A buffer solution (pH 4.74) contains acetic acid (0.05 mol/L) and sodium acetate (0.05 mol/L) i.e. it is a 0.1M acetate buffer. Calculate the pH after addition of 2 mL of 0.025M hydrochloric acid to 10 mL of the buffer.
4.65
Explanation
No of mole of Hydrochloric acid added = (0.025mol/1000ml)×2ml = 0.00005mol
No of mole of CH3COOH = (0.05mol/1000ml)×10ml = 0.0005mol
No of mole of CH3COO- = (0.05mol/1000ml)×10ml = 0.0005mol
HCl react with CH3COO-
HCl + CH3COO- ------> CH3COOH + Cl-
this is 1:1 reaction
Therefore, after HCl addition
No of mole of CH3COOH = 0.00050 + 0.00005 = 0.00050
No of mole of CH3COO- = 0.00050 - 0.00005 = 0.00045
total volume = 12ml
[ CH3COOH ] = (0.00055mol/12ml)×1000ml = 0.0458M
[ CH3COO- ] = (0.00045mol/12ml) ×1000ml = 0.0375M
Handerson equation
pH = pKa + log ([A-]/[HA])
= 4.74 + log( 0.0375/0.0458)
= 4.74 - 0.09
= 4.65
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