Tutored Practice Problem 18.2.6 COUNTS NOWASDS GRADD Prepare a buffer by direct

Question

Tutored Practice Problem 18.2.6 COUNTS NOWASDS GRADD Prepare a buffer by direct addition. Close Problem Consider how best to prepare one liter of a buffer solution with pH-7.87 using one of the weak acid/conjugate base systems shown here. Weak Acid Conjugate BaseK pKa 4.19 HC204 H2Pos HP2627.21 HCO3 C2042 HPO42- co2 48 x101 1032 6.4 x 10-5 6.2 x 10 many grams of the potassium salt of the weak acid must be combined with how many grams of the potassium salt of its conjugate base, to produce 1.00 L of a buffer that is 1.00 M in the grams potassium salt of weak acid- ate base - When performing a calculation to detemine how to make a buffer solution using an acid-base reaction, it is helpful to set up a stoichiometry table to assist in keeping track of the initial amounts of reactants and the

Explanation / Answer

To prepare pH = 7.87, using H2PO4-/HPO4-2 combination is best one.

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

pka2 of H3PO4 = 7.21

total no of mol of buffer = M*V = 1*1 = 1 mol

no of mol of K2HPO4 must take = x mol

no of mol of KH2PO4 must take = 1-x mol

7.87 = 7.21 + log(x/(1-x))

x = 0.82

no of mol of K2HPO4 must take = x = 0.82 mol

amount of K2HPO4 must take = n*Mwt = 0.82*174.2 = 142.84 g

no of mol of KH2PO4 must take = 1-x = 1-0.82 = 0.18 mol

amount of KH2PO4 must take = 0.18*136 = 24.48 g

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