Which of the following is not a correct formula?

Options

A. `H_2S`
B. `NaHSO_4`
C. `SiO_2`
D. CaCl

Explanation

To determine which of the given formulas is not correct, we need to analyze the valencies of the elements involved in each compound. Let's break down the analysis step by step:

### Step 1: Analyze H2S
- **Elements involved**: Hydrogen (H) and Sulfur (S).
- **Valency of Hydrogen**: +1 (H typically forms H⁺ ions).
- **Valency of Sulfur**: -2 (in the form of sulfide, S²⁻).
- **Combining the valencies**: To balance the charges, we need 2 H⁺ ions (total +2) to balance with 1 S²⁻ ion (total -2).
- **Correct formula**: H2S (This formula is correct).

### Step 2: Analyze NaHSO4
- **Elements involved**: Sodium (Na) and HSO4.
- **Valency of Sodium**: +1 (Na typically forms Na⁺ ions).
- **Valency of HSO4**: -1 (the sulfate ion HSO4⁻ has a charge of -1).
- **Combining the valencies**: The charges balance as +1 from Na and -1 from HSO4.
- **Correct formula**: NaHSO4 (This formula is correct).

### Step 3: Analyze CaCl
- **Elements involved**: Calcium (Ca) and Chloride (Cl).
- **Valency of Calcium**: +2 (Ca typically forms Ca²⁺ ions).
- **Valency of Chloride**: -1 (Cl typically forms Cl⁻ ions).
- **Combining the valencies**: To balance the charges, we need 1 Ca²⁺ ion (+2) and 2 Cl⁻ ions (-2).
- **Correct formula**: CaCl2 (The formula CaCl is incorrect).

### Conclusion
The incorrect formula among the options is **CaCl**. The correct formula should be **CaCl2**.

### Final Answer
The formula that is not correct is **CaCl**.

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