Question

Calculate the expected initial pH for Buffer 1 and for Buffer 2.

Buffer 1:

Add 15 mL of 0.10 M HC₂H₃O₂ to a 25 mL volumetric flask using the syringe. Rinse the syringe with water.

Add 10 mL of 0.10 NaC₂H₃O₂ to the same volumetric flask using the syringe.

Buffer 2:

Add 10 mL of 0.50 M HC₂H₃O₂ to a 25 mL volumetric flask using the syringe. Rinse the syringe with water.

Add 15 mL of 0.50 M NaC₂H₃O₂ to the same volumetric flask using the syringe.

Answer 1

We know, Moles = Molarity (M) x Volume (V in L) Molarity of HC2H302= 0.100 M So Moles HC2H302 = 0.100 * 0.015 Volume of HC2H302 = 15.00 mL = 0.002 mol = 0.015 L Moles C2H302- = 0.100 * 0.010 Molarity of C2H302-= 0.100 M = 0.0010 mol Volume of C2H302-= 10.00 mL Given Ka= 1.80E-05 = 0.010 L we know pKa = -log (Ka) = -log [ 1.8E-05 ] = 4.745 Henderson equation for buffer solution is pH=pKa + log [Base/Acid] here the Base is C2H302- and acid is HC2H302 + + pH = pka = 4.745 = 4.745 = 4.745 = 4.57 log (C2H302-/ HC2H302] log [ 0.0010 0.0015 ] log [ 0.6667 ] -0.176 + +

We know, Moles = Molarity (M) x Volume (V in L) Molarity of HC2H302= 0.500 M So Moles HC2H302 = 0.500 * 0.010 Volume of HC2H302 = 10.00 mL = 0.005 mol = 0.010 L Moles C2H302- = 0.500 * 0.015 Molarity of C2H302-= 0.500 M = 0.0075 mol Volume of C2H302-= 15.00 mL Given Ka= 1.80E-05 = 0.015 L we know pKa = -log (Ka) = -log [ 1.8E-05 ] = 4.745 Henderson equation for buffer solution is pH=pKa + log [Base/Acid] here the Base is C2H302- and acid is HC2H302 pH = pka = 4.745 = 4.745 = 4.745 = 4.92 + + + + log (C2H302-/ HC2H302] log [ 0.0075 0.0050 ] log [ 1.5000 ] 0.1761