Q97180
Solution: Let HA be a weak acid with ionization constant "\\alpha" .
"HA \\leftrightarrow H^+ + A^-" ---(reaction at equilibrium)
"c" ---(concentrations at "t=0")
"c(1-\\alpha) c\\alpha" "c\\alpha" ---(concentrations at "t=t" )
Thus, "K_a=[H^+][A^-]\/[HA]=6.2*10^{-4}" ----(given)
"\\implies (c\\alpha*c\\alpha)\/c(1-\\alpha)=6.2*10^{-4}"
"\\therefore c\\alpha^2\/(1-\\alpha)=6.2*10^{-4}" ----(1)
(a)
Given : "c=3*10^{-3}M"
Using (1), we get;
"(3*10^{-3})\\alpha^2+(6.2*10^{-4})\\alpha-6.2*10^{-4}=0"
Solving the above quadratic equation for "\\alpha" , we get;
"\\alpha=0.363" (Answer)
(negative root is neglected as percent ionization is always positive)
Thus, percent ionization is "36.3 %" %
Now, "pH =(-log_{10}[H^+])=(-log_{10}(c\\alpha))"
"=-log_{10}(3*10^{-3}*0.363)"
"=2.963" (Answer)
pH of the solution at a concentration of "3*10^{-3}M" is "2.963"
(b)
Now; solution is diluted 1000 times.
Thus, the concentration becomes; "c=3*10^{-6}M"
Again, using (1), we get;
"(3*10^{-6})\\alpha^2+(6.2*10^{-4})\\alpha-6.2*10^{-4}=0"
Solving the above quadratic equation for "\\alpha" we get;
"\\alpha=0.995" (Answer)
(negative root is neglected as percent ionization is always positive)
Thus, percent ionization is "99.5" %
Again, "pH =(-log_{10}[H^+])=(-log_{10}(c\\alpha))"
"=-log_{10}(3*10^{-6}*0.995)"
"=5.525" (Answer)
pH of the solution at a concentration of "3*10^{-6}M" is "5.525"
Comments
what is the PH of a solution prepared by adding 0.2g of Na2CO3 and o.2g of NaHCO3 to water and bring to a volume of 1 liter ? ( PKa2= 10.25).
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(a)Describe the preparation of 500mL of 0.08M acetate buffer (pH=5.4) from acetic acid [F.W.=60.05, specific gravity =1.05, 99.6%w/w] and solid NaOH (F.w.=40.0). (b) what is the concentration of Na+ in the buffer?
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