For the reaction, `4NH_(3)(g) + 5O_(2)(g)hArr4NO(g) + 6`
` H_(2)O(l)`, `Delta H =` positive. At equilibrium the factor that will not affect the concentration of `NH_(3)` is:

Options

A. change in pressure
B. change in volume
C. catalyst
D. None of these

Explanation

To solve the problem, we need to analyze the given reaction and determine which factor will not affect the concentration of ammonia (NH₃) at equilibrium. The reaction is:

\[ 4NH_3(g) + 5O_2(g) \rightleftharpoons 4NO(g) + 6H_2O(l) \]

Given that the change in enthalpy (\(\Delta H\)) is positive, this indicates that the reaction is endothermic.

### Step 1: Analyze the Effect of Pressure
- When pressure is applied to a gaseous equilibrium, the reaction will shift towards the side with fewer moles of gas.
- In this reaction, the total moles of gas on the reactant side is \(4 + 5 = 9\) moles, and on the product side, it is \(4 + 0 = 4\) moles (since water is a liquid and does not count).
- Therefore, increasing pressure will shift the equilibrium to the right, decreasing the concentration of NH₃. Thus, pressure does affect the concentration of NH₃.

**Hint:** Consider how changes in pressure influence the side of the reaction with fewer moles of gas.

### Step 2: Analyze the Effect of Volume
- Volume is inversely related to pressure. If the volume decreases, the pressure increases, and vice versa.
- A decrease in volume will similarly shift the equilibrium towards the side with fewer moles of gas, which again is the product side in this case.
- Hence, changing the volume will also affect the concentration of NH₃.

**Hint:** Remember that volume changes affect pressure and thus can shift the equilibrium.

### Step 3: Analyze the Effect of a Catalyst
- A catalyst speeds up the rate of both the forward and reverse reactions equally but does not change the position of equilibrium.
- It does not affect the concentrations of reactants or products at equilibrium; it only helps the system reach equilibrium faster.
- Therefore, the presence of a catalyst will not affect the concentration of NH₃.

**Hint:** Think about the role of a catalyst in a reaction and its effect on equilibrium.

### Step 4: Analyze the Effect of Temperature
- While the question does not explicitly mention temperature, it is important to note that changing temperature can affect the equilibrium position, especially in endothermic reactions.
- For an endothermic reaction, increasing the temperature shifts the equilibrium to the right, which would decrease the concentration of NH₃.

**Hint:** Consider how temperature changes influence the direction of the equilibrium shift.

### Conclusion
The factor that will not affect the concentration of NH₃ at equilibrium is the presence of a catalyst. Therefore, the correct answer is:

**Answer:** Catalyst