Hess’s law is based on :

Options

A. Law of conservation of mass
B. Law of conservation of energy
C. First law of thermodynamics
D. None of the above

Explanation

To determine what Hess's law is based on, we can analyze the options given in the question. Here’s a step-by-step solution:

### Step 1: Understanding Hess's Law
Hess's law states that the total enthalpy change for a chemical reaction is the same, regardless of whether the reaction occurs in one step or multiple steps. This means that the enthalpy change is a state function, which depends only on the initial and final states of the system.

**Hint:** Recall that Hess's law is related to the concept of state functions in thermodynamics.

### Step 2: Analyzing the Options
The options provided are:
1. Law of conservation of mass
2. Law of conservation of energy
3. First law of thermodynamics
4. None of the above

**Hint:** Think about how energy is conserved in chemical reactions and how that relates to Hess's law.

### Step 3: Identifying the Correct Basis
Hess's law is fundamentally based on the law of conservation of energy. This law states that energy cannot be created or destroyed, only transformed from one form to another. Since enthalpy is a measure of energy in a system, the total change in enthalpy for a reaction must remain constant regardless of the pathway taken.

**Hint:** Consider how the enthalpy changes in different steps of a reaction relate to the overall energy change.

### Step 4: Conclusion
Thus, the correct answer to what Hess's law is based on is the **law of conservation of energy**.

**Final Answer:** Option 2 - Law of conservation of energy.