H2S and Its Qualitative Analysis:
H2S, also known as hydrogen sulfide, is a toxic gas that is commonly recognized by its strong rotten egg-like smell. Despite its unpleasant odor, H2S has various applications in different industries, including its use in qualitative analysis.

Solubility of H2S in Water:
At standard temperature and pressure (STP), the solubility of H2S in water is stated to be 0.195 M. This means that 0.195 moles of H2S can dissolve in 1 liter of water at STP.

Henry's Law and Constants:
Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. Mathematically, it can be expressed as:
C = k * P

Where:
C is the concentration of the gas in the liquid (mol/L),
k is Henry's constant (mol/(L·atm)),
P is the partial pressure of the gas (atm).

Calculating Henry's Constant:
To calculate Henry's constant for H2S, we need to use the solubility value and the partial pressure of H2S at STP.

Given:
Solubility of H2S in water at STP = 0.195 M

At STP, the partial pressure of any gas is equal to 1 atm. Hence, the partial pressure of H2S is also 1 atm.

Using Henry's Law equation, we can rearrange it as:
k = C / P

Substituting the given values:
k = 0.195 M / 1 atm

Therefore, Henry's constant for H2S is 0.195 mol/(L·atm).

Conclusion:
H2S is a toxic gas with a rotten egg-like smell, and it is commonly used in qualitative analysis. The solubility of H2S in water at STP is 0.195 M. Henry's Law relates the solubility of a gas in a liquid to its partial pressure, and the constant of proportionality is known as Henry's constant. By using the solubility value and the partial pressure of H2S, we can calculate Henry's constant, which in this case is found to be 0.195 mol/(L·atm).