5. What is the [H3O] in a solution formed by dissolving in 1.00 g NH4CI (MM 53.5

Question

5. What is the [H3O] in a solution formed by dissolving in 1.00 g NH4CI (MM 53.5 g/mol) in 30.0 mL of 3.00 M NHs? Kb(NH3)-1.8 105 A. 2.7 * 109 M B. 5.510-10 C. 1.2 10-10 D. 1.4 1012 6. Which pair of solutes could be used to prepare an aqueous buffer solution with a pH>7? A. HCl and NH4CI Kb(NH3)-1.8 10 B. HF and NaF (Ka(HF) 6.3 10-4) C. NHs and NH4CI Kb(NHs) 1.8 10 D. NaOH and NaCl 7. Which is the best acid for the preparation of a buffer with pH 3.3? A)???, (Ka-4.5x10") B) HNo, C)H103 (Ka-1.7x10") D) HOI (Ka-20x10"1)

Explanation / Answer

5. Mass of NH4Cl = 1g.

Molar mass of NH4Cl = 53.5g/mol

Moles of NH4Cl = 1g/53.5g/mol = 0.019mol = 0.02mol

Moles of NH3 = volume*molarity = 3M*0.03L = 0.09mol

Hsing Henderson-Hasslebalch equation-

pOH = pKb + log[conjugate acid]/[base]

Kb = 1.8*10-5

pKb = -logkb = -log(1.8*10-5) = 4.74

pOH = 4.74 + log0.02/0.09

= 4.74 - 0.65

pOH = 4.09

pH = 14 - pOH = 14-4.09 = 9.91

pH = -logH3O+

H3O+ = 10-9.91 = 1.2*10-10

Option C.

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