Consider the titration of 40.0 mL 0.250 M ethylamine, C2H5NH2, with 0.350 M HCl.
Determine each of the following and sketch the titration curve.
Kb of ethylamine = 5.6x 10-4
a.
The volume of added acid required to reach the equivalence point.
b.
The initial pH.
c.
The pH when 5.0 mL of acid has been added.
d.
The pH at the half-way point.
e.
The pH at the equivalence point.
f.
The pH after adding 5.0 mL of acid beyond the equivalence point
You have a nightmare of a problem (actually 6 problems) here. It is important that you know how to do these. I don't mind helping you through it but first tell me what you know and what you don't understand. We can go from there. Please show your work.
confused on how to use the henderson-hasselbach equation. I am having trouble with part C. I got a pH of 10.75 for part C. Not sure if i'm doing it correctly
Thanks you. That helps a lot.
Let's call ethylamine just BNH2.
BNH2 + HCl ==> BNH3^+ + Cl^-
initial mols BNH2 = M x L = approx 0.01
mols HCl when adding 5 mL is M x L = 0.00175
mols. Put that on an ICE chart but I rather use millimols.. All of those zeros confuse me.
....BNH2 + HCl ==> BNH3^+ + Cl^-
I...10....0.......0
add.......1.75......
C.-1.75..-1.75....1.75
E...?.....0......1.75
Then substitute the E line into the HH equation and solve for pH.
Note: The HH equation uses CONCENTRATIONS (MILLIMOLS/ML) BUT SInce mL is the same for both, the you may use millimols by itself since the volume cancels.
Please follow up is this is not sufficient.
Check that post I made. I'm in a big hurry and I may have hit the wrong key on the calculator or misread an answer from the display. That is, confirm what I've written.
To determine each of the following and sketch the titration curve, you will need to follow a systematic approach using principles of acid-base titration and the relationships between pH, concentrations, and volumes of the solutions involved. Here is a step-by-step explanation of how to calculate each value:
a. The volume of added acid required to reach the equivalence point:
The equivalence point is reached when the moles of acid added equals the moles of base present.
- Calculate the moles of ethylamine using the formula: moles = concentration (M) × volume (L).
Moles of ethylamine = 0.250 M × 0.0400 L = 0.0100 mol
- Since the balanced equation between ethylamine and HCl is 1:1, the moles of HCl needed to reach the equivalence point are also 0.0100 mol.
- To calculate the volume of HCl solution needed, use the formula: volume = moles / concentration.
Volume of HCl = (0.0100 mol) / (0.350 M) = 0.0286 L = 28.6 mL
Therefore, the volume of added acid required to reach the equivalence point is 28.6 mL.
b. The initial pH:
The initial pH can be determined based on the concentration of ethylamine and its Kb value.
- Calculate the pOH using the formula: pOH = -log10(Kb).
pOH = -log10(5.6 × 10^-4) ≈ 3.25
- Calculate the initial concentration of hydroxide ions (OH-) using the formula: [OH-] = √(Kb × [ethylamine]).
[ethylamine] = 0.250 M (given)
[OH-] = √(5.6 × 10^-4 × 0.250) ≈ 0.026 M
- Calculate the initial pH using the formula: pH + pOH = 14.
pH + 3.25 = 14
pH ≈ 14 - 3.25 ≈ 10.75
Therefore, the initial pH is approximately 10.75.
c. The pH when 5.0 mL of acid has been added:
To determine the pH after adding a certain volume of acid, you need to calculate the remaining concentration of ethylamine and use it to determine the concentration of hydroxide ions.
- Calculate the moles of acid added using the formula: moles = concentration (M) × volume (L).
Moles of acid added = 0.350 M × 0.0050 L = 0.00175 mol
- Subtract the moles of acid added from the initial moles of ethylamine:
Moles of ethylamine remaining = 0.0100 mol - 0.00175 mol = 0.00825 mol
- Calculate the concentration of ethylamine remaining using the formula: [ethylamine] = moles remaining / volume.
Volume = 40.0 mL + 5.0 mL = 45.0 mL = 0.0450 L
[ethylamine] = 0.00825 mol / 0.0450 L ≈ 0.183 M
- Calculate the new concentration of hydroxide ions using the formula: [OH-] = √(Kb × [ethylamine]).
[OH-] = √(5.6 × 10^-4 × 0.183) ≈ 0.010 M
- Calculate the new pH using the formula: pH = 14 - pOH.
pH = 14 - (-log10(0.010)) ≈ 12
Therefore, the pH after adding 5.0 mL of acid is approximately 12.
d. The pH at the half-way point:
The half-way point in the titration is when half of the moles of the original acid or base have reacted.
- Calculate the moles of acid needed to reach the half-way point, which is half of the moles of ethylamine:
Moles of ethylamine remaining at half-way point = 0.0100 mol / 2 = 0.0050 mol
- Calculate the volume of acid needed to reach the half-way point using the formula: volume = moles / concentration.
Volume of acid needed = (0.0050 mol) / (0.350 M) = 0.0143 L = 14.3 mL
- Calculate the concentration of ethylamine remaining and the concentration of hydroxide ions as done in step c.
- Calculate the new pH using the formula: pH = 14 - pOH.
Therefore, the pH at the half-way point can be determined once you know the concentrations of ethylamine and hydroxide ions.
e. The pH at the equivalence point:
At the equivalence point, the moles of acid added equal the moles of base present.
- Calculate the moles of acid needed to reach the equivalence point (already done in part a): 0.0100 mol.
- Calculate the volume of acid needed to reach the equivalence point (already done in part a): 28.6 mL.
- Determine the number of moles of HCl remaining after adding 28.6 mL of acid. This is equal to the excess moles of acid added.
Moles of excess HCl = moles of acid added - moles at equivalence point
Moles of excess HCl = 0.0100 mol - 0.0100 mol = 0.0000 mol
- Calculate the remaining concentration of ethylamine and the concentration of hydroxide ions as done in step c.
- Calculate the new pH using the formula: pH = 14 - pOH.
Therefore, the pH at the equivalence point can be determined once you know the concentrations of ethylamine and hydroxide ions.
f. The pH after adding 5.0 mL of acid beyond the equivalence point:
After the equivalence point, the excess acid starts to determine the pH of the solution.
- Calculate the volume of excess acid beyond the equivalence point: Total volume of acid added - volume of acid at equivalence point.
Volume of excess acid = (40.0 mL + 5.0 mL) - 28.6 mL = 16.4 mL
- Determine the number of moles of HCl in the excess acid: moles = concentration × volume.
Moles of excess HCl = 0.350 M × 0.0164 L = 0.00574 mol
- Calculate the moles of ethylamine remaining using the balanced equation (1:1 ratio) and subtract the excess moles of acid.
Moles of ethylamine remaining = 0.0100 mol - 0.00574 mol = 0.00426 mol
- Calculate the concentration of ethylamine remaining and the concentration of hydroxide ions, similar to step c.
- Calculate the new pH using the formula: pH = 14 - pOH.
Therefore, the pH after adding 5.0 mL of acid beyond the equivalence point can be determined once you know the concentrations of ethylamine and hydroxide ions.
To sketch the titration curve, plot the pH on the y-axis and the volume of acid added on the x-axis. The curve will show the pH changes as the acid is added, from the initial pH to the equivalence point, and beyond. The points obtained from the calculations can be used to plot the curve accurately.