A solution of a weak acid was tested with the indicators used in this experiment

Question

A solution of a weak acid was tested with the indicators used in this experiment. The color obsereved were as follows: What is the approximate pH of the solution? 2. A solution of Na_2CO_3 has a pH of 10. The CO_3^2- ion is the conjugate base of the HCO_3^- ion. Write the net ionic equation for the reaction Which makes a solution of Na_2CO_3 basic (Eq.7) 3. The pH of a 0.10 M HCN solution is 5.2. a. What is [H^+] in that solution? b. What is [CN^-]? What is [HCN]? (where do the H^+ anb CN^- ions come form? c. What is the value of K_a for HCN? what is the value of pK_a?) 4. Formic acid, HFor, has a K_a value equal to about 1.8 times 10^-4. A student is asked to prepare a buffer having a pH of 3.4 from a solution of formic acid and a solution of sodium formate having the same molarity How many militers of the Na For solution should she add to 20 mL of the HFor solution to make the buffer?

Explanation / Answer

1. The approximate pH of the given solution here would be as based on the changes in the color of all the indicator is, 5.0

2. Net ionic equation for basic solution of Na2CO3

CO3^2-(aq) + H2O(l) <==> HCO3-(aq) + OH-(aq)

3. pH of 0.1 M HCN is 5.2

a. The concentration of [H+]

pH = 5.2 = -log[H+]

So [H+] = 6.31 x 10^-6 M

b. [H+] = [CN-] = 6.31 x 10^-6 M

[HCN] = 0.1 - 6.31 x 10^-6 = 0.1 M

c. Ka for HCN = [H+][CN-]/[HCN] = (6.31 x 10^-6)^2/0.1

    = 3.98 x 10^-10 M

pKa = -log[Ka] = 9.40

4. let 1 M be the concentration of both HCOOH and HCOONa

say x amount of HCOONa is added to prepare the buffer

pH = pKa + log(base/acid)

3.4 = 3.74 + log(x/1 M x 20 ml - x)

9.14 - 0.46x = x

x = 6.26 mmol

So volume of HCOONa to be added = 6.26 mmol/1 M = 6.26 ml

6. pKa for HF = 3.46

le x amount of NaOH is added

pH = pKa + log(base/acid)

3.4 = 3.46 + log(x/0.1 M x 20 ml - x)

1.742 - 0.871x = x

x = 0.93 mmol

ml of NaOH to be added = 0.93 mmol/0.1 M = 9.3 ml  

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