Predict the formula of neutral ionic compound formed by K+ and S2-?

K+ and S^2- .....

One would think that you might have potassium sulfide, K2S, but that's more difficult than you think. K2S won't form from a solution of the ions. Sulfide ion in water is HS^-. If you had any solid K2S and exposed it to air, it would react with the water in the air to make KOH and KHS.

As for being "ionic", K-S bonds have about 54% ionic character (46% covalent character), so it's close to the middle of the "ionic-covalent" spectrum. That also suggests that there are no actual K+ ions or S^2- ions in solid K2S. Instead, the charges on K and S would be less than +1 and -2.

Each K⁺ ion carries 1 positive charge, while each S²⁻ ion carries 2 negative charges.

To make the charge to be neutral, each formula unit should contains 2 K⁺ ions and 1 S²⁻ ion.

Hence, the formula is K₂S.