Question

Calculate the pH of a buffer prepared by mixing 500 ml of 0.38 M acetic acid with 500 ml of 0.25 M sodium acetate. pKA for acetic acid is 4.76

(a) Calculate the pH of a buffer prepared by mixing 400 ml of 0.24 M acetic acid with 300 ml of 0.48 M sodium acetate. (b) For the buffer in part a, calculate the new pH if to that buffer was added 10 ml of 6M NaOH. Ignore any volume change from the additional 30 ml.

Answer 1

pH = 4,76

a) pH = 4,94

b) pH = 5,51

Step-by-step explanation:

To calculate the pH of a buffer you must use Henderson-Hasselbalch formula thus:

pH = pka + log
`([A^-])/([HA])`

Where [A⁻] is acetate ion concentration and [HA] is acetic acid concentration

The concentration of acetic acid is:

0,5 L× 0,38 M ÷(0,5L+0,5L) =0,19 M

The concentration of acetate ion is:

0,5 L× 0,25 M ÷(0,5L+0,5L) = 0,125 M

The pH is:

pH = 4,76 + log
`([0,125])/([0,19])`

4,76

a) The concentration of acetic acid is:

0,4 L× 0,24 M ÷(0,4L+0,3L) =0,137 M

The concentration of acetate ion is:

0,3 L× 0,48 M ÷(0,4L+0,3L) = 0,206 M

The pH is:

pH = 4,76 + log
`([0,206])/([0,137])`

4,94

b) The reaction of NaOH with acetic acid produces acetate ions, thus, moles of NaOH will increase concentration of acetate ion and descrease acetic acid concentration, thus:

NaOH moles: 0,010L× 6M = 0,060 moles

The concentration of acetic acid is:

(0,4 L× 0,24 M) - 0,060 moles ÷(0,4L+0,3L) =0,0514 M

The concentration of acetate ion is:

(0,3 L× 0,48 M) + 0,060 moles ÷(0,4L+0,3L) = 0,291 M

The pH is:

pH = 4,76 + log
`([0,291])/([0,0514])`

5,51

I hope it helps!