Кислотные дожди

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 rejoicethis. 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} /(0.1x)=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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Weak acids
The question then arises, and how to calculate the acidity of solutions of weak acids. In fact, because if the acid dissociates completely, then how do we know which molecules dissociate and what is not.
Thus, we come to the concept of the acidity constant. Assume that there is a weak acid HA, which dissociates into H ^{+} ion and A ^{.} Based on the fact that we have a weak acid and it dissociates completely, we understand that at some point we have to establish some form of concentration of the undissociated acid and ions. We call such a concentration equilibrium. Suppose that the equilibrium concentration of protons is x, then the equilibrium concentration of the anions of acidic residues is also equal to x. The equilibrium concentration of the acid is equal to the difference between the initial concentration of acid (e.g. C _{0)} and the concentration of dissociated molecules (i.e., x). The ratio of x * x / (C _{0} x) called constant acidity. These data are presented in different tables. How to use them in the calculation of pH  the next time
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It's strange the word "pH"
So, what is a notorious acidity, and how is it measured?
Imagine that we took a solution of a strong acid. This acid, in particular hydrochloric acid is (i.e. hydrogen chloride in water). HCl in contact with water is fully dissociated, i.e. splits into ions. Ie each particle of HCl in water gives one hydrogen ion (H +) and 1 chloride ion (Cl). Imagine that the hydrochloric acid concentration was 1 mol / l (i.e. in 1 liter of a solution containing 1 mole of acid). This means that the concentration of hydrogen ions (or protons) is the same. If the concentration of hydrochloric acid was 0.1 mol / l, and then the concentration of protons is the same.
Slightly more complicated in the case of sulfuric acid. She, as you probably know, contains 2 hydrogen atoms (its formula H2SO4). In the sulfuric acid solution with a concentration of 1 mol / l concentration of protons will be 2 mol / L (after dissociation with one molecule of sulfuric acid, a hydrogen atom 2).
The proton concentration (ions H +) and determines the acidity of the medium. The higher it is, the more acidic environment. However, it so happened that in chemistry began to enjoy not only the concentration and negative logarithm of concentration.
What is a logarithm? Recall the construction of the power. For example, 10 ^{2} = 10 * 10 = 100. Now we will try to solve the equation x ^{2} = 100. We know that to find x, it is necessary to take the square root of 100, and now we solve the equation ^{x} = 10 100. To find x, we must calculate the logarithm to the base ten hundred. Ie logarithm  is the degree to which you want to build a base to give the desired number. If the logarithm of the decimal, so the base is 10.
If the concentration is equal to 1, the logarithm of unity is zero. In fact, to get one, you want to raise ten to the power of 0 If the concentration is equal to 0.1, the logarithm is 1 (after 10 ^{1} = 0.1). However, the acidity was positive to the logarithm pririsovyvayut minus sign. Negative logarithm in physics and chemistry denoted by the letter p. Ie pH  it is simply the negative logarithm of hydrogen ion concentration (H ^{+).}
Of course, you may wonder what to do with the type of concentration 0.5 0.2 etc. To do this, there are scientific calculators which logarithm is denoted by an lg
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Dissociation of acids
As we remember from chemistry course grade 7 (and someone probably read about it for the first time), the acid is a complex substance containing a hydrogen atom capable of substitution on the metal atom and the acid residue. It turns out that the ability to replace the hydrogen atom to the metal atom is linked to the process of dissociation. If we imagine that the acid is composed of n atoms of hydrogen and acid residue A, it turns out that these n hydrogen atoms by dissolving the acid in water, one detached from the acid residue A in the form of positively charged hydrogen ions (H +). Depending on the nature of the acid residue, the process may proceed to 100%, i.e. completely irreversible (and we are talking about strong acids), and it happens that it takes place completely. Then we are talking about weak acids
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Two words about the dissociation
Let's try to sort out the most features that a theory of dissociation, why it is needed.
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Alternative measure of acidity. What is measured?
So we stayed on the fact that the measurement medium acidity using indicator paper may be inaccurate. How can I avoid this?
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Again, the scientific and not quite
So, we still continue to talk about and not very scientific.
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Scientific and not very
Alas, that the participants in this project is very small, and the conversation is not yet glued. Still, I very much hope that the project participants will join in the conversation, and we will launch a fruitful discussion.
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The first questions
So, in this project begin to appear the first results. I am very grateful to the children and their teachers, they decided to participate in this project. Initial support  it is very important. Thank you very much.
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