Question

Using a 0.30 M phosphate buffer with a pH of 6.4, you add 0.75 mL of 0.49 M HCl to 46 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.) WebAssign will check your answer for the correct number of significant figures. Incorrect: Your answer is incorrect. Using a 0.30 M phosphate buffer with a pH of 6.4, you add 0.75 mL of 0.49 M NaOH to 46 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.)

Answer 1

pH of phosphate buffer = pka2 + log(HPO4^-2/H2PO4^-)

   pka2 phosphate buffer = 7.21

total no of mol of mol of buffer = 46*10^-3*0.3 = 0.0138 mol

no of mol of HPO4^-2 = x mol

no of mol of H2PO4^- = 0.0138-x mol

6.4 = 7.21 + log(x/(0.0138-x))

x = 0.00185

after addtion of HCL

no of mol of hcL added = 0.75*10^-3*0.49 = 0.0003675 mol

pH of phosphate buffer = pka2 + log(HPO4^-2 - HCl/H2PO4^- + HCl)

               = 7.21+log((0.00185-0.0003675)/(0.0138-0.00185+0.0003675))

   pH = 6.29

after addition of NaOH

no of mol of NaOH added = 0.75*10^-3*0.49 = 0.0003675 mol

pH of phosphate buffer = pka2 + log(HPO4^-2 + NaOH/H2PO4^- - NaOH)

               = 7.21+log((0.00185+0.0003675)/(0.0138-0.00185-0.0003675))

pH = 6.50