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# In A Solution At 25°c, The [h+] is 3.5 × 10−6 m. What Is The [oh−] ?

The pH of a solution can be determined by the concentration of H+ ions in that solution. The pH scale ranges from 0 to 14, with lower numbers indicating increasing acidity and higher numbers indicating increasing alkalinity. The pH of pure water is 7, which is neither acidic nor alkaline, but neutral (neither acidic nor alkaline). The pH scale is logarithmic: for example, if the pH of a solution changes from 7 to 6, there are 10 times as many H+ ions present (not 1/10th).

In this problem, we are given the concentration of H+ ions in an unknown solution; we need to find out what value this represents on the pH scale by using the relationship between these two concentrations.

This is a simple question. The answer is 7.5 × 10−9 m.

You start by knowing that the pH of this solution is 3.5. So you can use the Henderson Hasselbalch equation to find out how much OH− there is in the solution:

pH = pK + log10(OH−)

pH = 3.5 + log10(7.5 × 10−9)

pH = 3.5 + (-3)

pH = 0

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