# Double titration and law of equivalence

• Krushnaraj Pandya
In summary, the conversation discusses the concept of titration and the use of different indicators in titrations. It also explains the equations for calculating equivalents in titrations and the protonation of CO32- to HCO3-. The conversation also touches upon the concept of chemical and ionic equilibria.
Krushnaraj Pandya
Gold Member

## Homework Statement

1) for a given reaction to consume one reactant completely, must the equivalents of both reactants be same? for example, I know in the reaction of HCl + NaOH - the equivalents of HCl=equivalents of NaOH for a titration, is it the same for Na2CO3 + HCl?
2) the following is an excerpt from my textbook-
Double titration: This is a titration of specific compound using different indicators. When the solution containing NaOH and Na2CO3 is titrated using phenolpthalein indicator, the following reaction takes place at the phenolpthalein end point.
NaOH + HCl - NaCl + H2O
Na2CO3 + HCl - NaHCO3 + NaCl
Here, Equivalents of NaOH + 1/2 equivalents of Na2CO3= equivalents of HCl...(i)
When methyl orange is used Na2CO3 is converted into NaCl + CO2 + H2O
Hence equivalents of NaOH + eqs. of Na2CO3=eqs. of HCl...(ii)

I dont't understand how we got the first equation and this titration in general, can someone please explain?

Delta2
Krushnaraj Pandya said:
When methyl orange is used Na2CO3 is converted into NaCl + CO2 + H2O

No. After phenolphthaleine endpoint there were no Na2CO3 in the solution. CO32- was converted into HCO3- and it is HCO3- that reacts in the second titration stage.

Borek said:
No. After phenolphthaleine endpoint there were no Na2CO3 in the solution. CO32- was converted into HCO3- and it is HCO3- that reacts in the second titration stage.
ok...but my doubts still remain,can you explain how we got both these equations and the titration in some detail? I'd be really grateful, thank you

Not clear to me what it is that you don't understand, it is just about knowing what reacts and balancing reaction equations.

What reacts can be predicted knowing the final pH and Ka values of acids present.

Borek said:
Not clear to me what it is that you don't understand, it is just about knowing what reacts and balancing reaction equations.

What reacts can be predicted knowing the final pH and Ka values of acids present.
why is the coefficient of Na2CO3 in the first equation 1/2? I know its n-factor is 2 but how does that come into play here?

Krushnaraj Pandya said:
why is the coefficient of Na2CO3 in the first equation 1/2? I know its n-factor is 2 but how does that come into play here?
hello?

Write equation for the reaction in which CO32- is protonated to HCO3-.

Krushnaraj Pandya said:
for a given reaction to consume one reactant completely, must the equivalents of both reactants be same?For example, I know in the reaction of HCl + NaOH - the equivalents of HCl=equivalents of NaOH
No.
H2SO4 +2NaOH →Na2SO4 + 2H2O
For this, 2(Eq. of NaOH)=Eq. of H2SO4
This is because H2SO4 ionizes as
H2SO4→2H+ + SO42- ......*
Hence, as you see, for each mole of H2SO4, there are 2 moles of H+ ions formed. So, you need an equal quantity (2 mol) of OH- ions (to form the water), which are furnished by NaOH. And hence,
2(Eq. of NaOH)=Eq. of H2SO4
*You will understand this better after you do the topic of Chemical & Ionic Equilibria

baldbrain said:
For this, 2(Eq. of NaOH)=Eq. of H2SO4

I suggest you check the definition of the equivalent.

Got it! Thanks a lot

Borek said:
I suggest you check the definition of the equivalent.
All right, my bad. Let's just say that NaOH & H2SO4 react in the ratio 2:1.
I was just saying from the point of view of the H+ & OH- ions in the reaction.

Delta2

## 1. What is double titration?

Double titration is a chemical analytical method that involves using two separate titrations to determine the concentration of a substance in a solution. It is useful for determining the amount of acid or base in a solution, as well as for identifying unknown substances.

## 2. How is double titration performed?

The first titration is performed with a known concentration of an acid or base, which reacts with the unknown substance to form a salt. The second titration is then performed with a different known concentration of the same acid or base, which reacts with the remaining salt. By measuring the volume of each titrant used, the concentration of the unknown substance can be calculated.

## 3. What is the law of equivalence in double titration?

The law of equivalence states that in a chemical reaction, the number of moles of one substance equals the number of moles of the other substance involved. In double titration, this means that the number of moles of the unknown substance is equal to the number of moles of the titrant used in the second titration.

## 4. What is the purpose of using double titration?

Double titration is used to determine the concentration of an unknown substance in a solution. It is a more accurate method than single titration, as it takes into account any impurities or other substances that may be present in the solution.

## 5. What are some common sources of error in double titration?

Some common sources of error in double titration include incorrect measurements of titrant volumes, presence of impurities in the solution, and incomplete reactions. It is important to carefully follow the procedure and use precise measuring tools to minimize these errors.

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