How do you calculate the equilibrium constant?

Write the balanced equation, then build a fraction: each product's equilibrium concentration raised to its coefficient on top, each reactant's on the bottom, leaving out pure solids and pure liquids. For H2 + I2 = 2 HI with [HI] at 0.156 M and both reactants at 0.0220 M, that is 0.156 squared divided by 0.0220 times 0.0220, which is about 50.3. The one non-negotiable input: the concentrations must be measured at equilibrium, not at the start.

Why are solids and pure liquids left out of the expression?

Because their activity is 1. A pure solid or a pure liquid has a fixed composition, so its effective concentration is a property of the substance itself and does not change as the reaction proceeds. A term that never changes cannot carry any information about where the equilibrium sits, so it multiplies through as 1 and disappears. Doubling the amount of solid changes nothing about the equilibrium concentrations around it, which you can verify in any lab.

What does a big or small K mean?

K much greater than 1 means the products dominate at equilibrium: the reaction has mostly gone. K much less than 1 means the reactants dominate: the reaction has barely started. K near 1 means both sides are present in comparable amounts. It says nothing about speed. A reaction with an enormous K can still take years to get there without a catalyst, which is why a match does not light itself.

What is the difference between Q and K?

Same expression, different moment. K uses concentrations measured at equilibrium and is a constant for that reaction at that temperature. Q uses whatever concentrations you have right now, equilibrium or not, and is a snapshot. Comparing them gives the direction: Q below K and the reaction runs forward to make more products, Q above K and it runs in reverse, Q equal to K and the mixture is already at equilibrium.

Does the equilibrium constant have units?

Formally no. K is defined through activities, which are dimensionless ratios against a standard state, so the true constant is a pure number. In practice, textbooks compute Kc from concentrations in mol/L and the units appear to survive whenever the total coefficients on top and bottom differ. Both habits are common; this page follows the convention and reports K as a plain number, which is also what makes taking its logarithm legal in later chemistry.

Does K change with temperature?

Yes, and only with temperature. Adding reactant, removing product, changing the pressure or the volume, or adding a catalyst all shift the position of the equilibrium without moving K by a hair: the concentrations rearrange themselves until the same ratio holds again. Change the temperature and K itself moves, in the direction Le Chatelier predicts: heating favors the endothermic direction. That is why every published K comes with a temperature attached.

What is the difference between Kc and Kp?

Kc is built from molar concentrations and Kp from partial pressures, so Kp only applies to reactions with gases in them. They are related by Kp = Kc(RT)^dn, where dn is the moles of gas on the product side minus the moles of gas on the reactant side. When dn is zero, as in H2 + I2 = 2 HI, the two constants are equal. This page works in concentrations throughout.

Equilibrium Constant Calculator: Kc and Q

Enter the equilibrium concentrations of up to two reactants and two products with their coefficients, and get Kc with the full expression built and substituted in front of you. Mark a species as a pure solid or liquid and it drops out of the expression, with the reason shown. Or hand the same numbers a known K and they become Q, plus a verdict on which way the reaction runs.

How this equilibrium constant calculator works

Every reversible reaction settles into a balance point, and the equilibrium constant is the number that describes where that balance sits. This page builds the mass action expression from your balanced equation, one species at a time: give each reactant and product its coefficient and its equilibrium concentration, and the expression is assembled, substituted, and evaluated in front of you. Anything your reaction does not have, you simply leave blank, and it drops out of the expression the way it drops out of the chemistry.

This page computes K for any reaction from equilibrium concentrations. One specific equilibrium comes up so often it has earned its own tool: a sparingly soluble salt dissolving in water. That constant is called Ksp, and converting between it and molar solubility has its own moves, so it lives on our solubility calculator. General reaction here, dissolving salt there.

The formula

aA + bB ⇌ cC + dD
K_c = [C]^c[D]^d ÷ [A]^a[B]^b

The square brackets mean concentration in mol/L, measured at equilibrium, and the lowercase letters are the coefficients from the balanced equation, which become exponents. Products go on top, reactants on the bottom. Pure solids and pure liquids never appear at all, for a reason worth its own section below. The concentrations must come from a system that has stopped changing: feed this expression starting concentrations and you get Q, a different and also useful number, not K.

Worked example

Hydrogen and iodine make hydrogen iodide: H₂ + I₂ ⇌ 2 HI. A hot flask of the three gases settles down, and the equilibrium concentrations are measured as [HI] = 0.156 M, [H₂] = 0.0220 M, [I₂] = 0.0220 M.

