Need 200 ml of 0.1 M potassium phosphate, monobasic - 1 group prepare (also prep

Question

Need 200 ml of 0.1 M potassium phosphate, monobasic - 1 group prepare (also prepare 5 & 6) What is the formula weight of KH_2PO_4? (1) How many grams of KH_2PO_4 do you need? (3) How many ml of water will you dissolve the KH2PO4 in? (1) What is the expected pH of the original solution? (3) Need 200 ml of 0 1 M potassium phosphate, dibasic - 1 group prepare this (also prepare 5 & 6) What is the formula weight of K_2HPO_4? (1) How many grams of K2HPO4 do you need? (3) How many ml of water will you dissolve the K_2HPO_4 in? (1) What is the expected pH of the original solution? (3)

Explanation / Answer

3. For 200 ml of 0.1 M KH2PO4 solution

a. formula weight of KH2PO4 = 136.086 g/mol

b. grams of KH2PO4 required = 0.1 x 136.086 x 0.2 = 2.72 g

c. Volume of water required = 200 ml

d. pH of solution

H2PO4- <==> HPO4^2- + H+

let x amount has dissociated

Ka2 = 6.2 x 10^-8 = x^2/0.1

x = [H+] = 7.87 x 10^-5 M

pH = -log[H+] = 4.10

4. For 200 ml of 0.1 M K2HPO4 solution

a. formula weight of K2HPO4 = 174.2 g/mol

b. grams of K2HPO4 required = 0.1 x 174.2 x 0.2 = 3.48 g

c. Volume of water required = 200 ml

d. pH of solution

HPO4^2- + H2O <==> H2PO4- + OH-

let x amount has hydrolyzed

Kb2 = Kw/Ka2 = 1 x 10^-14/6.2 x 10^-8 = x^2/0.1

x = [OH-] = 1.27 x 10^-4 M

pOH = -log[OH-] = 3.89

pH = 14 - pOH = 10.10

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