# Acid Rain

## So, consider the pH of a weak acid

Colleagues, well, first of all, I congratulate all of you on the beginning of the school year. Second, the beginning of autumn. So, soon the rains. And then it's time, it's time to rejoice-this. We will collect rainwater, measure its pH =). And there - and winter is not far off. We verify the acidity of snow.

In the last article we stayed on the fact that there is a weak acid acidity constant. And using this value can be calculated pH acid solution. How do we do it?

Take for example a weak acetic acid. Assume that its concentration in the solution was 0.1 mol / l. For her, the acidity constant is known (for example, that here it is written) and is equal to 1.7 × 10 ^{-5.}

We write the equation of dissociation of acetic acid: CH _{3} -COOH = CH _{3} COO ^{- +} H +. Formally, an equal sign here is incorrect. It is necessary to portray the character of equilibrium (look at the first reaction here ), but I do not do that, so as not to overload the text images. Assume that x mol / L acetic acid dissociated. Then the concentration of CH _{3} COO ^{-} and H ^{+} is the same and equal to x. Hence we obtain (see previous post =)) x ^{2} /(0.1-x)=1.7*10 ^{-5.} Anyone can convert this equation in the usual square, but I would suggest such a course. Acid called weak if it dissociates at low power. Ie x is small. If we assume that x is much less than 0.1, it appears that the 0.1 's - it's almost 0.1. PLEASE NOTE! I do not equate x to zero. Then the whole fraction is reset. I equate assume 0.098 to 0.1. Then it turns out that x ^{2} = 1.7 × 10 ^{-6.} Or x = 1.3 * 10 ^{-3} mol / l. This proton concentration corresponds pH 2,9.

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