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Sagot :
Answer:
The answers are in the explanation. The pH is 5.91
Explanation:
The CH3NH2 reacts with HCl as follows:
CH3NH2 + HCl → CH3NH3⁺ + Cl⁻
When 200mL of HCl are added, the moles of CH3NH2 and HCl are reacting completely producing CH3NH3+ and Cl-. That means the species present are:
no H+. All reacted
yes H2O. Because the water is present in the solutions of HCl and CH3NH2
yes Cl-. Is a product of the reaction
Yes CH3NH2. Is consumed in the reaction but comes from the equilibrium of CH3NH3+
yes CH3NH3+. Is the other product of the reaction. MAJOR SPECIES
When 300.00mL of HCl are added, 100mL are in excess:
yes H+. Is in excess: H+ + Cl- = HCl in water. MAJOR SPECIES. Determine the pH of the solution.
yes H2O. Is present because the reactants are diluted.
yes Cl-. Is a product of reaction and comes from HCl.
Yes CH3NH2. The reactant is over but comes from the equilibrium of CH3NH3+
yes no CH3NH3+. Yes. Is a product and remains despite HCl is in excess.
To find the pH:
At equivalence point the ion that determines pH is CH3NH3+. Its concentration is:
0.100L * (0.200mol/L) = 0.0200 moles / 0.300L = 0.0667M CH3NH3+
The equilibrium of CH3NH3+ is:
Ka = Kw/kb = 1x10-14/4.4x10-4 = 2.273x10-11 = [H+] [CH3NH2] / [CH3NH3+]
As both [H+] [CH3NH2] comes from the same equilibrium:
[H+] = [CH3NH2] = X
2.273x10-11 = [X] [X] / [0.0667M]
1.5159x10-12 = X²
X = 1.23x10-6M = [H+]
As pH = -log [H+]
pH = 5.91
The pH at the equivalent point for this titration is "5.91".
pH Calculation:
[tex]CH_3NH_2 = 0.200\ M\\\\ \text{volume} = 100.0\ mL = 0.100\ L\\\\HCl = 0.100\ M\\\\[/tex]
We must now quantify the pH well at the equivalence point.
We know that even at the point of equivalence, moles of acid and moles of the base are equivalent. As such, first, we must calculate the number of moles of the given base.
Calculating the Moles in [tex]CH_3NH_2 = 0.200\ M \times 0.100\ L = 0.0200\ moles[/tex]
Calculating the Moles in [tex]HCl = 0.0200 \ moles[/tex]
Calculating the volume of [tex]HCl[/tex]:
[tex]\to \text{Molarity} = \frac{ \text{moles}}{\text{volume \ (L)}} \\\\\to \text{Volume} = \frac{\text{moles}}{\text{molarity}}\\\\[/tex]
[tex]= \frac{0.0200 \ moles}{ 0.100\ M}\\\\= 0.200 \ L\\\\= 200 \ mL\\\\[/tex]
Calculating the reaction among the acid and base:
[tex]CH_3NH_2 + HCl \longrightarrow CH_3NH_3^{+} + Cl^-[/tex]
[tex]0.0200 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 0.0200 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 0.0200[/tex]
Therefore the conjugate acid of the bases exists at the standard solution.
Then we must calculate the new molar mass of [tex]CH_3NH_3^+[/tex].
Total volume[tex]= 100 + 200 = 300\ mL = 0.300\ L[/tex]
[tex][CH_3NH_3^+] = \frac{0.0200\ mole}{ 0.300\ L}= 0.0667\ M[/tex]
Using the ICE table
[tex]CH_3NH_3^+ + H_2O \longrightarrow CH_3NH_2 + H_3O^+[/tex]
[tex]I \ \ \ \ \ \ \ \ \ \ 0.0667 \ \ \ \ \ \ \ \ \ 0\ \ \ \ \ \ \ \ \ 0\\\\C\ \ \ \ \ \ \ \ -x\ \ \ \ \ \ \ \ +x \ \ \ \ \ \ \ \ +x\\\\E \ \ \ \ \ \ \ \ \ \ \ \ 0.0667-x \ \ \ \ \ \ \ \ \ \ \ \ +x \ \ \ \ \ \ \ \ \ \ \ \+x\\\\\to Ka = \frac{[CH_3NH_2] [H_3O^+] }{[CH_3NH_3^+]}[/tex]
Calculating [tex]K_a[/tex] from [tex]K_b[/tex]
[tex]\to K_a \times K_b = 1\times 10^{-14}\\\\\to K_a = \frac{1\times 10^{-14}}{4.4\times 10^{-4}} = 2.27\times 10^{-11}\\\\[/tex]
[tex]= 2.27\times 10^{-11} \\\\= x\times \frac{x}{(0.0667-x)}[/tex]
The x in the 0.0667-x can be ignored since the Ka value is just too small and it also does not follow the five percent criteria.
[tex]\to 2.27 \times 10^{-11} \times 0.0667 = x_2\\\\\to x_2 = 1.515\times 10^{-12}\\\\\to x = 1.23\times 10^{-6}\ M\\\\\to [H_3O^+] = x = 1.23\times 10^{-6}\ M\\\\[/tex]
We have the formula to calculate pH.
[tex]\to pH = - \log [H_3O^+] = - \log 1.23\times 10^{-6}\ M= 5.91[/tex]
The pH at the equivalent point for this titration is "5.91".
Find out more information about the pH here:
brainly.com/question/15289741
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