To determine which ionic compound is water-soluble, we need to consider the solubility rules in chemistry. Let's examine each of the given compounds in detail:
1. CuS (Copper(II) sulfide)
- Sulfides (S^2-) are generally insoluble in water except for those containing alkali metals (like Li^+, Na^+, K^+, Rb^+, Cs^+) and ammonium (NH4^+). CuS is not one of these exceptions and is therefore insoluble in water.
2. NH4NO3 (Ammonium nitrate)
- Nitrates (NO3^-) are generally soluble in water, with no common exceptions. Hence, ionic compounds that contain the nitrate anion (NO3^-) are soluble. Since NH4NO3 contains the nitrate ion, it is soluble in water.
3. SrSO4 (Strontium sulfate)
- Sulfates (SO4^2-) are generally soluble in water, but there are a few exceptions. Sulfates of barium (Ba^2+), strontium (Sr^2+), lead (Pb^2+), calcium (Ca^2+), and mercury (Hg2^2+) are notably insoluble. Therefore, SrSO4 is insoluble in water.
4. ZnF (Zinc fluoride)
- Fluorides (F^-) are generally soluble in water, but there are exceptions. Some fluorides, such as those of magnesium (Mg^2+), calcium (Ca^2+), strontium (Sr^2+), barium (Ba^2+), and lead (Pb^2+), are insoluble. However, ZnF2 (zinc fluoride) is soluble in water, so this would be another candidate. But considering our initial examination, NH4NO3 follows a clear-cut rule of solubility without exceptions.
Based on the solubility rules discussed, the water-soluble ionic compound from the given list is:
[tex]\[
NH_4NO_3
\][/tex]
Therefore, the answer is:
[tex]\[
2 \quad \text{(Index of NH4NO3 in the given list)}
\][/tex]
The index of NH4NO3 in the given list is 2.
