Reaction Quotient Calculator

Compute Q from stoichiometric coefficients and molar activities, or solve any of them in reverse.

Builds up to six species per side and solves for Q, any activity, or any coefficient.

Updated October 2, 2026

CreatorFrank Zhao

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What the reaction quotient is

A reversible reaction never simply stops. It keeps running in both directions while the two rates settle toward each other. The reaction quotient $Q$ is the single number that tells you which side is currently winning, and it can be evaluated at any moment — not just at the end.

It is built the same way as an equilibrium constant: each activity is raised to its stoichiometric coefficient, the products are multiplied together, and the reagents are divided out. The one difference is what the activities represent. At equilibrium they are the equilibrium values. Everywhere else they are whatever you measured a moment ago.

Up to six species per side

Each species is a stoichiometric coefficient paired with an activity, for as many as six reagents and six products.

Three concentration scales

Pick mol/L, mmol/L or μmol/L per activity. Mixed units are converted onto one scale before any arithmetic happens.

Every row is optional

A species only joins the expression once you fill in both boxes of its row, so the form stays as short as your reaction.

The equation checks itself

As rows are filled, the reaction is written out above the result so a mistyped coefficient is visible at a glance.

You will need this whenever a problem hands you a mixture that is not yet at equilibrium and asks which way it will move: a titration partway through, a solution freshly prepared from a solid, a gas mixture after a pressure change, or a sample analysed shortly after a reaction starts.

Quick start guide

Work through the reagents first, then the products, then read the quotient.

1Write down the balanced equation, and separate the species to the left of the arrow from those to the right.
2In the Reagents block, fill in a coefficient and an activity for each species on the left. Pick the unit that suits the size of your number.
3Do the same in the Products block for the species on the right.
4Read the Reaction quotient at the bottom. Compare it with the equilibrium constant for the same reaction to decide the direction.

Rows appear as you go

Two pairs are offered on each side to begin with. Once a pair is complete, the next one appears underneath it. You never have to scroll past a wall of empty boxes, and you can stop at two species per side if that is all your reaction has.

A worked case: cadmium complex formation

Suppose you have 0.0100 mol/L cadmium(II) and 0.100 mol/L chloride, and the complex $\mathrm{CdCl_4^{2-}}$ has formed at 0.00200 mol/L. $\mathrm{Cd^{2+}} + 4\,\mathrm{Cl^-} \rightleftharpoons \mathrm{CdCl_4^{2-}}$

Q = \frac{[\mathrm{CdCl_4^{2-}}]}{[\mathrm{Cd^{2+}}]\,[\mathrm{Cl^-}]^{4}}

= \frac{0.00200}{0.0100 \times (0.100)^{4}}

= \frac{0.00200}{0.0100 \times 0.0001}

= 2000

The activity box for the chloride is raised to the fourth power because the coefficient in front of it is 4. Enter those four species and the calculator returns the value below, while the reaction summary above the result reads $1[\mathrm{A}] + 4[\mathrm{B}] \rightleftharpoons 1[\mathrm{C}]$ .

Quotient

Q = 2000

How Q is built

For a general reaction written with the reagents on the left, the quotient is

Q = \frac{\prod_i [i]_{p_i}}{\prod_j [j]_{a_j}}

QThe reaction quotient: a pure ratio, so it carries no unit.

[i]Activity of a product species, taken here as its molar concentration.

[j]Activity of a reagent species, on the same standard scale.

p, aStoichiometric coefficients, used as the exponent on each activity.

Every species that actually takes part appears once, raised to its stoichiometric coefficient. Nothing else appears at all. That last point is where most mistakes happen, so it is worth being precise about what counts as taking part.

A coefficient of 0 removes the species

Any activity raised to the power of zero is exactly 1, so the species drops out of the expression without changing the result. That is what an absent species is — the calculator shows those boxes blank and treats them as zero.

An activity of 1 is the standard state

1 mol/L is the reference state, and any species sitting at 1 mol/L contributes a factor of exactly 1. Pure solids and pure liquids are permanently there, which is why they are left out of the expression altogether.

The two numbers that mean nothing to the quotient

A coefficient of $0$ and an activity of $1$ are the neutral choices. A species sitting at both is invisible to the expression, which is exactly right for a solid precipitate or the solvent itself.

Water is the classic case. When it appears on both sides of an equation it cancels entirely and never needs to be entered.

Solving backwards

You can also work the other way round. Enter everything except one activity, type the quotient you want into the Reaction quotient box, and leave that one activity empty. The missing value is then solved for you.

[\mathrm{Cl^-}] = \left(\frac{[\mathrm{CdCl_4^{2-}}]}{Q \cdot [\mathrm{Cd^{2+}}] \times \prod_{\text{others}}}\right)^{1/4}

The same works for a coefficient, with one caveat. A coefficient only shows up inside a logarithm, and $\ln 1 = 0$ , so an activity of exactly 1 mol/L leaves the coefficient undetermined. Leave that box at some other value and the coefficient can be recovered.

Units are only ever a display convenience

Each activity carries its own unit, and you are free to mix them — one species in mol/L and the next in mmol/L. Everything is put onto a single scale before the arithmetic happens, so a four-power term entered in mmol/L gives the same quotient as the same value entered in mol/L.

Worked examples

Weak acid partway through dissociation

You have just prepared 0.050 mol/L of a weak acid and measured the ionisation that has already happened. This is the usual reason somebody needs a quotient at all: the mixture is not at equilibrium, and you want to know which way it is still moving.

