From the calculations, the pH of the final solution is 9.04.
We can use the Henderson Hasselbach equation to obtain the final pH of the solution in terms of the pKb and the base concentration.
Number of moles of salt = 250/1000 L * 0.5 M = 0.125 moles
Number of moles of base = 150/1000 L * 0.5 M = 0.075 moles
Total volume of solution = 250ml + 150ml = 400ml or 0.4 L
Molarity of base = 0.075 moles/ 0.4 L = 0.1875 M
Molarity of salt = 0.125 moles/ 0.4 L = 0.3125 M
pOH = pKb + log[salt/base]
pKb = -log(1.8 x 10^-5) = 4.74
pOH = 4.74 + log[0.3125/0.1875 ]
pOH = 4.96
pH = 14- 4.96
pH = 9.04
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