Question

Chemistry lab question?!?

3.The titration procedure you have just performed is one that is carried out, exactly as you did it, by water-quality workers throughout the world. They use this exact kit, so what you have done is on-the-job training. There is a difference, however. These kits were set up using water samples provided for you. In real-life situations, workers collect water samples in a lake or stream instead of using Samples A and B that you tested. For this lab to work, sodium carbonate was added to make the water samples alkaline. In lakes and streams it is calcium carbonate (CaCO3) that causes alkalinity. Calcium carbonate acts as a buffer, a substance that serves to resist small changes in acidity or alkalinity in a solution. When acid rain enters lake water, it can be neutralized by the bases present and thus the lake does not become too acidic. Since the key ion is the hydroxide ion, the nature of the cation doesn't matter in this case. All over the world, right now, pollution workers are measuring calcium carbonate in lakes and streams.

If a worker studied two lakes and Lake X had a greater ppm of calcium carbonate than Lake Z, which of the two lakes would be able to withstand a greater amount of acid rain? Explain your answer.

Answers

Calcium carbonate is an ionic salt that is slightly soluble in water according to the following equation:

CaCO3(s) ? Ca^2+(aq) + CO3^2-(aq)

The carbonate ions (CO3^2-) further interact with water molecules forming hydrogen carbonate ions and hydroxide ions according to the following equilibrium equation:

CO3^2-(aq) + H2O(l) ? HCO3^-(aq) + OH^-(aq)

When acid rains falls, whatever acids are dissolved in it contribute the H^+ ions that react with hydroxide ions (OH^-) forming water molecules according to the following equation:

H^+(aq) + OH^-(aq) → H2O(l)

The above equation tends to decrease the amount of available hydroxide ions making the above equilibrium to shift to the right, trying to replace the missing hydroxide ions. This in turn, uses up the available carbonate ions (from calcium carbonate present). As long as there is a steady supply of extra carbonate ions, the concentration of hydroxide ions will remain fairly constant in spite of addition of acid rain. That in turn will determine the concentration of hydrogen ions, and the pH of the water.

Answer: If Lake X has a greater parts-per-million amount of calcium carbonate than Lake Z, it will be able to withstand a greater amount of acid rain than Lake Z, because it will have a greater supply of carbonate ions.

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