7.21? the pH of 14. Phosphate solutions. The pH of a phosphate solution is 7.51.

Question

7.21? the pH of 14. Phosphate solutions. The pH of a phosphate solution is 7.51. The analytical concentration ), Section mL of of phsophate is 0.12 M. (a) What are the two principal species in the M HC 0 mL of 0.04 M solution? (b) Calculate the concentration of each spe cies M HCl; 0 mL of15. Phosphate buffer. A phosphate buffer is pre- olutions. ppropri- lic acid 0 mL of NaOH; pared by dissolving 8.52 g of Na2HPO4 and 10.8 g of NaH2PO in 500 mL of solu- tion. (a) Calculate the pH of the buffer. (b) 20 mL of 0.30 M NaOH is added to 100 mL of the buffer. Calculate the pH of the resulting solution.

Explanation / Answer

a)

pH ig vien byu

mol of Na2HPO4 = mass/MW = 8.52/141.9588 = 0.060

mol of NaH2PO4 = mass/MW = 10.8/119,9770 = 0.090

then

pH = pKa2 + log(Na2HPO4 /NaH2PO4 )

pH = 7.21 + log(0.060/0.090) = 7.0339

b)

V = 100 mL of 500 --> 1/5

mol of Na2HPO4 = 0.060*1/5 = 0.012

mol of NaH2PO4 = 0.090*1/5 = 0.018

after

mol of NaOH = MV =(0.3)(20*10^-3) = 0.006

reactions

mol of Na2HPO4 = 0.012 + 0.006 = 0.018

mol of NaH2PO4 = = 0.018 - 0.006 = 0.012

pH = pKa2 + log(Na2HPO4 /NaH2PO4 )

pH = 7.21 + log(0.018/0.012) = 7.386

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