## Titration and pH computation

Chemistry and homework help forum.

Organic Chemistry, Analytical Chemistry, Biochemistry, Physical Chemistry, Computational Chemistry, Theoretical Chemistry, High School Chemistry, Colledge Chemistry and University Chemistry Forum.

Share your chemistry ideas, discuss chemical problems, ask for help with scientific chemistry questions, inspire others by your chemistry vision!

Please feel free to start a scientific chemistry discussion here!

Discuss chemistry homework problems with experts!

Ask for help with chemical questions and help others with your chemistry knowledge!

Moderators: expert, ChenBeier, Xen

Dhamnekar Winod
Sr. Staff Member
Posts: 149
Joined: Sat Nov 21, 2020 10:14 am
Location: Mumbai[Bombay],Maharashtra State,India

### Titration and pH computation

Calculate the pH at the beginning, at the midpoint, and at the equivalence point for the titration also choose an indicator:
a) of 50 mL of 0.1M HCl with 0.1 M NaOH
b) of 50 mL of 0.1 M CF3COOH with 0.1 M NaOH
c) 50 mL of 0.1 M triethylamine with 0.1 M HCl

I am working on these questions.

My answer to a) : At the beginning, pH of NaOH will be very large, but when we start dropping 50 mL of HCl, pH will start decreasing. At some time, time will come when 0.005 mol of HCl will be equal to moles of NaOH i-e 0.005 mol. This point is called equivalence point. At this point, the pH of the solution will be 7.
Last edited by Dhamnekar Winod on Tue Apr 27, 2021 8:33 am, edited 1 time in total.
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
ChenBeier
Distinguished Member
Posts: 743
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

### Re: Titration and pH computation

Please be aware that M is a concentration and not the same like moles. You mix it up.
The solution is 0,1 M equal to 0,1 mol/ l. The moles in 50 ml is 0.005 mol and not 0.005 M.
Dhamnekar Winod
Sr. Staff Member
Posts: 149
Joined: Sat Nov 21, 2020 10:14 am
Location: Mumbai[Bombay],Maharashtra State,India

### Re: Titration and pH computation

Thanks for pointing out my mistake. I corrected my mistake in my post. What would be indicator for the reaction in a) Will it be Phenol red?
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
ChenBeier
Distinguished Member
Posts: 743
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

### Re: Titration and pH computation

Indicator pH must be around +- 1 pH around equivalent point. Strong acid and strong Base means pH 6 to 8. Phenol red would be fine 6.4 - 8.2
Bromine thymolblue, neutral red,rosol acid, m-nitrophenol also possible.
Dhamnekar Winod
Sr. Staff Member
Posts: 149
Joined: Sat Nov 21, 2020 10:14 am
Location: Mumbai[Bombay],Maharashtra State,India

### Re: Titration and pH computation

Answer to a) At the beginning, pH of the solution, before adding any amount of NaOH, is 1(pH=-log(0.1 )=1). After adding 0.0025 moles of NaOH to 50.0 mL of HCl, at the mid-point, the pH will be 1.48(pH=-log(0.0025/0.0525)=1.32). At the equivalence point pH will be 7.

Is the titration in question b) strong acid strong base or weak acid strong base titration?

Is the titration in question c), weak base strong acid titration?
Last edited by Dhamnekar Winod on Wed Apr 28, 2021 6:58 am, edited 1 time in total.
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
ChenBeier
Distinguished Member
Posts: 743
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

### Re: Titration and pH computation

a is correct
b no. Check properties of Trifluorine acetic acid.
c yes
Dhamnekar Winod
Sr. Staff Member
Posts: 149
Joined: Sat Nov 21, 2020 10:14 am
Location: Mumbai[Bombay],Maharashtra State,India

### Re: Titration and pH computation

At the beginning, pH of the solution, before adding any amount of NaOH is 1.15(pH=-log(0.0707)=1.15). After adding 0.0025 mol of NaOH, to the 50 mL of CF3COOH, at the mid point, the pH of the solution is 1.48(pH=-log(0.0025/0.075)=1.48). At the equivalence point, pH will be 7

Last edited by Dhamnekar Winod on Wed Apr 28, 2021 8:53 am, edited 3 times in total.
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
ChenBeier
Distinguished Member
Posts: 743
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

### Re: Titration and pH computation

a) is now wrong, why you changed to 0,00525
You have 50 ml + 25 ml = 75 ml = 0,075 l
b) is wrong too
Dhamnekar Winod
Sr. Staff Member
Posts: 149
Joined: Sat Nov 21, 2020 10:14 am
Location: Mumbai[Bombay],Maharashtra State,India

### Re: Titration and pH computation

Thanks, you are correct. I wrongly computed the new volume after adding NaOH. I corrected my last post . You may check it now. Is my answer to b) is correct now?
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
ChenBeier
Distinguished Member
Posts: 743
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

### Re: Titration and pH computation

Now its correct. What is your opinion to trifluorine acetic acid.
Dhamnekar Winod
Sr. Staff Member
Posts: 149
Joined: Sat Nov 21, 2020 10:14 am
Location: Mumbai[Bombay],Maharashtra State,India

### Re: Titration and pH computation

Trifluoroacetic acid is a strong organic acid with pKa =0

My answer to c) At the beginning, the pH of 50 mL C6H15N is 7.49. At the midpoint, pH of 0.075 mL mixture of HCl and C6H15N will be log(0.025/0.075)=1.48 At the equivalence point, pH will be less than 7.
Note: Kb of C6H15N= 5.3e-4

Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
ChenBeier
Distinguished Member
Posts: 743
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

### Re: Titration and pH computation

No, its a weak base so you have to calculate with HENDERSON Hasselbalch equation.
Dhamnekar Winod
Sr. Staff Member
Posts: 149
Joined: Sat Nov 21, 2020 10:14 am
Location: Mumbai[Bombay],Maharashtra State,India

### Re: Titration and pH computation

So, my corrected answer to c)

At the beginning, the pH of 50 mL solution of C6H15N(triethyamine) is 10.72
At the mid-point, pH of 0.075 mL mixture of C6H15N and HCl is 10.72

At the equivalence point, pH of 0.100 mL solution of 0.1M C6H15N and 0.1 M HCl is 6.01.
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
ChenBeier
Distinguished Member
Posts: 743
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

### Re: Titration and pH computation

Show your calculation. At midpoint its wrong. HCl is not the point, which acid we are dealing with.
Dhamnekar Winod
Sr. Staff Member
Posts: 149
Joined: Sat Nov 21, 2020 10:14 am
Location: Mumbai[Bombay],Maharashtra State,India

### Re: Titration and pH computation

Yes. My pH computation at the midpoint for C6H15N + HCl reaction was wrong. Now, after working some more computations, at the mid-point, my answer for pH of 0.075mL mixture of HCl and C6H15N is 6.1.