Amphoteric Solutions

A chemical compound able to react with both an acid or a base is amphoteric.   

Water is amphoteric. The two acid-base couples of water are H3O+/H2O and H2O/OH-

It behaves sometimes like an acid, for example

And sometimes like a base :

Hydrogen carbonate ion HCO3- is also amphoteric, it belongs

to the two acid-base couples H2CO3/HCO3- and HCO3

-/CO32-

Neutralization Problems

If an acid and a base combine in a 1 to 1 ratio, then the volume of the acid multiplied by the concentration of the acid is equal to the volume of the base multiplied by the concentration of the base

VaMa = VbMb

Vb = volume of the base (in L) Mb = concentration of the base (in moles/L) VbMb = L x moles/L = moles of base Va = volume of the acid (in L) Ma = concentration of the acid (in moles/L) VaMa = L x moles/L = moles of acid

VbMb = VaMa is the same as Moles of base = Moles of acid!

Example:

You have 25 mL of HCl, with an unknown concentration. It takes _____ mL of 1.0 M NaOH to titrate (or neutralize) the acid. What is the concentration of the HCl? Tip: figure out what the variables are here (Vb,

Mb, Va, and Ma). Figure out what you’re solving for, and then solve!

Answer:

You have 25 mL of HCl, with an unknown concentration. It takes _____ mL of 2.0 M NaOH to titrate (or neutralize) the acid. What is the concentration of the HCl?

What is titration?

Quantitative method used to find the concentration of a substance.

Finding Unknown Solution

Standard Solution – 0.1M HCl Titrant – unknown concentration NaOH Indicator – Phenolphthalein End point – Where the indicator changes color We are trying to reach the equivalence point

Where moles Acid = moles Base

In our lab, equivalence point and end point are the same!

What does this mean?

If you use 100mL of NaOH that has a concentration of 0.1 moles/L, how many moles of base have you used?

0.100 L x 0.1 moles/L = .01 moles of base

So how many moles of acid were in the unknown acid? .01 moles!! Because moles base = moles acid in a titration.

Neutralization Problems Example 1: Hydrochloric acid reacts with potassium hydroxide

according to the following reaction:

HCl + KOH KCl + H2O If 15.00 mL of 0.500 M HCl exactly neutralizes 24.00 mL of KOH

solution, what is the concentration of the KOH solution? Solution:

Vacid Cacid = Vbase Cbase

(15.00 mL )(0.500 M) = (24.00 mL ) Cbase

Cbase = (15.00 mL )(0.500 M) (24.00 mL ) Cbase = 0.313 M

Neutralization Problems Whenever an acid and a base do not combine in a 1 to 1

ratio, a mole factor must be added to the neutralization equation

n Vacid C acid = V base C base

The mole factor (n) is the number of times the moles the acid side of the above equation must be multiplied so as to equal the base side. (or vice versa)

Example

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

The mole factor is 2 and goes on the acid side of the equation. The number of moles of H2SO4 is one half that of NaOH. Therefore the moles of H2SO4 are multiplied by 2 to equal the moles of NaOH.

Titrations

VaMa = VbMb

What the HECK is that?!?!?!?