2. O -4.5 points TCCGenChem2Lt 6, PRE,002, 0/3 Submissions Used My Notes Conside

Question

2. O -4.5 points TCCGenChem2Lt 6, PRE,002, 0/3 Submissions Used My Notes Consider 50 mL of a solution that is 0.25 M in both H2PO4 and HPO (a) What is the pH of this solution? (Use the table of Acid Dissociation Constants. Enter your answer to two decimal places.) (b) What would be the effect of adding 5 mL of 1.0 M HCl? o The pH would decrease dramatically O The ph would increase dramatically. O The pH would decrease slightly. O The pH would increase slightly. O The pH would not change at all. (c) What would be the effect of adding 2 mL of saturated NaOH (50 mass density 1.5 g/mL)? O The pH would increase dramatically. O The pH would increase slightly. O The pH would decrease dramatically. O The pH would not change at all. O The pH would decrease slightly.

Explanation / Answer

a)

millimoles of H2PO4- = 50 x 0.25 = 12.5

millimoles of HPO42- = 50 x 0.25 = 12.5

pH = pKa + log [HPO42- / H2PO4-]

pH = 7.21 + log [12.5 / 12.5]

pH = 7.21

b)

millimoles of HCl added = 5 x 1 = 5

pH = pKa + log [salt - C / acid + C]

    = 7.21 + log [12.5 - 5 / 12.5 + 5]

   = 6.84

pH = 6.84

c)

millimoles of NaOH = 2 x 18.75 = 37.5

H2PO4-   +   NaOH   -------------> HPO4-

12.5              37.5                           0

0                  25                             12.5

[OH-] = 25 / (50 + 2) = 0.481 M

pOH = 0.318

pH = 13.68

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