Question

1. What is the pH of a solution with 15 mL of 0.5 M acetic acid and 15 mL of 0.5 M sodium acetate in solution.

2. What is the pH if we were to take 5 mL of the above solution and add it to 25 mL of water.

Please explain and work all steps.

Answer 1

1.

Form the Henderson equation we know the p​​​​​​H of a buffer solution is

p​​​​​​H = pK​​​​​a +log [salt]/ [Acid] ([ salt] = concentration of salt ,[ acid ] = concentration of acid )

Now concentration of acetic acid = 15×0.5/30

=0.25

Now the concentration of salt = 15 × 0.5 /30

= 0.25

15 ml = volume of the acid / salt

30ml = total volume of the solution

0.5 M = strength of the acid / salt

Now the  

pH = p​​​​​​k​​​​​​​​​​a + log [ salt] / [ acid ]

4.74 + log( 0.25/0.25 ). [p​​​​​​Ka of acetic acid value =

4.74]

= 4.74

2.

FROM the above questions we get pH = 4.74

Now concentraion of H ​​​​+  of the above solutions = ?

p​​​​​H = -log[H+]

Now [H+] = 10-pH

= 1.8× 10-5

Now take the solution of 5 ml and add 25 ml of water then total volume of solution is 30 ml.

Concentration of [H+] = 5× 1.8 × 10-5/30

=3×10-6. ( Neglected the term dissotion of water.

Then , p​​​​​​H = -log [3×10-6]

=5.52 ( ans).