Question

Calculate the pH when 65.0 mL of 0.269 M hydrobromic acid is mixed with 65.0 mL of 0.269 M sodium hydroxide solution at 25

Answer 1

HBr + NaOH ----> NaBr + H2O

65 mL of 0.269M HBr is completely neutralized by 65 mL of 0.269M NaOH solution and NaBr is highly soluble in water. Therefore pH is 7.00

Given that for a weak acid, Ka = 1x10^-5

after adding 65 mL of 0.269M NaOH to 65 mL of 0.269M HA solution, it has 0.017485 moles A- in 130 mL. Therefore the concentration is 0.1345M

Kb = Kw/Ka= [HA][OH-]/[A-] or 1x10^-14/1.5x10^-5=x^2/0.1345-x.

Assuming x << 0.1345 and solving for x,

x = 9.469x10^-6 M

pH = 14 - (-logx) = 8.976.