Natural Salt Solutions 2: Analysis of Seawater Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Appendix Experiments Index ChemLab Home		FAQ Index Question 1 What do I do if I overshot the endpoint and added too much NaOH? Question 2 Is KHP a monoprotic or polyprotic acid? What is the mole/mole ratio when KHP reacts with NaOH in a titration? Question 3 I am having trouble with prelab problem one. How do I know which ion in parts c and d replaces the H+ ion in the column? Is it Mg+2 or Al+3 in part c; the Fe+2 or NH4+ in part d? Question 4 In prelab problem 3, how are we suposed to know the expected number of cations in our sea water sample? Question 5 How do I calculate the endpoint? Question 6 What is the concentration of the base that I will use? Question 7 What value are we supposed to use for "t" to calculate the 95% confidence interval? The lab manual only lists "t" for a limited number of samples, and we have 38, which is not included in the table. Submissions Submit a question or an answer to the FAQ Questions & Answers Question 1 What do I do if I overshot the endpoint and added too much NaOH? Professor Milde Unfortunately, there is nothing you can do except try again with another sample. Use the experience to nail the endpoint the next time around. Question 2 Is KHP a monoprotic or polyprotic acid? What is the mole/mole ratio when KHP reacts with NaOH in a titration? Professor Milde KHP, or potassium hydrogen phthalate, has one ionizable proton, so acts as a monoprotic acid. This means that one mole of KHP will react with one mole of NaOH, in your titration. We use KHP as a primary standard because it is a stable solid, easily and accurately measured by mass. The hydrogen pthalate ion is actually the conjugate base of phthalic acid, which is a diprotic acid. Question 3 I am having trouble with prelab problem one. How do I know which ion in parts c and d replaces the H+ ion in the column? Is it Mg+2 or Al+3 in part c; the Fe+2 or NH4+ in part d? Professor Milde The sulfonic acid groups of the cation exchange resin will exchange with ANY and ALL positive ions. So for part c, BOTH Mg+2 AND Al+3 will exchange, giving protons (H+ ions) in the effluent in a charge-conserving way. To complete the problem, you need to decide how many milliequivalents of charge will be in the eluent solution, using the formula of the compound. From that, you can calculate how many millimoles of H+ will be present in the effluent and then the mmoles of NaOH required to neutralize the effluent in the titration. Question 4 In prelab problem 3, how are we suposed to know the expected number of cations in our sea water sample? Professor Milde The appendix at the end of the experiment, which describes the content of seawater in general terms, includes a value for the milliequivalents of cations per mL of seawater. Remember that your sample will be 2 mL of seawater. As a practical matter, the value in the appendix is probably greater than that of the samples you will analyze. These samples are collected on beaches, so are generally diluted with incoming fresh water and thus have a lower cation concentration than the "typical" seawater, described in the appendix. Question 5 How do I calculate the endpoint? Professor Milde The endpoint is when the moles of base added is equal to the moles of acid being titrated. So for the KHP titrations, you need to calculate the moles of KHP being titrated first. This is equal to the moles of OH- needed for the titration, since the KHP and OH- react in a 1:1 ratio. From the moles of base and the approximate molarity, you can calculate the approximate number of mL of solution needed to reach the endpoint. The example problem on page 72-73 of the manual show how to calculate the endpoint for the titration of an effluent solution from an ion exchange experiment. Question 6 What is the concentration of the base that I will use? Professor Milde The approximate concentration of base is given in the manual in the prelab problems. The actual concentration will be on the labels of the solution in the lab. It is determined by the lab staff and is slightly different each time the experiment is done. Question 7 What value are we supposed to use for "t" to calculate the 95% confidence interval? The lab manual only lists "t" for a limited number of samples, and we have 38, which is not included in the table. Professor Milde Use the value of "t" for the number closest to your number of samples. The "t" value for 38 samples is within a percent of the value for 40 samples. As the number of measurements increases, the t statistic approaches the limit for an infinite number of measurements. If you have more samples than the maximum number listed in the table, use the "t" value for an infinite number of samples. 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