The reaction quotient Q can be calculated at any point in the reaction using an algebraic expression that depends on the balanced equation for the reaction.

The form Q takes for the general equation: aA + bB → cC + dD is shown below.
A, B, C and D are substances, and a, b, c and d are the numbers required to balance the equation.

Qc	=	[C]c[D]d
		[A]a[B]b

• If A, B, C and D are all solutes (followed by (aq) in aqueous systems) or all gases
  concentrations in 
  divided by the standard state concentration (
  ) are substituted as at the right.

• Qp	=	p(C)cp(D)d
  		p(A)ap(B)b

  If A, B, C and D are all gases
  partial pressures divided by the standard state pressure (100 kPa)
  are substituted as shown at the right.
• Pure solids and liquids do not appear in Q expressions because the standard state for pure solids and liquids is pure solid and pure liquid. Therefore when these are divided by their standard state, the result is 1.

For a system at equilibrium at a particular temperature, Q equals the number K, referred to as the equilibrium constant.

• Reactions having K greater than 1 are said to be product-favoured as products are present in higher concentration than reactants at equilibrium.
• Reactant-favoured reactions have K less than 1.