`x = sqrt(K_a*F)`

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The equation for weak acid is used to calculated the unknown concentration of weak acid after dissociation in solution. This equation is paired with fraction of dissociation of an acid equation. 

The equation is derived from 

  where the original weak acid dissociation reaction is . 

Solving for x , we assume that x is going to be a very small number thus (F-x)=F. Then the equation becomes. Multiplying both sides by F (formal concentration), we get . Taking the square root of both sides we get . 

Description

The equation is 

where 

• x is the [A^-] concentration of the dissociated acid particle in units of (mol/L)
• F is the formal concentration in units of (mol/L)
• K_a is the acid dissociation constant, an extended list can be found here.

Related Topics

• Fraction dissociation for weak acid (for the Spanish site click here)
• Acid equilibrium constant (for the Spanish site click here)

References

Harris, 9th Edition.  Pp.193