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How to identify a weak acid?

June 24, 2019

 

How to identify a weak acid?

A weak acid is an acid that does not dissociate completely. If we are given a Ka for an acid that is small, the acid must be weak. For strong acids, Kat are not even usually given, unless the professor is trying to trick you.

There are three definitions of acids:

Arrhenius acid- Hydrogen ion producer when in water.

Bronsted Lowry acid - proton (H+) donor

Lewis acid - electron pair acceptor.

 

In order to identify a weak acid, students must first memorize strong acids.

Strong acids are : hydrochloric acid (HCl) , nitric acid (HNO3), sulfuric acid (H2SO4), hydrobromic acid (HBr), hydroiodic acid (HI), perchloric acid (HClO4), and chloric acid (HClO3).

 

Weak acids will have hydrogen that they are able to donate, but will not belong to the strong acid list.

Some examples of weak acids are organic acids (molecules with a COOH group) such as acetic acid, CH3CH2COOH. Other weak acids are HF, H2CO3, H3PO4 and many more. As you can see most weak acids can be identified by having hydrogen in front of the formula, except for organic acids.

 

Exercise:

Please circle all of the weak acids:

 

a. KCl

b. NaOH

c. HCN

d. CH3COOH

e. H2SO3

f. MgO

 

Answer:

The weak acids are c, d, and e.

A is the wrong answer because its a salt (ionic compound) and doesn't even have hydrogens. B is a strong base. E is another salt that doesn't have hydrogens. 

 

For more help with general chemistry, please contact Transformation Tutoring at 646-407-9078. We provide tutoring in NYC, Brooklyn, and online. 

 

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