I think the easy way to do this is to graph it as mv on the y axis and M on the x axis. If you do that you can estimate just on scratch paper, that it is about 2.7 x 10^-3 M for the unknown. If you want to do it without graphing you can try this
You have -211.3 mv for 1.00E-3M and -238.6. mv for 4.00E-3. So that's (238.6-211.3 = 27.3 mv for a difference in M of 4.00E-3 - 1.00E-3 = 3.00E-3 or
3.00E-3/27.3 = 1.09E-4M (and you may want to carry that out more) for each 1 mv.
(I've ignored the - sign for the voltage. You can keep it straight in yur head.)
So how far are you from the 1E-3 standard. That's 226.5-211.3 = 15.2 mv away and at 1.09M/mv x 15.2 mv = about 1.67E-3M and that + 1.00E-3 from the starting point makes about 2.67M for the unknown. We should get close to that if we try the top end and come down. You can do that yourself if you wish but it's something like this.
Difference is 238.5-226.5 = 12.10 x 1.09E-4 = 1.32M so we should be down that far from the 4.00E-3.
4.00E-3-1.32E-3 = about 2.68E-3 for the unknown. If you will put this on a graph and determine the slope of the line and the intercept for y = mx+b you can calculate exactly what it should be for the unknown.
2. A fluoride electrode is used to determine fluoride in a water sample. Standards and samples are diluted 1:10 with TISAB solution. For a 1.00 x10-3M(before dilution) standard, the potential reading relative to the reference electrode is -211.3mV and for a 4.00 x10-3M standard it is -238.6mV.The reading with the unknown is -226.5mV.What is the concentration of fluoride in the sample?
1 answer