1. When 50.00 mL of 0.100 M NaOH reacts with 25.00 mL of 0.300 M HX, a weak monoprotic acid, the pH of the resulting solution is 3.74. Calculate the Ka of the acid.
2. Will the equivalence point of ammonia plus HCl be higher, lower, or equal to 7? Explain.
1) [NaOH ] = molarity x volume in Litres = 0.1 M x 0.05 L = 0.005 mol
[HX] = 0.3 M x 0.025 L = 0.0075 mol
HX + NaOH ---------------> NaX + H2O
0.0075 mol 0.005 mol 0
-----------------------------------------------------------------------------
0.0075 - 0.005 0 0.005 mol
= 0.0025 mol
Hence,
[HX] = 0.0025 mol
[NaX] = 0.005 mol
pH = -logKa + log [NaX] /[HX]
3.74 = -logKa + log [0.005]/[0.0025]
logKa = log [0.005]/[0.0025] - 3.74
Hence,
Ka = 0.000364
Therefore,
Ka of the acid = 0.000364
2) Equivalence point of ammonia plus HCl
It will be lower than 7.
Because, HCl is a strong acid and ammonia is weak base.
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