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Weak acid (HX) dissociates in 10%. At equilibrium, concentration of non-dissociated acid is [tex]9\cdot 10^{-3} M[/tex]. Respective pH and Kd are:

A. 3 and 1.11 × 10-5,
B. 4 and 1.11 × 10-6,
C. 3 and 1.11 × 10-4,
D. 4 and 1.11 × 10-5,
E. 2.2 and 1.11 × 10-6

Svaret er C.

Takk!

duodenum wrote:Hallo! Fin søndag å lese kjemi på! Håper noen kan hjelpe meg med denne!
Weak acid (HX) dissociates in 10%. At equilibrium, concentration of non-dissociated acid is [tex]9\cdot 10^{-3} M[/tex]. Respective pH and Kd are:
A. 3 and 1.11 × 10-5,
B. 4 and 1.11 × 10-6,
C. 3 and 1.11 × 10-4,
D. 4 and 1.11 × 10-5,
E. 2.2 and 1.11 × 10-6
Svaret er C.
Takk!

[tex]HX+H_2O <=>H^+ + X^-[/tex]
[tex]y.................0.......0[/tex]
[tex]9*10^{-3}........0.1y...0.1y[/tex]
thus

[tex]y-0.1y=9*10^{-3}[/tex]
[tex]y=0.01[/tex]
so

[tex]K_a=\frac{(0.01*0.1)^2}{9*10^{-3}}\,(M)=1.11*10^{-4}\,(M)[/tex]

and

[tex]pH=-\lg(0.001)=3[/tex]

or

[tex]pH=0.5(pK_a - \lg(9*10^{-3}))=3[/tex]
):
C.