How do you calculate the pH of a weak acid?

Strong acids dissociate fully in solution. Examples of strong bases include HCl and H2SO4. Organic acids, such as methanoic acid or oxalic acid, as well as some inorganic acids such as hydrogen fluoride, are weak acids.

A weak acid differs to a strong acid in that it does not fully dissociate in solution (it does not release all of it's potential hydrogen ions). This means that you cannot use the usual formula (pH=-log
10[H^+]) by simply letting [H^+] equal the concentration of the acid. Instead, you must use the acid dissociation constant, Ka. Where, at equilibrium, [H^+] is the hydrogen ion concentration, [A^-] is the dissociated acid concentration, and [HA] is the concentration of the undissociated acid:

Ka = ([H^+][A^-])/[HA]

If we assume that when the weak acid dissociates, [H^+]=[A^-], then:

Ka = ([H^+]^2)/[HA]

If you know the equilibrium concentration of the acid and the Ka (which you will usually be given), you can rearrange this formula to give [H^+], which should be all you need to work out the pH.

Joe M. GCSE Biology tutor, A Level Biology tutor, A Level Chemistry t...

2 years ago

Answered by Joe, an A Level Chemistry tutor with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


£26 /hr

Lorenzo C.

Degree: Biomedical Sciences with Honours in Infectious Diseases (Bachelors) - Edinburgh University

Subjects offered: Chemistry, Physics+ 1 more


“Hey! I'm Lorenzo and I study Biomedical Sciences with an honours in Infectious Diseases at the University of Edinburgh. I'd be happy to help you with Biology, Chemistry and Physics at GCSE or at A-Level.  I have previous experience tu...”

£20 /hr

Ed W.

Degree: Medicine (Bachelors) - Edinburgh University

Subjects offered: Chemistry, Maths+ 3 more

Further Mathematics
-Medical School Preparation-

“Hi! I'm Ed and I'm currently studying Medicine at the University of Edinburgh. I tutor at GCSE and A Level forBiology, Chemistry and Maths, and at GCSE level forFurther Maths. These are subjects I'm very comfortable in and enjoy. With ...”

MyTutor guarantee

£20 /hr

Dominika K.

Degree: Biomedical sciences (Bachelors) - Edinburgh University

Subjects offered: Chemistry, Spanish


“I am a first year student of Biomedical Sciences in Edinburgh. I was born in Spain but my parents are from Poland, so I speak both languages. Last year I finishedstudying the International Baccalaureate and the Spanish Baccalaureate a...”

MyTutor guarantee

About the author

£24 /hr

Joe M.

Degree: Biology (Bachelors) - Durham University

Subjects offered: Chemistry, Science+ 2 more


“Second year Biology student tutoring science and maths in a relaxed and friendly environment”

You may also like...

Other A Level Chemistry questions

How do mass spectrometers work?

What is optical isomerism and how can you distinguish between optical isomers?

What are optical isomers?

How do ionic charge and radius affect lattice enthalpy?

View A Level Chemistry tutors


We use cookies to improve our service. By continuing to use this website, we'll assume that you're OK with this. Dismiss