Lake Lianna is fed by two rivers, the Matthew and the Andrew. You have made care
ID: 601601 • Letter: L
Question
Lake Lianna is fed by two rivers, the Matthew and the Andrew. You have made careful water chemistry measurements throughout the year and found that the two rivers have the following pH and alkalinity:Matthew River: pH = 8.2; alkalinity = 10-3 equivalents/liter
Andrew River: pH = 5.7; alkalinity = 10-5 equivalents/liter.
Your studies of river fluxes show that Lake Lianna is formed by mixing 2 parts of the Matthew with 3 of the Andrew. Calculate its pH, assuming that Lake Lianna is equilibrated with the atmosphere. Your measured pH value for Lake Lianna is 7.6. Why might this differ from the value you calculate based on mixing of the two sources?
Explanation / Answer
calculated pH = (8.2*2 + 5.7*3)/5 = 6.7 Alkalinity is the measure of the buffering capacity of water = the capacity of bases to neutralize acids. It is the ability of the water to resist the pH. alkalinity = [(10^-3)*2 + (10^-5)*3]/5 = 4.06x10^-4 This alkalinity, is responsible for the increase in pH from 6.7 to 7.6. It neutralizes some amount of acids in the mixture system of the two rivers into lake lianna.
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