A buffer is prepared by mixing 50.0 mL of 0.050 mol/L, sodium bicarbonate, NaHCO
ID: 954838 • Letter: A
Question
A buffer is prepared by mixing 50.0 mL of 0.050 mol/L, sodium bicarbonate, NaHCO3, and 10.7 mL of 0.10 mol/L, NaOH.
a) What is the pH
b) What mass of HCl must be added to 25.0 mL of the buffer to change the pH by 0.07 units?
Be sure to answer all parts A buffer is prepared by mixing 50.0 mL of 0.050 mol/L sodium bicarbonate, NaHCO, and 10.7 mL of 0.10 mol/L NaOH. (Ka = 4.7 x 10-11) (a) What is the pH? pH 10.2 (b) What mass of HCl must be added to 25.0 mL of the buffer to change the pH by 0.07 units? g HCIExplanation / Answer
a) PKa = -logKa = -log(4.7*10^(-11)) = 10.33
No of mol of NaHCO3 = 50/1000*0.05 = 0.0025 mol
No of mol of NaOH = 10.7/1000*0.1 = 0.00107 mol
pH = pka + log(salt/acid)
= 10.33+log(0.00107/(0.0025-0.00107))
= 10.2
b) pH = pka + log(salt-HCl/acid+HCl)
10.2-0.07 = 10.33+log((0.00107-x)/(0.00143+x))
x = 0.0001 mol
50+10.7 = 60.7 ml
for 60.7 ml of buffer = 0.0001 mol HCl needed
25 ml of buffer = 0.0001*25/60.7 = 4.12*10^-5 mol
mass of HCl = 4.12*10^-5*36.5 = 0.0015038 grams
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