If you add acid precipitation to a lake, will it always reduce the pH of the lak
ID: 482721 • Letter: I
Question
If you add acid precipitation to a lake, will it always reduce the pH of the lake water? Why or why not?
What is the molecular weight of CaCO3?
How many moles of H+ can be neutralized per mole of CaCO3 (creating carbonic
acid)?
In our lab, we will determine the alkalinity of samples of water where the alkalinity may be dominated by the following possible five combinations of anions:
i) carbonate [CO3-2],
ii) bicarbonate [HCO3 ]
iii) hydroxide only [OH ]
iv) carbonate [CO3-2] and bicarbonate [HCO3 ] v) carbonate [CO3-2] and hydroxide [OH ]
The combination of alkalinity dominated by bicarbonate [HCO3 ] and hydroxide [OH ] wasn’t included. Why not? Consider the range of pH over which bicarbonate dominates the total carbonate system.
Explanation / Answer
It depends on the nature of the soil and water of the lake.
If the lake water is having basic nature then its pH will be reduced by adding an acid.
Molecular weight of CaCO3 is --------------(40+12+16x3) = 100g/mol
Calcium is dibasic , hence it can neytralise two moles of H+
CaCO3 + 2H+ ------------> Ca+2 + H2CO3(carbonic acid)
Alkalinity is primarily a way of measuring the acid neutralizing capacity of water.
In other words, its ability to maintain a relatively constant pH.
The possibility to maintain constant pH is due to the hydroxyl, carbonate and bicarbonate ions present in water.
The ability of natural water to act as a buffer is controlled in part by the amount of calcium and carbonate ions in solution.
Carbonate ion and calcium ion both come from calcium carbonate or limestone.
So water that comes in contact with limestone will contain high levels of both
Ca++ and CO32- ions and have elevated hardness and alkalinity.
When a water sample that has a pH of greater than 4.5 is titrated with acid to a pH 4.5 end point, all OH-, CO32-, and HCO3-will be neutralized.
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