How many of the following statements are correct for green vitriol?
1. it can be used for the preparation of Mohr's salt.
2. It is a hydrated salt and its anhydrous form is white in colour.
3. The decomposition reaction of its anhydrous form is redox in nature.
4. It is used for making fenton's reagent used as a catalyst in organic chemistry.
5. It can be used for the preparation of Potassium ferrocynide.
6. It has same number of molcules of water of crystallization as white vitriol or blue vitriol
7. It turns brownish-yellow on exposure to atmosphere.
8. Its aqueous solution is acidic in natrue due to hydrolysis.
9. Out of the decomposition products of its anhydrous form, only one can turn acidified `K_(2)Cr_(2)O_(7)` solution green.

Explanation

To determine how many of the statements about green vitriol (ferrous sulfate, FeSO4·7H2O) are correct, we will analyze each statement one by one.

1. **It can be used for the preparation of Mohr's salt.**
- Mohr's salt is represented as FeSO4·(NH4)2SO4·6H2O. Since green vitriol (FeSO4·7H2O) contains FeSO4, it can indeed be used to prepare Mohr's salt.
- **Conclusion:** Correct.

2. **It is a hydrated salt and its anhydrous form is white in color.**
- Green vitriol is indeed a hydrated salt with the formula FeSO4·7H2O. The anhydrous form (FeSO4) is white in color.
- **Conclusion:** Correct.

3. **The decomposition reaction of its anhydrous form is redox in nature.**
- The decomposition of FeSO4 can be represented as:
\[ 2FeSO4 \rightarrow Fe2O3 + SO2 + SO3 \]
In this reaction, the oxidation state of iron changes from +2 to +3 (oxidation), and sulfur changes from +6 to +4 (reduction). Thus, it is a redox reaction.
- **Conclusion:** Correct.

4. **It is used for making Fenton's reagent used as a catalyst in organic chemistry.**
- Fenton's reagent is typically made using hydrogen peroxide and iron (II) salts, including FeSO4. Therefore, green vitriol can be used to prepare Fenton's reagent.
- **Conclusion:** Correct.

5. **It can be used for the preparation of potassium ferrocyanide.**
- Potassium ferrocyanide (K4[Fe(CN)6]) can be prepared from FeSO4 and potassium cyanide (KCN). Thus, green vitriol can be used for this preparation.
- **Conclusion:** Correct.

6. **It has the same number of molecules of water of crystallization as white vitriol or blue vitriol.**
- White vitriol (ZnSO4·7H2O) has 7 water molecules, but blue vitriol (CuSO4·5H2O) has only 5 water molecules. Therefore, green vitriol does not have the same number of water molecules as blue vitriol.
- **Conclusion:** Incorrect.

7. **It turns brownish-yellow on exposure to atmosphere.**
- When exposed to air, FeSO4 can oxidize to Fe2O3, which has a brownish-yellow color. Thus, this statement is correct.
- **Conclusion:** Correct.

8. **Its aqueous solution is acidic in nature due to hydrolysis.**
- The aqueous solution of FeSO4 contains Fe²⁺ ions, which can hydrolyze to produce H⁺ ions, making the solution acidic. Therefore, this statement is correct.
- **Conclusion:** Correct.

9. **Out of the decomposition products of its anhydrous form, only one can turn acidified K2Cr2O7 solution green.**
- The decomposition products include Fe2O3, SO2, and SO3. Among these, only SO2 can reduce acidified K2Cr2O7 to Cr²⁺, which is green.
- **Conclusion:** Correct.

### Summary of Correct Statements:
- Correct statements: 1, 2, 3, 4, 5, 7, 8, 9 (8 statements are correct).
- Incorrect statement: 6.

### Final Answer:
**Total correct statements: 8**