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December 19, 2014

December 19, 2014

Posted by **Radha** on Sunday, May 11, 2014 at 8:02pm.

(How can this problem be solved?)

The final answer is 4.41

However, I keep getting 3.76

- Chemistry 104 -
**Devron**, Sunday, May 11, 2014 at 9:35pmKa of HSO3^-=6.3 x 10^-8

HSO3^- --------> H^+ + SO3^-

Ka=[H^+][SO3^-]/[HSO3^-]

E of the ICE chart is the following:

[H^+]..[SO3^-]...[HSO3^-]

x............x............0.3M-x

Plug in values to Ka equation:

Ka=[x][x]/[0.3M-x]

I am going to assume x is small and ignore, but will have to check the assumption afterwards.

6.3 x 10^-8=x^2/[0.3M]

([0.3M]*Ka)^1/2=x

x=1.37 x 10^-4 M

1.37 x 10^-4 M/0.3M=0.04%

The assumption checks out.

x=H^+

and pH=-log[H^+]

pH=-log[1.37 x 10^-4 M]=3.86

pH=3.86

Now, pay attention and see what happens when I change the concentration from 0.3M to 0.03M:

E of the ICE chart is the following:

[H^+]..[SO3^-]...[HSO3^-]

x............x............0.3M-x

Plug in values to Ka equation:

Ka=[x][x]/[0.03M-x]

I am going to assume x is small and ignore, but will have to check the assumption afterwards.

6.3 x 10^-8=x^2/[0.03M]

([0.03M]*Ka)^1/2=x

x=4.35 x 10^-5 M

4.35 x 10^-5 M/0.03M=0.14%

The assumption checks out.

x=H^+

and pH=-log[H^+]

pH=-log[4.35 x 10^-5 M]=4.36

pH=4.36

I have never seen a HSO3^- concentration that high, which is were the error has occurred. Next time, include the Ka value that is given to you. This seems like a made up problem that your teacher didn't check or you made a typo. I don't think that a 0.3M solution of HSO3^- can be made.

- Chemistry 104 -
**Devron**, Sunday, May 11, 2014 at 9:37pmKa of HSO3^-=6.3 x 10^-8

HSO3^- --------> H^+ + SO3^-

Ka=[H^+][SO3^-]/[HSO3^-]

E of the ICE chart is the following:

[H^+]..[SO3^-]...[HSO3^-]

x............x............0.3M-x

Plug in values to Ka equation:

Ka=[x][x]/[0.3M-x]

I am going to assume x is small and ignore, but will have to check the assumption afterwards.

6.3 x 10^-8=x^2/[0.3M]

([0.3M]*Ka)^1/2=x

x=1.37 x 10^-4 M

(1.37 x 10^-4 M/0.3M)*100=0.04%

The assumption checks out.

x=H^+

and pH=-log[H^+]

pH=-log[1.37 x 10^-4 M]=3.86

pH=3.86

Now, pay attention and see what happens when I change the concentration from 0.3M to 0.03M:

E of the ICE chart is the following:

[H^+]..[SO3^-]...[HSO3^-]

x............x............0.03M-x

Plug in values to Ka equation:

Ka=[x][x]/[0.03M-x]

I am going to assume x is small and ignore, but will have to check the assumption afterwards.

6.3 x 10^-8=x^2/[0.03M]

([0.03M]*Ka)^1/2=x

x=4.35 x 10^-5 M

(4.35 x 10^-5 M/0.03M)*100=0.14%

The assumption checks out.

x=H^+

and pH=-log[H^+]

pH=-log[4.35 x 10^-5 M]=4.36

pH=4.36

I have never seen a HSO3^- concentration that high, which is were the error has occurred. Next time, include the Ka value that is given to you. This seems like a made up problem that your teacher didn't check or you made a typo. I don't think that a 0.3M solution of HSO3^- can be made.

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