((0.03445-x)/(0.052+x))
0.03445= moles of NH3
0.052= moles of NH4Br
I calculated the grams to be .11 g but it was wrong. Did i set up my H-H equation wrong?
It appears to me that your set up is ok. But when I work the problem I don't get exactly 0.11 grams. More like 0.122 g. And if I divide 0.122/36.46 I get 0.00335 moles. And 9.25 + log[(34.45-3.35)/(52.0+3.35)] = 8.9997 which I rounded to 9.0. Actually, if you use 0.003348 it gives 9.0.
To determine if you set up the Henderson-Hasselbalch equation correctly, we can check by following the steps you described.
The Henderson-Hasselbalch equation relates the pH of a buffer solution to the concentrations of the weak acid and its conjugate base. It is given by:
pH = pKa + log([A-]/[HA])
Where:
pH = desired pH of the buffer
pKa = dissociation constant of the weak acid
[A-] = concentration of the conjugate base
[HA] = concentration of the weak acid
In your case, you set up the Henderson-Hasselbalch equation as:
9 = 9.24551 + log((0.03445 - x)/(0.052 + x))
To find the mass of HCl that could be handled before the pH fell below 9.00, we need to calculate the value of x, which represents the amount of HCl added. By finding x, we can determine the change in concentrations of NH3 and NH4Br in the buffer solution.
To solve this equation, you would typically rearrange it to isolate the logarithmic term and then solve it using numerical methods. However, this calculation is unnecessary since we only need to check if the equation was correctly set up.
Upon inspecting your equation, it appears to be set up correctly according to the Henderson-Hasselbalch equation. Therefore, the problem may lie in the subsequent calculations, rather than the equation itself.
Double-check your calculations to ensure that you correctly accounted for the moles of NH3 and NH4Br and performed the logarithmic calculation accurately. Reevaluate your approach and recalculate the value for x.