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8.1 Ions in aqueous solution

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In a solution of ethanoic acid (or acetic acid), the following equilibrium is established:

CH 3 COOH (aq) + H 2 O ⇌ CH 3 COO - (aq) + H 3 O +

The concentration of CH 3 COO - ions is found to be 0,003 mol · dm - 3 . Calculate the pH of the solution.

  1. According to the balanced equation for this reaction, the mole ratio of CH 3 COO - ions to H 3 O + ions is the same, therefore the concentration of these two ions in the solution will also be the same. So, [H 3 O + ] = 0,003 mol · dm - 3 .

  2. pH = -log[H 3 O + ]

    = -log(0,003)

    = 2,52

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Understanding pH is very important. In living organisms, it is necessary to maintain a constant pH so that chemical reactions can occur under optimal conditions.

It may also be useful for calculations involving the pH scale, to know that the following equation can also be used:

[H 3 O + ][OH - ] = 1 × 10 - 14

Interesting fact

A build up of acid in the human body can be very dangerous. Lactic acidosis is a condition caused by the buildup of lactic acid in the body. It leads to acidification of the blood (acidosis) and can make a person very ill. Some of the symptoms of lactic acidosis are deep and rapid breathing, vomiting and abdominal pain. In the fight against HIV, lactic acidosis is a problem. One of the antiretrovirals (ARV's) that is used in anti-HIV treatment is Stavudine (also known as Zerit or d4T). One of the side effects of Stavudine is lactic acidosis, particularly in overweight women. If it is not treated quickly, it can result in death.

In agriculture, farmers need to know the pH of their soils so that they are able to plant the right kinds of crops. The pH of soils can vary depending on a number of factors such as rainwater, the kinds of rocks and materials from which the soil was formed and also human influences such as pollution and fertilisers. The pH of rain water can also vary and this too has an effect on agriculture, buildings, water courses, animals and plants. Rainwater is naturally acidic because carbon dioxide in the atmosphere combines with water to form carbonic acid. Unpolluted rainwater has a pH of approximately 5,6. However, human activities can alter the acidity of rain and this can cause serious problems such as acid rain.

Calculating ph

  1. Calculate the pH of each of the following solutions:
    1. A 0,2 mol · dm - 3 KOH solution
    2. A 0,5 mol · dm - 3 HCl solution
  2. What is the concentration (in mol · dm - 3 ) of H 3 O + ions in a NaOH solution which has a pH of 12?
  3. The concentrations of hydronium (H 3 O + ) and hydroxyl (OH - ) ions in a typical sample of seawater are 10 - 8 mol · dm - 3 and 10 - 6 mol · dm - 3 respectively.
    1. Is the seawater acidic or basic?
    2. What is the pH of the seawater?
    3. Give a possible explanation for the pH of the seawater.
    (IEB Paper 2, 2002)

The following simulation allows you to test the pH of various substances.

run demo

Acid rain

The acidity of rainwater comes from the natural presence of three substances (CO 2 , NO, and SO 2 ) in the lowest layer of the atmosphere. These gases are able to dissolve in water and therefore make rain more acidic than it would otherwise be. Of these gases, carbon dioxide (CO 2 ) has the highest concentration and therefore contributes the most to the natural acidity of rainwater. We will look at each of these gases in turn.

Questions & Answers

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Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
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Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
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"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
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Source:  OpenStax, Siyavula textbooks: grade 10 physical science. OpenStax CNX. Aug 29, 2011 Download for free at http://cnx.org/content/col11245/1.3
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