1. How many grams of sodium fluoride should be added to 500.mL of a .02M hydrofl
ID: 960495 • Letter: 1
Question
1. How many grams of sodium fluoride should be added to 500.mL of a .02M hydrofluoric acid solution to create a buffer of pH = 3.16?Assume the change in volume is negligible.
Ka = 6.9x10-4
2 .Calculate the pH of a buffer solution prepared by adding 20.5g of CH3COOH and 17.8g of CH3COONa to enough water to make 5.00x102 mL of solution.
3 .Calculate the pH of a solution made by adding 10.0 g of sodium benzoate, NaC6H5CO2 , to 3.0g of benzoic acid, C6H5COOH, and dissolving in water to make1.00 L of solution. Ka benzoic acid=6.3X10-5
4. Calculate the pH of the buffer system made up of 0.15 M NH3/0.35 M NH4Cl.
Kb for NH3 =1.8X10-5.
5. Calculate the pH of the following buffer solutions: 1.0 M CH3COONa/1.0 M CH3COOH. ] (Ka acetic acid = 1.8x10-5)
Explanation / Answer
1.pH = Pka + log [NaF]/[HF]
3.16 = 3.16+log[NaF]-log0.02
log[NaF] = -1.69
[NaF] =10^-1.69 =0.02M
2. [CH3COOH] =20.5/60*1000/500 = 0.682M
[CH3COONa] = 17.8/82*1000/500 = 0.433M
pH = pKa + log[CH3COONa]/[CH3COOH]
pH = 4.75+log 0.433/0.682 = 4.75-0.197=4.55
pH = 4.55
3. [PhCOOH] =3.0/122.12*1000/1000 = 0.0245M
[PhCOONa] = 10.0/144.11*1000/1000 = 0.069M
pH = pKa + log[PhCOONa]/[PhCOOH]
pH = 4.20+log 0.024/0.069 = 4.20-0.45=3.75
pH = 3.75
4.pH = pKa + log[NH4Cl]/[NH3]
pH = 4.74 + log 0.35/0.15 = 4.74+0.367 = 5.10
5.pH = pKa + log[CH3COONa]/[CH3COOH]
since concentartion os salt = acid, pH = pKa
pH = 4.74+log 1.0/1.0 = 4.74
pH = 4.74
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