**Solubility of Potassium Sulphate**

**Definition of Solubility:**
Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature and pressure. It is usually expressed in grams of solute per 100 grams of solvent.

**Given Information:**
- Mass of potassium sulphate (K2SO4) = 12g
- Mass of water (H2O) = 75g
- Temperature = 40°C

**Calculating Solubility:**
To calculate the solubility of potassium sulphate, we need to determine how much of the solute (K2SO4) can dissolve in the given amount of solvent (H2O).

1. Determine the molar mass of potassium sulphate (K2SO4):
- Atomic mass of potassium (K) = 39.10 g/mol
- Atomic mass of sulfur (S) = 32.07 g/mol
- Atomic mass of oxygen (O) = 16.00 g/mol
- Molar mass of K2SO4 = (2 * 39.10) + 32.07 + (4 * 16.00) = 174.26 g/mol

2. Calculate the number of moles of potassium sulphate:
- Moles = Mass / Molar mass
- Moles of K2SO4 = 12 g / 174.26 g/mol = 0.0688 mol

3. Calculate the solubility of potassium sulphate:
- Solubility = Moles of solute / Mass of solvent (in g) * 100
- Solubility = 0.0688 mol / 75 g * 100 = 0.0917 g/100g

**Answer:**
The solubility of potassium sulphate (K2SO4) in water at 40°C is 0.0917 g/100g.

**Explanation:**
The solubility of a substance depends on various factors such as temperature, pressure, and the nature of the solute and solvent. In this case, we are given the mass of potassium sulphate (K2SO4) and water (H2O) at a specific temperature.

By calculating the molar mass of potassium sulphate and the moles of solute, we can determine the solubility of potassium sulphate in water. The solubility is expressed as grams of solute per 100 grams of solvent.

In the given scenario, 12 grams of potassium sulphate can dissolve in 75 grams of water at 40°C. By calculating the solubility using the number of moles and mass of the solvent, we find that the solubility of potassium sulphate in water at 40°C is 0.0917 g/100g.

It is worth noting that solubility can vary with temperature. Generally, solubility increases with an increase in temperature for most solids, including potassium sulphate. However, this relationship may not hold true for all substances, as some exhibit different solubility trends with temperature changes.