To determine the concentration of [tex]\( \text{OH}^- \)[/tex] ions in the ammonia solution with a pH of 11.8, let's follow these steps:
1. Calculate the pOH:
According to the given information, the relationship between pH and pOH in a solution is:
[tex]\[
\text{pH} + \text{pOH} = 14
\][/tex]
Therefore:
[tex]\[
\text{pOH} = 14 - \text{pH}
\][/tex]
Substituting the given pH value:
[tex]\[
\text{pOH} = 14 - 11.8 = 2.2
\][/tex]
2. Calculate the concentration of [tex]\( \text{OH}^- \)[/tex] ions:
The concentration of [tex]\( \text{OH}^- \)[/tex] ions can be calculated using the formula:
[tex]\[
[\text{OH}^-] = 10^{-\text{pOH}}
\][/tex]
Substituting the calculated pOH value:
[tex]\[
[\text{OH}^-] = 10^{-2.2}
\][/tex]
3. Determine the exact value:
Calculating [tex]\( 10^{-2.2} \)[/tex] gives us:
[tex]\[
[\text{OH}^-] \approx 0.00631 \text{ M}
\][/tex]
Hence, the concentration of [tex]\( \text{OH}^- \)[/tex] ions in the solution is [tex]\( 0.00631 \text{ M} \)[/tex].
Among the given options, the correct answer is:
[tex]\[
6.31 \times 10^{-3} \text{ M}
\][/tex]
