# Solving Copper (II) Salt Titration Problem

• Wrichik Basu
In summary, the conversation discusses a method for determining the percentage of copper in a Copper (II) salt using thiosulphate titration. The conversation includes a calculation using both stoichiometry and equivalents, with the conclusion that the correct answer is 25.5.
Wrichik Basu
Gold Member

## Homework Statement

:[/B]

The percentage of copper in a Copper (II) salt can be determined by using a thiosulphate titration. 0.305g of a copper (II) salt was dissolved in water and added to an excess of KI solution liberating Iodine. The liberated Iodine required 24.5 ##dm^3## of a 0.1 mole ##dm^{-3}## solution of sodium this sulphate. The percentage of copper, by mass, in the copper (II) salt is:

1. 64.2
2. 51.0
3. 48.4
4. 25.5

:[/B]

## The Attempt at a Solution

:[/B]

0.1 mole per ##dm^3## means 0.1 mole per litre.

Valence factor for sodium thiosulphate is 2.

So, there are 0.1×2=0.2 eqs of thiosulphate in 1 litre, which basically means the normality is 0.2.

No. of eqs of ##Cu^{2+}## = No. of eqs of Iodine liberated = No. of eqs of thiosulphate used = ##\frac {24.5}{1000} ×0.2 = 4.9 ×10^{-3}##.

So, weight of ##Cu^{2+}## ions = ##4.9 ×10^{-3} ×63.5=0.31115g##, which is greater than the weight of the sample.

Where am I going wrong?

Start by finding the equation of the reaction between iodine and thiosulfate.

Borek said:
Start by finding the equation of the reaction between iodine and thiosulfate.
The balanced equations are:
$$2Cu^{2+}+4I^{-} \rightarrow 2CuI+I_2$$
$$2S_2 O_3 ^{2-} +I_2 \rightarrow 2I^{-} +S_2 O_6^{2-}$$

Borek said:
Can't I do it by equivalents? Or does that concept fail here?

It doesn't fail when applied correctly.

Borek said:
It doesn't fail when applied correctly.
Where am I going wrong?

Follow the stoichiometry and you will find out.

Borek said:
Follow the stoichiometry and you will find out.
By stoichiometry I get answer 25.5.

But if I follow equivalents and do ##4.9×10^{-3}×\dfrac {63.5}{2}## (without reason why I divided by 2), I get 51.0, which is double the first answer.

Which is correct?

I told you - stoichiometry is right. Equivalents are a proxy for stoichiometry, and they can be quite convenient, but not when applied blindly. Your calculation of what the equivalent is was wrong (you calculated an equivalent for the reaction with H+, which is not what is happening here) so you got the wrong answer.

Wrichik Basu

## 1. What is a copper (II) salt titration?

A copper (II) salt titration is a laboratory technique used to determine the concentration of a solution containing copper ions. This is done by adding a solution of known concentration (titrant) to the copper solution until the reaction is complete, and then calculating the concentration of the copper solution based on the amount of titrant used.

## 2. What equipment is needed for a copper (II) salt titration?

The equipment needed for a copper (II) salt titration includes a burette, pipette, conical flask, volumetric flask, and a stirring rod. Other common laboratory equipment such as beakers, graduated cylinders, and a balance may also be used.

## 3. How do you determine the endpoint in a copper (II) salt titration?

The endpoint in a copper (II) salt titration is determined by the use of an indicator, which changes color when the reaction is complete. Common indicators for copper titrations include potassium chromate and potassium iodide. The endpoint is reached when the indicator changes color permanently, indicating that all of the copper ions have reacted with the titrant.

## 4. What are some sources of error in a copper (II) salt titration?

Some sources of error in a copper (II) salt titration may include inaccurate measurements of the solutions, human error in reading the burette or pipette, and contamination of the solutions. Other factors such as temperature changes or improper mixing of the solutions can also affect the accuracy of the titration.

## 5. How can the results of a copper (II) salt titration be used in real-world applications?

The results of a copper (II) salt titration can be used in many real-world applications, including environmental monitoring, quality control in industries such as mining and electroplating, and in the production of medicines and other products that require precise measurements of copper concentrations. Titration techniques are also commonly used in academic research to study chemical reactions and analyze unknown substances.

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