A buffer solution contains 0.479 M CH3NH3Cl and 0.313 M CH3NH2 (methylamine). De
ID: 994857 • Letter: A
Question
A buffer solution contains 0.479 M CH3NH3Cl and 0.313 M CH3NH2 (methylamine). Determine the pH change when 0.063 mol NaOH is added to 1.00 L of the buffer. pH after addition pH before addition = pH change =
A buffer solution contains0.337 M NaHCO3 and0.244 M K2CO3. Determine the pH changewhen 0.071 mol HNO3 is added to 1.00 L of the buffer.
pH change =
A buffer solution contains0.351 M NH4Br and0.300 M NH3 (ammonia). Determine the pH changewhen 0.068 mol HClO4 is added to 1.00 L of the buffer.
pH after addition pH before addition = pHchange =
Explanation / Answer
pH of buffer is calculated using Henderson Hasselbalch Equation
pH = pKa + log base/acid
Ka of methyl ammonium is 2.3 x 10-11
pKa = - log Ka
pKa = - log 2.3 x 10-11
pKa = 10.64
pH = pKa + log base/acid
pH = 10.64 + log 0.313/0.479
pH = 10.64 -0.185
pH = 10.45 before adding NaOH
When 0.063 mol of NaOH is added the base concentration becomes 0.376 M and the ammonium concentration reduces to 0.416 M.
So the new pH is
pH = pKa + log base/acid
pH = 10.64 + log 0.376/0.416
pH = 10.64 -0.044
pH = 10.60 after adding NaOH
pH change = 10.60 - 10.45 = 0.15
2) Bicarbonate pKa = 10.3
pH before adding HNO3 is
0.337 M NaHCO3 and0.244 M K2CO3
pH = pKa + log base/acid
pH = 10.3 + log 0.244/0.337
pH = 10.3 -0.14
pH = 10.16
After adding 0.071 mol HNO3
0.337 + 0.071 = 0.408M NaHCO3 and 0.244 - 0.071 = 0.173 M K2CO3
pH = pKa + log base/acid
pH = 10.3 + log 0.173/0.408
pH = 10.3 -0.372
pH = 9.93
pH Change = 10.16-9.93 = 0.23
3) pKa for ammonium = 9.24
pH before adding HClO4
0.351 M NH4Br and0.300 M NH3
pH = pKa + log base/acid
pH = 9.24 + log 0.3/0.351
pH = 9.24 -0.068
pH = 9.17
pH after adding 0.068 HClO4
0.351 + 0.068 = 0.419 M NH4Br and 0.300 -0.068 = 0.232 M NH3
pH = pKa + log base/acid
pH = 9.24 + log 0.232/0.419
pH = 9.24 -0.26
pH = 8.98
pH change = 9.17 - 8.98 = 0.19
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