Acid-Base Titration: Solve Question w/ Phenolphthalein & Methyl Orange

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In summary, the question is about an acid-base titration involving a 25cm^3 solution containing 0.5M sodium carbonate and 0.5M sodium hydrogencarbonate being titrated with 1M HCl using phenolphthalein and methyl orange indicators. After 12.5cm^3 of HCl is added, the phenolphthalein indicator is decolourized. The volume of HCl needed to change the colour of methyl orange from yellow to orange is either 25cm^3 or 37.5cm^3, depending on whether it is added to the solution that has reached the first end point or to the remaining solution. The methyl orange changes color at a pH of
  • #1
Clari
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Hello! Here is a acid-base titration question which I am a bit confused at, please help me with it.

If 25cm^3 solution containing 0.5M sodium carbonate and 0.5M sodium hydrogencarbonate is titrated with 1M HCl using phenolphthalein indicator. Upon the addition of 12.5cm^3 HCl, the phenolphthalein is decolourized. After the 1st end point, methyl orange indicator is added and the solution is further titrated with HCl. What is the volume of HCl needed the change the colour of methyl orange from yellow to orange?

My teacher told me it was 37.5 cm^3, but I don't understand it. I think it is 25 cm^3...
 
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  • #2
Write down reaction that takes place before first endpoint.

How much HCl do you need to reach first endpoint?

Write down reaction that takes place before second endpoint.

How much HCl do you need to reach second endpoint?

Calculate total amount of HCl needed.

Try these questions and - if you will be still not able to find out why it is 37.5 cc - post your answers so that we will be able to show you where you have gone wrong.

Unless someone decide to show you results of calculations before checking out if you are not able to do it by yourself :(


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  • #3
God I swear why do they still do A-B titrations in school. They are totally worthless, not one chemist in the real world does A-B titrations like they are taught to undergraduates.
 
  • #4
Hello Borek, Thank you for your help. however, I am still not able to understand why it is 37.5 cc. Here are my steps:
The reaction that happens to reach the first end point is:
sodium carbonate + HCl --> sodium hydrogencarbonate + NaCl
Since there are 0.025*0.5= 0.0125 mol of sodium carbonate, 0.0125 mol of HCl is needed. That is, 12.5 cc of HCl is needed to reach the first end point.

The reaction that happens to reach the second end point it:
sodium hydrogencarbonate + HCl --> NaCl + water + carbon dioxide
Since there are 0.0125 mol sodium hydrogencarbonate originally,adds up with the 0.0125 mol of sodium hydrogencarbonate formed before the first end point, there will be totally 0.025 mol of sodium hydrogencarbonate. Therefore, 0.025 mol, and thus, 25 cc of HCl is needed to reach the second end point.

Therefore the total amount of HCl added is 37.5 cc.
But, in my notes, it says " Upon the addition of 37.5 cc of HCl, the methyl orange changes from yellow to orange". I don't think it is right, because that just means 50 cc of HCl is needed in total...or I have mistaken something? please tell me.
 
  • #5
Clari said:
Hello! Here is a acid-base titration question which I am a bit confused at, please help me with it.

If 25cm^3 solution containing 0.5M sodium carbonate and 0.5M sodium hydrogencarbonate is titrated with 1M HCl using phenolphthalein indicator. Upon the addition of 12.5cm^3 HCl, the phenolphthalein is decolourized. After the 1st end point, methyl orange indicator is added and the solution is further titrated with HCl. What is the volume of HCl needed the change the colour of methyl orange from yellow to orange?

My teacher told me it was 37.5 cm^3, but I don't understand it. I think it is 25 cm^3...

Just as a question what does methyl orange actually indicate exactly? Like when does it actually change color?
It looks like you might be right... though, maybe your teacher had mistaken the total HCl added instead...
 
  • #6
Clari said:
Therefore the total amount of HCl added is 37.5 cc.
But, in my notes, it says " Upon the addition of 37.5 cc of HCl, the methyl orange changes from yellow to orange". I don't think it is right, because that just means 50 cc of HCl is needed in total...or I have mistaken something? please tell me.

It is not 'upon addition of 37.5 cc more'!

Titration goes as follows:
0 cc titrant added - phenolophtaleine visible
12.5 cc titrant added - phenolophtaleine color disappears
now you add methyl orange and the solution is yellow
total of 37.5 cc titrant added - solution changes to orange

So you have to add either 25 cc of titrant to the solution that have reached first end-point, or total of 37.5 cc of titrant to the not yet titrated solution.

Sorry if my explanation is not readable or wrong - I did my best :blushing: but English is my second language and perhaps I have missed something.


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  • #7
apchemstudent said:
Just as a question what does methyl orange actually indicate exactly? Like when does it actually change color?...
I think it is about 4.0 that makes the methyl orange to change from yellow to orange.

borek said:
So you have to add either 25 cc of titrant to the solution that have reached first end-point, or total of 37.5 cc of titrant to the not yet titrated solution.
It is not 25 cc of the titrant added which makes the solution to reach the first end point...it should be the second end point.. :wink:
 

1. What is acid-base titration?

Acid-base titration is a laboratory technique used to determine the concentration of an acid or base in a solution. It involves adding a known amount of a base or acid to a known volume of the solution being tested, until the reaction reaches its equivalence point.

2. What is the purpose of using phenolphthalein and methyl orange in acid-base titration?

Phenolphthalein and methyl orange are indicators commonly used in acid-base titration. They change color at different pH levels, allowing the experimenter to visually determine when the reaction has reached its equivalence point.

3. How do you calculate the concentration of an acid or base using acid-base titration?

To calculate the concentration of an acid or base, you need to know the volume and concentration of the other solution used in the titration, as well as the volume of the solution being tested. By using the formula C1V1 = C2V2, you can determine the concentration of the solution being tested.

4. What is the difference between a strong acid-strong base titration and a weak acid-strong base titration?

In a strong acid-strong base titration, the reaction between the acid and base is complete, resulting in a rapid change in pH at the equivalence point. In a weak acid-strong base titration, the reaction is not complete, resulting in a gradual change in pH at the equivalence point.

5. What are some sources of error in acid-base titration experiments?

Some sources of error in acid-base titration experiments include inaccurate measurements of the solutions, improper calibration of equipment, and the presence of impurities in the solutions. Other factors such as temperature changes and human error can also affect the accuracy of the results.

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