K_c = [HI]² ÷ ([H₂][I₂]) = (0.156)² ÷ (0.0220 × 0.0220) = 0.02434 ÷ 0.000484 = 50.28

The coefficient 2 on HI shows up as the exponent 2, and nowhere else. K is about 50, comfortably above 1, so at this temperature the products are favored: when this system comes to rest, most of what is in the flask is HI. This is the classic general chemistry example, and the measured value near 450 degrees C really is close to 50.

The rule everyone loses points on: pure solids and liquids

If a species in your reaction is a pure solid or a pure liquid, it does not go into the expression. Not with its concentration, not with a placeholder: it is simply absent. The reason is that a pure phase has an activity of 1. Its effective concentration is a property of the substance itself (solid calcium carbonate is exactly as dense as solid calcium carbonate) and it does not change as the reaction proceeds. A term that cannot change cannot tell you anything about where the equilibrium sits, so it multiplies through as 1 and vanishes.

The classic case is limestone decomposing in a hot kiln: CaCO₃(s) ⇌ CaO(s) + CO₂(g). Two of the three species are solids, so the whole expression collapses to K = [CO₂]. At a given temperature the CO₂ concentration above the solids is fixed, and it does not matter whether the kiln holds a pebble of limestone or a tonne. Mark a species as a pure solid or liquid above and this page removes it the same way, with the reason written into the steps. Water gets the same treatment when it is the solvent: in a dilute aqueous reaction, water is effectively a pure liquid at about 55.5 M, and that number no more belongs in K than the limestone does.

Q against K: which way does it run

Take the same expression and feed it concentrations that are not at equilibrium. The number that comes out is called the reaction quotient, Q, and comparing it to K answers the most practical question in the subject: which way does this mixture move?

Q less than K: the fraction is too small, so there is too little product. The reaction runs forward. Q greater than K: too much product, and the reaction runs in reverse. Q equal to K: the mixture is already at equilibrium and nothing net happens. For the HI reaction with K near 50.3, a fresh mixture with all three gases at 0.100 M has Q = (0.100)² ÷ (0.100 × 0.100) = 1, far below 50.3, so the reaction makes more HI. Switch the calculator to Q mode, enter a known K, and it renders this comparison for your own numbers.

Where equilibrium concentrations come from: the ICE table

Most homework does not hand you equilibrium concentrations; it hands you starting ones and a K, and the bridge between them is the ICE table: Initial, Change, Equilibrium. Write the initial concentrations, express every change in terms of one unknown x scaled by the coefficients, add the rows, and substitute the equilibrium line into the K expression.

A quick one, using the reaction above. Start with 0.500 M each of H₂ and I₂ and no HI, with K = 50.28. The changes are -x, -x, and +2x, so at equilibrium the concentrations are (0.500 - x), (0.500 - x), and 2x. Then K = (2x)² ÷ (0.500 - x)², and because both sides are perfect squares you can take the square root of the whole equation: 2x ÷ (0.500 - x) = 7.09, which gives x = 0.390. The equilibrium concentrations are [H₂] = [I₂] = 0.110 M and [HI] = 0.780 M, and feeding those three numbers into the calculator above returns exactly the K we started from, which is the check that the algebra behaved. This calculator takes the equilibrium values, the E row; the ICE table is how you earn them.

What K does and does not tell you

Reading the size of K is a skill worth having. K far above 1 (thousands and up): the reaction goes essentially to completion, and at equilibrium you would need good instruments to find any reactant. K far below 1 (thousandths and down): the reaction barely starts. K near 1: both sides are present in force, and this is the regime where equilibrium calculations earn their keep, because intuition alone will not tell you the mix.

Three honest boundaries. First, K is formally dimensionless: it is defined through activities, which are ratios against a standard state, though textbooks often carry mol/L units through informally and this page will not scold anyone for that. Second, K says nothing about speed. Diamond converting to graphite has K greater than 1 at room conditions and takes geological time; a large K is a destination, not a schedule. Third, for gas reactions there is a sibling constant built from partial pressures: K_p = K_c(RT)^Δn, where Δn is the change in moles of gas; this page works in concentrations, and our gas law calculator handles the pressure side of that conversion. And if your concentrations started life as a mass in a flask, our molarity calculator turns grams and volumes into the mol/L this page eats.

Equilibrium Constant Calculator