Mapoob sapling leaming Strong base is dissolved in 535 mL of 0.200 M weak acid (

Question

Mapoob sapling leaming Strong base is dissolved in 535 mL of 0.200 M weak acid (Ka 3.39 x105) to make a buffer with a pH of 3.96. Assume that the volume remains constant when the base is added. HA(aq) + OH-(aq) H,O(1) + A-(aq Calculate the pKa value of the acid and determine the number of moles of acid initially present. Number Number mol HA Tools When the reaction is complete, what is thel 10 ratio of conjugate base to acid? Number HA How many moles of strong base were initially added? Number mol OH

Explanation / Answer

pKa = -log(Ka) = -log(3.39*10^(-5)) = 5 - log(3.9) = 4.408

Number of moles initial present?

Molarity = Number of moles/Volume of Solution(in L)

Number of moles = 0.200 * 0.535 = 0.107 moles

Using the equation,

pH = pKa + log(base/acid)

3.96 = 4.408 + log(base/acid)

-0.448 = log(base/acid)

Base/acid = 10^(-0.448) = 0.35645

Number of moles of base = number of moles of acid * 0.35645 = 0.03814 moles

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