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Correct statements about the percentage ionization of weak acids in water include...

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lak-86 | Student, Undergraduate | (Level 1) Salutatorian

Posted August 10, 2013 at 4:18 AM via web

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Correct statements about the percentage ionization of weak acids in water include which of the following?

I The percentage ionization increases as the ionization constant of the acid becomes larger.

II The percentage ionization increases as the concentration of the acid becomes smaller.

(A)

I only

(B)

II only

(C)

Both I and II

(D)

Neither I nor II

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llltkl | College Teacher | (Level 3) Valedictorian

Posted August 10, 2013 at 4:44 AM (Answer #1)

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The ionization of a weak acid, HA in water, can be represented as:

`HA stackrel larr rarr H^(+) (aq.) + A^(-) (aq.)`

`K_a=([H^(+)(aq.)][A^(-)(aq.)])/([HA])`

It follows from the expression that, at a given experimental condition, larger the value of `K_a` , greater is the concentration of free ions in solution, hence greater is the extent of ionization.

Therefore, statement I is correct.

Again, applying degree of dissociation concept to the dissociation equilibrium:

`HA stackrel larr rarr H^(+)(aq.) + A^(-)(aq.)`

`c(1-alpha)`             `alpha*c`         ` alpha*c`

`K_a=((alpha*c)(alpha*c))/(c(1-alpha))`

=`alpha^2c` (`alpha` is very small, so `(1-alpha)~~1` ` ` )

Therefore, `alpha=sqrt(K_a/c)`

alpha, the degree of dissociation increases when c, the molar concentration decreases. So, statement II is also correct.

Hence, correct answer is C).

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