# A2 OCR Planning Help: Determine Chlorine Concentration by Back Titration

• vikhas786
In summary, the conversation is about determining the concentration of a saturated aqueous solution of chlorine through back titration. The process involves measuring out specific amounts of chlorine and iron (II) and combining them in a conical flask. The reaction between the two substances requires a 2:1 molar ratio, with the concentration of iron needing to be twice that of chlorine. KMnO4 is then titrated into the solution until it turns pink, and the volume of KMnO4 used is recorded. The results are analyzed and the concentration of chlorine in the solution is calculated. The use of safety precautions and the importance of repeating the procedure multiple times for accuracy are also mentioned.
vikhas786
Have a read and post what you think,

Question - Determine the concentration of a saturated aqueous solution of chlorine by back titration.

1. Using measuring cylinders, measure out 25cm3 (enough for reliable results but not wasteful) of chlorine and 40cm3 (excess amount) of reducing agent Iron (II) and place into separate beakers.

Add the contents of the two beakers into one conical flask. Swirl the conical flask thoroughly for a minute.

The half-equations when chlorine and Iron (II) react are:

Fe2+--> Fe3++ 1e-
Cl2+ 2e- --> 2Cl-

doubling the first (Chlorine requires 2 electrons) and adding the 2 half equations together:

2Fe2++ Cl2 --> 2Fe3++ + 2Cl-

Therefore for this reaction to take place completely you need to have a 2:1 molar ratio.

Concentration of chlorine = mass/RMM = 7g/71 =0.1moldm-3

Concentration of Iron needs to be twice the amount of chlorine, 0.1moldm-3 x2 = 0.2moldm-3.

2. Fill a burette with KMnO4 (which contains 3.50g dm-3 of solid) and titrate into the conical flask containing Iron (II) and chlorine solution. When the solution turns pink, stop the titration and record the volume of KMnO4 used.

Since you used an excess of FeSO4, you will have some left over Fe(II) ions in solution. This will be titrated against KMnO4.

3.Repeat this procedure two more times.A mean volume value of KMnO4 is far more reliable than a one-off result.

Risk Assessment

Wear safety gloves and goggles to prevent getting KMnO4 (an irritant) on your skin. In the event of broken glass, tell a supervisor who will clear it up.

Analysis of results

Step 1

Amount of chlorine used (in moles): 0.1 moldm-3 x 0.025 = 0.0025 moles
Amount of Iron (II) used (in moles): 0.2 moldm-3 x 0.040 = 0.008 moles

Step 2

The reaction of the titration is:

5Fe2++ + MnO4- + 8H+ --> Mn2+ + 5Fe3+ + 4H2O

Therefore 5 moles of Iron (II) reacts with 1 mole of Manganate. To calculate the amount of excess Fe2+ ions, first work out the moles of indicator used.

Concentration of indicator is Mass/RMM = 3.5g/158 = 0.0222 moldm-3

Number of moles in indicator = 0.0222 x mean volume of indicator used in titration

Multiply this value by 5 to give the amount of Fe+2 moles.

Subtracting the total original moles of Fe+2 used (0.008 moles) by the moles of excess Fe2+ gives moles of iron that reacted with chlorine.

Divide this value by 2 (due to the 2:1 molar ratio) to give the moles of chlorine used

Finally, to answer the aim, the concentration of chlorine in an aqueous solution = moles of chlorine/ 0.025 dm3 (volume of chlorine)

Is it correct? Is it ok that I assumed the mass of KmnO4 to be 3.5g?

It has to be in by next week, so please help!

You'll get more replies if you would be more specific.

## 1. What is back titration and why is it used in determining chlorine concentration?

Back titration is a technique used in chemistry to determine the concentration of a substance that is present in a low quantity. It involves adding an excess of a known concentration of a reactant to the sample, allowing it to react, and then determining the remaining amount of the reactant through a titration. This method is used for substances that are difficult to directly titrate, such as chlorine, as it allows for more accurate results.

## 2. What materials are needed for a back titration to determine chlorine concentration?

The materials needed for a back titration to determine chlorine concentration include a burette, a pipette, a conical flask, a standard solution of known concentration, an indicator, and the sample containing the chlorine.

## 3. What is the step-by-step process for conducting a back titration to determine chlorine concentration?

The first step is to measure out a known volume of the sample containing the chlorine using a pipette. Next, add an excess of a standard solution of known concentration to the sample in a conical flask. Allow the reaction to occur, and then titrate the remaining excess of the standard solution with a burette using an indicator. The endpoint of the titration is when the color of the solution changes, indicating that all of the reactants have been used up. Finally, use the volume and concentration of the standard solution to calculate the concentration of the chlorine in the sample.

## 4. What are some potential sources of error when using a back titration to determine chlorine concentration?

Potential sources of error when using a back titration to determine chlorine concentration include inaccurate measurements of the sample and standard solution, incomplete reactions, and errors in reading the burette or pipette. It is important to carefully measure and record all volumes and concentrations to minimize errors.

## 5. Are there any safety precautions that should be taken when conducting a back titration to determine chlorine concentration?

Yes, some safety precautions to take include wearing appropriate protective gear, such as gloves and goggles, as the chemicals used can be harmful if they come into contact with skin or eyes. It is also important to work in a well-ventilated area and dispose of all chemicals properly after the experiment.

Replies
2
Views
3K
Replies
5
Views
5K
Replies
10
Views
2K
Replies
8
Views
5K
Replies
1
Views
2K
Replies
2
Views
21K
Replies
1
Views
3K
Replies
1
Views
16K
Replies
1
Views
27K
Replies
3
Views
11K