Question

pH and Buffers IV. pH of a Buffer Solution Mass of sodium acetate, CH,COONa, g 4.2009 COOM Calculated pH Measured pH buffer solution prepared with dissolved CHCOONa+ 8.5 mL CH, COOH(aq) 5.54 40 mL buffer + 1.0 mL of 6.0 M HCl(aq) 14.05 40 mL buffer + 1.0 mL of 6.0 M NaOH(aq) 5.02 Calculated pH Measured pH 5.54 1063 deionized water 40 mL DI water + 1.0 mL of 6.0 M HCl (aq) 40 mL DI water + 1.0 mL of 6.0 M NaOH (aq) 13.54 Compare the change in pH you observed when 1.0 mL of 6.0 M HCl is added to 40 mL of the buffer versus 40 mL of water. Is there a difference, and if so, why? Do the same for your observation of the change in pH of the buffer versus the water upon the addition of 1.0 mL of 6.0 M NaOH solution. REPORT SHEET. 127

Answer 1

Solution:

A buffer solution resists the change in pH on addition of small amount of acid or base.

A) Addition of 1 mL of 6 M HCl:

Addition of 1 mL HCl in 40 mL of buffer does not contribute a significant change in pH because acetate ion present in buffer reacts with HCl and forms unionized acetic acid and therefore no significant change in pH observed.

HCl + CH3COONa = NaCl + CH3COOH

The addition of HCl in deionized water increases H+ concentration and therefore pH decreases.

B) Addition of 1mL of 6 M NaOH:

When NaOH is added in buffer solution, the acetic acid reacts with NaOH and forms water molecules hence no significant change in pH observed.

NaOH + CH3COOH = CH3COONa + H2O

When NaOH added in deionized water, it increases OH- concentration therefore increases pH of deionized water.