# How do I know what if an acid is strong or weak?

• student34
In summary, the conversation discusses using the modified Arrhenius theory to classify HNO3(aq), HI(aq), and HF(aq) as strong or weak acids. The book only provides the definition of the modified Arrhenius theory and it seems like there is not enough information to determine the strength of the acids. It is suggested to measure the conductivity or rate of hydrogen liberation to determine the strength.

## Homework Statement

The question asks me to use the modified Arrhenius theory of acids to classify the following as strong or weak acids, HNO3(aq), HI(aq) and HF(aq).

## Homework Equations

But the book only says that the modified Arrhenius theory for acids is, "an Arrhenius acid reacts with water to produce H3O+ (aq) in aqueous solution".

## The Attempt at a Solution

There just doesn't seem to be enough information for me to know what percent of the solution is H3O+ ions.

I don't see how to answer the problem without already knowing strength of these acids (their dissociations constants).

student34
Just googled "Modified Arrhenius Theory" and it looks like "edubabbled" nonsense to rationalize something Arrhenius couldn't explain. Don't give your instructor too much backtalk about it, though --- he might be the idiot who invented it.

student34
Thanks, it's probably just bad wording in the textbook.

Well if that is what your book says, then stick with it. You can measure the conductivity of the three acids (same number of moles and same volumes) by inserting two inert electrodes in the solution with a particular potential difference across them and connect an ammeter to the circuit, and the acid with the least resistance clearly has the highest H3O+ concentration, since the stronger an acid, the more strongly it ionizes the water molecules(and the more conductive it gets). You can also alternatively use a gas syringe to measure the rate of hydrogen liberation at the cathode, with the acid giving the highest rate being the strongest.

## 1. What is the definition of a strong acid?

According to the Arrhenius definition, a strong acid is a substance that completely dissociates into ions when dissolved in water, producing a high concentration of hydrogen ions (H+).

## 2. How do I determine if an acid is strong or weak?

You can determine the strength of an acid by looking at its dissociation constant (Ka). A strong acid will have a high Ka value, indicating that it completely dissociates in water. On the other hand, a weak acid will have a low Ka value, meaning it only partially dissociates.

## 3. What are some examples of strong acids?

Some common examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3).

## 4. How does the strength of an acid affect its pH?

The strength of an acid is directly related to its pH. Strong acids have a low pH, typically below 3, while weak acids have a higher pH, usually between 3 and 6. The lower the pH, the stronger the acid.

## 5. Can an acid be both strong and weak?

No, an acid cannot be both strong and weak. It is either one or the other based on its dissociation in water. However, some acids may be considered strong in one context and weak in another. For example, hydrofluoric acid (HF) is a weak acid in water, but a strong acid in the presence of a base.