Standard addition | Chemistry homework help

The selenium content in a 0.145 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. 3.00 mL of the cyclohexane solution were placed in a cuvette for fluorescence determination. Several small additions of a 2.76 ?g Se/mL standard solution containing the fluorescent product were added to the 3.00 mL solution in the cuvette as shown in the table below. The fluorescence was measured at 518 nm.

The Table is attached.

Determine the weight percent (wt%) of selenium in the soil sample and find the absolute uncertainty in the wt% Se. Report the weight percent and absolute uncertainty with three significant figures.