Why can\'t you just measure the pH of a water sample to tell if it is susceptibl
ID: 1066345 • Letter: W
Question
Why can't you just measure the pH of a water sample to tell if it is susceptible to acid rain? How can you account for the differences in the ANC values between your samples (what environmental factors contribute to ANC)? If you titrated to an endpoint pH of 4.1 by accident would this make your ANC value appear higher or lower than it truly is in the sample? What type of error is this? Why is the HCI you used in this experiment standardized? Give a procedure for making and standardizing a 0.02 M solution of HCI from concentrated (12 M) HCI.Explanation / Answer
1. Water is amphoteric in nature. It is a weak buffer. The concentration of H+ and OH- ions in water at any given time remains constant. Thus measuring pH of it becomes difficult.
2. The ANC vaue for the samples would vary depending upon the type of impurity or acid present. If a strogn acid sample is present, the ANC is directly proportional to the moles of acid present. Whereas, in case of weak polyprotic acid, depending upon the temperature, concentration of solution, the value changes.
3. If we titrated a sample to its end point, the ANC would be higher than the actual value. This is experimental error.
4. HCl is standarized in the experiment as it is used as the titrant to determine the exact moles of base used. So one of the solutions either acid or base concentration must be known to calculate and run the experiment.
Preparation of 0.02 M HCl starting with 12 M HCl solution, say the final volume of 0.02 M HCl be 500 ml
volume of 12 M HCl solution required = 0.02 M x 500 ml/12 M = 0.83 ml
So take 0.83 ml of 12 M HCl and dilute upto 500 ml with water in a volumetric flask.
HCl solution thus preparaed can be standarized with Na2CO3 solution. 1 mole of Na2CO3 required 2 mole of HCL for complete reaction.
Prepare a 0.02 M Na2CO3 solution and titrate it against HCl to calculate the exact molarity of HCl solution.
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