The reaction reads $\mathrm{H_aA} \rightleftharpoons \mathrm{H^+} + \mathrm{A^-}$ , and the readout confirms it as $1[\mathrm{A}] \rightleftharpoons 1[\mathrm{B}] + 1[\mathrm{C}]$ . With $[\mathrm{H^+}] = [\mathrm{A^-}] = 0.00200$ mol/L:

Q = \frac{[\mathrm{H^+}][\mathrm{A^-}]}{[\mathrm{H_aA}]}

= \frac{(0.00200)(0.00200)}{0.050}

= 0.00008

Quotient

Q = 0.00008

That very small quotient says the acid has barely ionised, so the overwhelming majority of molecules are still undissociated — the expected picture for a weak acid at 0.050 mol/L.

Why a small error matters more than you think

Return to the cadmium case. The chloride activity is raised to the fourth power, so measurement error is amplified rather than passed through. Raise the chloride by ten percent, from 0.100 to 0.110 mol/L, and watch what happens to the quotient.

Q = \frac{0.00200}{0.0100 \times (0.110)^{4}}

= 1366.03

(1.1)^{4} = 1.4641

+10%

concentration change

×1.4641

factor on Q

−31.7%

resulting shift

A ten percent shift in one concentration moved the quotient by roughly 46 percent, and in the opposite direction, because chloride sits in the denominator. When you are comparing two quotients that differ by less than a factor of two, a concentration measured only to one significant figure is not enough to tell them apart.

Reading Q against K

A quotient on its own is just a number. It becomes a prediction the moment you compare it with the equilibrium constant for the same reaction.

Q < K

Too little product

Runs forward

Reagents keep converting into products until the quotient climbs to meet K.

Q = K

Nothing more to move

At equilibrium

Both rates match, so the composition of the mixture stays exactly as it is.

Q > K

Too much product

Runs backward

Product converts back into reagent, pushing the quotient down toward K.

To get the other half of the comparison you need $K$ for your specific reaction. The equilibrium constant calculator builds it from equilibrium concentrations, and it solves backwards for any missing concentration or coefficient in exactly the same way.

One structural difference is worth keeping straight: the quotient depends only on the mixture you have in front of you, so it changes every time the mixture changes. The equilibrium constant belongs to the reaction at a given temperature and does not. Raise the temperature and $K$ moves while $Q$ does not, which is precisely why a rising temperature can shift the equilibrium without the quotient moving at all.

If your species are gases, the activities should be partial pressures rather than concentrations, and the comparison uses the pressure form of the constant. The Kp calculator handles that version, including the conversion between the two forms and the temperature dependence of the gas constant.

Common mistakes

Do not enter solids, liquids or the solvent

A pure solid or pure liquid has an activity of exactly 1, so it can never change the quotient. Adding it to the expression at best does nothing and at worst tempts you to invent a number. Leave those rows empty.

Check that the coefficient is the one you meant

The exponent is the coefficient in the balanced equation, not the charge or the stoichiometric ratio you happened to compute. The reaction summary above the result restates your equation so a typo is visible immediately.

Keep the coefficients balanced before anything else

An unbalanced equation produces a confident, completely meaningless number. Balance first, then enter coefficients.

Keep the digit count honest on high powers

Because a coefficient of 4 quadruples the sensitivity, a concentration quoted to one significant figure can swamp the comparison you are trying to make. Carry enough digits to resolve the difference between Q and K.

Do not expect a coefficient to fall out of a standard-state activity

If a species sits at exactly 1 mol/L the calculator cannot recover its coefficient, because every exponent gives the same result there. Enter the actual measured value instead.

Frequently asked questions

Why are the boxes blank instead of showing zero?

An empty coefficient box means zero, and an empty activity box means 1 mol/L. Those are the two neutral values:

[i]^0 = 1

and

[i]^1 = 1

. A blank box therefore means this species has no effect on the quotient, which is the honest description of a species that is not taking part.

Why does it say I need a species on both sides?

Because a quotient built from one side alone is not a reaction quotient. With only reagents filled in, the denominator exists but the numerator is an empty product, and the result would be a meaningless reciprocal rather than a statement about the reaction. The warning appears as soon as you have entered something, and clears once both sides are represented.

My concentration seems high — is the calculator wrong?

Almost certainly a unit or exponent slip rather than a real sample. No liquid solution approaches 100 mol/L, so a value that large is flagged while still being displayed so you can correct it. The usual causes are entering a millimolar number in a molar box, or a coefficient on the wrong species.

How do I get from a mass to a concentration?

Convert it first, then enter the result. The molarity calculator handles mass, volume and molar mass in one step, and the solution dilution calculator covers the case where you are diluting a stock solution to a target concentration.

Limitations

Activities are entered as concentrations

This calculator treats every activity as a molar concentration relative to the 1 mol/L standard state. That is an excellent approximation for dilute solutions and a poor one for concentrated ones, where ions interact and the effective activity departs noticeably from the concentration. If your concentrations are well above 0.1 mol/L, treat the result as indicative rather than precise.

Also worth knowing

Coefficients must be positive whole numbers as they appear in a balanced equation. A species that is not part of the reaction is left blank rather than entered.

Activities must be greater than zero. A concentration of exactly zero is a genuine mathematical break in the expression, not a small number, and cannot be represented here.

The calculator checks that your quotient is meaningful, not that your equation is balanced. Balancing is your responsibility.

Results reflect the mixture you entered at the temperature you entered it. They describe that state and do not predict how far the reaction will travel.

An educational tool, not an authority

For safety-critical, regulatory or commercial decisions, confirm with experimental data and a qualified professional. Nothing here substitutes for measured results.

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