Question

The selenium content in soil can be determined fluorometrically upon conversion of the selenium to the fluorescent product shown, followed by extraction in cyclohexane. Emission from the fluorescent product is observed at 518 nm. The selenium content in a 0.101 g soil sample was converted to the fluorescent product and extracted in 50.0 mL of cyclohexane. The method of standard additions was used to determine the selenium content in the soil sample. Initially, 3.00 mL of the cyclohexane solution were placed in a cuvette for fluorescence determination. Several small additions of a 1.75 ug Se/mL standard solution containing the fluorescent product were added to the 3.00 mL solution in the cuvette. The fluorescence was measured at 518 nm. The results are shown in the table. Total added Se standard volume (ul) 0.00 20.0 Fluorescence intensity (arbitrary units) 54.3 64.5 76.7 91.1 108.3 40.0 60.0 80.0 Determine the weight percent (wt%) of selenium in the soil sample and find the absolute uncertainty in the weight percent of selenium. Report the weight percent and absolute uncertainty with three significant figures. Se wt% = do +

Answer 1

The Selenium in the Soil Sample is extracted in to a Soml Solution 3.00ml of this solution was taken and analyzed for fluoroscence. Several volumes cha torniga 1.75419 Selmu sclitors were added. Consider two solutions : 200L 64.5 YouL 16.7 Let concentration of se in the soil sample scle be weg Selml• 2001 : 208.109 mb Calculate the concentration of se in the solution in the cuvette as follows: 1.75 ligimL x 20x109 mL = 302 ml x concentration Concentration = 0.0116 og Selmi Fluorosence- kx Concentration 64.5= kx(0.016+w)_ O Similoudy, The sclition with 40.0.4L added gives 767 So we get 76.] = %x10.0232 +2) -- Use both the equ. to calculate the value of ne From equ-Oi ka __64:5_ O-0116+n Substitute in 76-7= 64.5_ osa. 0.012+ = 0.04973 So.concentration of se in the unknown Solution = 0.04973.leg Selml Mass ch Se in Soma 50mL X 0.04973 lg Selml= 2.486549 Se or wt% 0 Ser ( Weight se_ )x100 I weight of sample) (2048X10-62-)x100 = 0.00245%) T 0.1019 2.48X1056g

To calculate the uncentainity, the Uncentainity of the volumes adde mass of the sample must be known for multiplication, the resultant Lincentainity - sumo squares of percent percent uncertainity - Absolute uncentainity = x 100 O magnitude dy measure theret Hope this helps Chcertainities