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5. A) If a solution at pH 5 undergoes a 1000-fold increase in [H+], what is the resulting pH?B) If a solution at pH 5 undergoes a 1000-fold increase in [OH-], what is the resulting pH?

Sagot :

REmember that pH is the negatige logarithem of the concentration of protons:

[tex]pH=-\log _{10}\lbrack H^+\rbrack[/tex]

when we make the invers of the logarithm:

[tex]\lbrack H^+\rbrack=10^{-pH}[/tex]

in this case:

[tex]\lbrack H^+\rbrack=10^{-5}[/tex]

if the concentration of protons undergoes a 1000-fold increase the new concentration is:

[tex]\lbrack H^+\rbrack=10^{-5}\times10^3=10^{-2}[/tex]

therefore the new pH is calculated as follows:

[tex]pH=-\log _{10}\lbrack H^+\rbrack=-\log (10^{-2})=2[/tex]

B.

For the second part we need to remember that [H+] and [OH-] are related according the following equation:

[tex]\lbrack H^+\rbrack\times\lbrack OH^-\rbrack=10^{-14}[/tex]

We have calculated before that

[tex]\lbrack H^+\rbrack=10^{-5}[/tex]

Then we can calculate [OH-]:

[tex]10^{-5}\times\lbrack OH^-\rbrack=10^{-14}\text{ }\rightarrow\lbrack OH^-\rbrack=10^{-9}\text{ }[/tex]

1000 fold that concentration is

[tex]\lbrack OH^-\rbrack=10^{-9}\times10^3=10^{-6}[/tex]

Again we use the relation between [H+] and [OH-] with the new value:

[tex]\lbrack H^+\rbrack\times\lbrack OH^-\rbrack=10^{-14}[/tex][tex]\lbrack H^+\rbrack\times10^{-6}=10^{-14}[/tex][tex]\lbrack H^+\rbrack=10^{-8}[/tex]

And once again the pH formula:

[tex]pH=-\log _{10}\lbrack H^+\rbrack=-\log (10^{-8})=8[/tex]

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