Partial Pressure at Equilibrium Calculator

Calculator Inputs

Use A, B, C, and D as placeholders for gaseous species. Set any unused species coefficient to zero.

Kp

Coefficient of A

Coefficient of B

Coefficient of C

Coefficient of D

Initial Pressure of A (atm)

Initial Pressure of B (atm)

Initial Pressure of C (atm)

Initial Pressure of D (atm)

Reset

Example Data Table

Reaction Form	Kp	a	b	c	d	PA₀	PB₀	PC₀	PD₀	Expected Shift
A + B ⇌ 2C	1.80	1	1	2	0	1.20	0.80	0.10	0.00	Toward products
2A ⇌ C + D	0.65	2	0	1	1	1.60	0.00	0.20	0.30	Toward reactants
A ⇌ C	3.40	1	0	1	0	0.90	0.00	0.15	0.00	Toward products

Formula Used

For a general gas reaction, use:

aA + bB ⇌ cC + dD

The equilibrium constant in pressure form is:

Kp = (P_C^c × P_D^d) / (P_A^a × P_B^b)

Let x be the reaction extent. Then each equilibrium pressure becomes:

P_A,eq = P_A,0 − ax
P_B,eq = P_B,0 − bx
P_C,eq = P_C,0 + cx
P_D,eq = P_D,0 + dx

The calculator substitutes these expressions into the Kp equation and solves for x numerically. After finding x, it calculates every equilibrium partial pressure.

How to Use This Calculator

Enter the equilibrium constant, Kp, for your temperature.
Enter stoichiometric coefficients for reactants and products.
Enter the initial partial pressure of each gas species.
Set any unused species coefficient to zero.
Click the calculation button.
Read the result table, direction prediction, and plotted pressure lines.
Use the CSV or PDF buttons for reporting.

Frequently Asked Questions

1) What does this calculator solve?

It solves equilibrium partial pressures for gaseous reactions using Kp, stoichiometric coefficients, and initial pressures. It also estimates the reaction shift direction and plots pressure changes against reaction extent.

2) Can I leave a species unused?

Yes. Set that species coefficient to zero. Its pressure then stays unchanged, and it will not affect the equilibrium expression.

3) Why does the calculator use x?

x represents the extent of reaction. It links stoichiometric change to every species pressure, making one solvable equation from many related pressure changes.

4) What units should I use?

Use consistent pressure units for all species. The form labels atm, which is the most common choice for Kp problems.

5) Why can the predicted direction change?

The calculator compares the initial reaction quotient, Qp, with Kp. If Qp is smaller, the reaction moves toward products. If Qp is larger, it moves toward reactants.

6) Does this work for real named gases?

Yes. A, B, C, and D are placeholders. You can map them to gases like H₂, I₂, HI, N₂O₄, or NO₂.

7) Why might I get an invalid interval message?

That usually means the entered coefficients and initial pressures produce an impossible extent range. Negative equilibrium pressures are not physically allowed.

8) Can I use decimals for coefficients?

You can. Integer stoichiometric values are most common, but the solver accepts decimal entries when you need a generalized model or fitted relation.