Which of the following is the formula of Sorel’s cement?
A. KCL.MgCl2.6H2O

B. MgCl2.5MgO.XH2O

C. CaSO4.2H2O

D. MgCO3.CaCO3.

Sorel’s cement is a type of magnesium oxychloride cement. Its formula is typically expressed as a combination of magnesium chloride and magnesium oxide.

Let’s break down the options:

**A. KCl.MgCl2.6H2O**
- This is a mixture of potassium chloride, magnesium chloride, and water. It’s not Sorel’s cement.

**B. MgCl2.5MgO.XH2O**
- This formula corresponds to magnesium chloride and magnesium oxide with water. This is consistent with Sorel’s cement, where \( X \) typically represents water of hydration.

**C. CaSO4.2H2O**
- This is gypsum, not Sorel’s cement.

**D. MgCO3.CaCO3**
- This is a mixture of magnesium carbonate and calcium carbonate, not related to Sorel’s cement.

So the correct answer is **B. MgCl2.5MgO.XH2O**.