While demonstrating acid-base reactions, Ms. Prabha, a science teacher added sodium oxide to HCl until it was in excess. Which of the following graphs correctly represents the change in pH?

Options

A.
B.
C.
D.

Explanation

To solve the question about the change in pH when sodium oxide is added to hydrochloric acid (HCl), we can follow these steps:

### Step 1: Understand the Reaction
Sodium oxide (Na2O) is a basic oxide and will react with hydrochloric acid (HCl) to form sodium chloride (NaCl) and water (H2O). The reaction can be represented as:
\[ \text{Na}_2\text{O} + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} \]

### Step 2: Initial pH of HCl
Hydrochloric acid is a strong acid, and its pH is typically less than 7. The exact pH depends on the concentration of HCl, but we can assume it starts at a low value (e.g., around 1-3).

### Step 3: Adding Sodium Oxide
As sodium oxide is added to the HCl solution, it will start to neutralize the acid. Initially, the pH will begin to rise slowly as the acid is neutralized.

### Step 4: Reaching Neutralization Point
At a certain point, all the HCl will be neutralized by the sodium oxide, and the pH will reach around 7, which is neutral. This is the point of neutralization.

### Step 5: Excess Sodium Oxide
Once all the HCl is neutralized, any additional sodium oxide will cause the pH to increase further, as the solution will now be basic. The pH will continue to rise but at a slower rate until it stabilizes at a higher value (greater than 7).

### Step 6: Graph Interpretation
The graph that represents this scenario would show:
- A gradual increase in pH as sodium oxide is added.
- A sharp increase at the neutralization point.
- A plateau or constant increase after the neutralization point due to the excess sodium oxide.

Based on these observations, the correct graph would be the one that shows a gradual increase in pH followed by a plateau, indicating that after the neutralization point, the pH stabilizes with excess sodium oxide.

### Conclusion
The correct graph that represents the change in pH when sodium oxide is added to hydrochloric acid until it is in excess is **Graph 1**.

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