Question

1) What mass of ammonium chloride should be added to 2.50 L of a 0.160 M NH3 in order to obtain a buffer with a pH of 9.55?
2) A 350.0 −mL buffer solution is 0.140 M in HFand 0.140 M in NaF. What mass of
NaOH could this buffer neutralize before the pH rises above 4.00? If the same volume of the buffer was 0.360 M in HF and 0.360 M in NaF, what mass of NaOHcould be handled before the pH rises above 4.00?
3) A 5.65 −g sample of a weak acid with Ka=1.3×10−4 was combined with 5.20 mL of 6.10 M NaOH and the resulting solution was diluted to 750 mL. The measured pH of the solution was 4.25.What is the molar mass of the weak acid?

Answer 1

1)

this is a buffer

so

pOH = pKb+ log(NH4+/NH3)

pKb for NH3 = 4.75

then

pOH = 4.75 + log(NH4+/NH3)

and M = 0.16 of NH3

pOH = 14-pH = 14-9.55 = 4.45

pOH = 4.75 + log(NH4+/0.16)

4.45 = 4.75 + log(NH4+/0.16)

solve for NH4+

NH4+ = 0.16*10^(4.45-4.75) = 0.080189M

since V = 2.5 L

then

mol = MV = 0.080189*2.5 = 0.2004725 mol of NH4+

that is

0.2004725 mol of NH4Cl

mass = mol*MW = 0.2004725*53.491 = 10.723 g of NH4Cl are needed

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