Write formulas for the following compounds
(i) Mercury (II) chloride (ii) Nickel (II) sulphate
(iii) Tin(IV) oxide (iv) Thallium (I) sulphate
(v) Iron (III) sulphate (vi) Chromium (III) oxide.

Explanation

To write the formulas for the given compounds, we will follow a systematic approach, identifying the symbols and oxidation states of the elements involved, and then applying the criss-cross method to determine the correct formulas.

### Step-by-Step Solution:

1. **Mercury (II) Chloride**:
- **Identify the symbols**: Mercury is represented as **Hg** and has an oxidation state of **+2**. Chloride is represented as **Cl** and has a charge of **-1**.
- **Apply the criss-cross method**: The formula will be HgCl₂ (the 2 from Cl becomes the subscript for Hg).
- **Final formula**: **HgCl₂**
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