Concentration of Acid and pH?

Question

I want to know answers to following questions:-

1) How is the concentration of hydronium ion in an acidic solution change when water is added to it? Please stae the underlying Plrinciple along with explanation.

2) How is concentration of Hydroxide ions in a basic solution affected when excess base is dissolved in it?

Answer 1

1) To start assume that you have 1 litre if 1M HCl which dissociates completely.

HCl <=> H+  +  Cl-

You can see that you have a concentration of 1mol H+ per litre. pH is defined as:

pH = -log[H+]

pH = -log[1.00]

pH=0

Now dilute this solution to 10litres. You now have a concentration of H+ions of 1Mper 10 litres or 0.1M

pH = -log [H+]

pH = -log 0.1

pH = 1.00

Dilute afurther 10 fold, concentration is 1M H+ per 100 litres or 0.01M

pH = 2

And so on. As you add more and more water, the [H+] or [H3O+] ion decreases

2) If you add excess OH- ions to a basic solution, the concentration of OH- ions must increase.

Answer 2

With both strong acids and strong bases, they dissociate completely in water.  When excess base is added to an already alkaline solution the concentration of hydroxide (OH-) ions increases and the pH of the base increases.