Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples351Practice Exercise 7.5If we collect a single sample (cost $20), then we can analyze that sample13 times (cost $650) and stay within our budget. For this scenario, thepercent relative error is2 2s se = samptn+ methn n= 2 179 010 .1+ 0.20..1× 13= 074sampsamprepwhere t(0.05, 12) is 2.179. Because this percent relative error is larger than±0.50%, this is not a suitable sampling strategy.Next, we try two samples (cost $40), analyzing each six times (cost $600).For this scenario, the percent relative error is2 2s se = samptn+ methn n= 2 2035 010 ..2+ 020 .057 .2× 6=sampsamprepwhere t(0.05, 11) is 2.2035. Because this percent relative error is largerthan ±0.50%, this also is not a suitable sampling strategy.Next we try three samples (cost $60), analyzing each four times (cost$600). For this scenario, the percent relative error is2 2s se = samptn+ methn n= 2 2035 010 ..3+ 020 .049 .3× 4=sampsamprepwhere t(0.05, 11) is 2.2035. Because both the total cost ($660) and thepercent relative error meet our requirements, this is a suitable samplingstrategy.There are other suitable sampling strategies that meet both goals. Thestrategy requiring the least expense is to collect eight samples, analyzingeach once for a total cost of $560 and a percent relative error of ±0.46%.Collecting 10 samples and analyzing each one time, gives a percent relativeerror of ±0.39% at a cost of $700.Click here to return to the chapter.Practice Exercise 7.6The fluoride ion, F – , is a suitable masking agent as it binds with Al 3+ toform the stable AlF 6 3– complex, leaving iron in solution.Click here to return to the chapter.Practice Exercise 7.7The relevant reactions and equilibrium constants are2Fe + 2( aq) + 3phen( aq) Fe( phen)+ ( aq)β = 5×103320