I also had a problem with this type of questionthe initial pressure of NOCl(g) is 4.329 atm, calculate the % of NOCl(g) left over after the reaction reaches equilibrium according to the balanced equation. The value of Kp at 400.0 °C is 1.99. The initial pressure of the reaction products is 0 atm.

2NOCl(g) = 2NO(g)+Cl2(g)

initial:

NOCl = 4.329
NO = 0
Cl2 = 0

change:
NOCl = -2x
NO = +2x
Cl2 = x

equilibrium:
NOCl = 4.329 - 2x
NO = 2x
Cl2 = x

Substitute into Kp and solve for x, multiply by 2 and subtract from original. That gives pressure of NOCl at equilibrium. I don't know if percent is to be based on pressure or not. I would think it would be based on grams but I don't see a way to quickly convert to grams.One way might be to convert Kp to Kc and find concns at equilibrium, then tak a liter.

1.99 = (3x)/(4.329-2x)

8.615 - 3.98x = 3x
8.615 = 6.98x
x = 1.23

NO = 2.46 atm
Cl2 = 1.23 atm

The question now is, how to find the % NOCl left over?

To calculate the % of NOCl(g) left over after the reaction reaches equilibrium, we need to make use of the equilibrium constant (Kp) and the initial pressure of NOCl(g) (denoted as P_initial).

Step 1: Convert the initial pressure of NOCl(g) to the concentration (in atm) by dividing it with the total pressure of the system. Since the initial pressure of the reaction products is 0 atm, the total pressure is solely equal to the initial pressure of NOCl(g). Let's call this concentration [NOCl].

[NOCl] = P_initial / P_total

Step 2: Use the balanced equation and stoichiometry to determine the relationship between the concentrations of NOCl(g), NO(g), and Cl2(g).

From the balanced equation: 2NOCl(g) = 2NO(g) + Cl2(g)

This implies that the concentrations of NO(g) and Cl2(g) are each half of the concentration of NOCl(g) at equilibrium.

[NO] = [Cl2] = [NOCl] / 2

Step 3: Substitute the concentrations into the expression for the equilibrium constant (Kp) and solve for [NOCl].

Kp = ([NO] / [NOCl])^2 * ([Cl2] / [NOCl])

Since [NO] = [Cl2] = [NOCl] / 2, we can rewrite the equation as:

Kp = ([NO] / [NOCl])^2 * ([NO] / [NOCl]) = ([NO] / [NOCl])^3

Rearranging the equation gives us:

[NOCl] = ([NO] / Kp)^(1/3)

Step 4: Plug in the values of Kp and [NO] into the equation to calculate [NOCl].

[NOCl] = ([NO] / Kp)^(1/3)

Step 5: Calculate the percentage of NOCl(g) remaining by expressing [NOCl] as a percentage of the initial concentration of NOCl(g).

% remaining = ([NOCl] / P_initial) * 100

By following these steps, you should be able to calculate the % of NOCl(g) left over after the reaction reaches equilibrium.