Problem Set 1

Problem set1 : 1. In spectrophotometry, we measure the concentration of analyte by its absorbance of light. A lowconcentration sample was prepared, and nine replicate measurements gave absorbances of 0.0047, 0.0054, 0.0062, 0.0060, 0.0046, 0.0056, 0.0052, 0.0044, and 0.0058 (these data gave a better estimate of standard deviation compared to blank data). Nine reagent blanks gave values of 0.0006, 0.0012, 0.0022, 0.0005, 0.0016, 0.0008, 0.0017, 0.0010, and 0.0011. (a) Find the minimum detectable signal (b) The calibration curve is a graph of absorbance versus concentration. Absorbance is a dimensionless quantity. The slope of the calibration curve is m= 2.24 x 104 M-1. Find the concentration detection limit. (c) Find the lower limit of quantitation 2. Low concentrations of Ni2+-EDTA near the detection limit gave the following counts in a mass spectral measurement: 175, 104, 164, 193, 131, 189, 155, 133, 151, 176. Ten measurements of a blank had a mean of 45 counts. A sample containing1.00M Ni2+-EDTA gave 1 797 counts. Estimate the detection limit for Ni2+EDTA. 3. Pure water containing no arsenic was spiked with 0.40 μg arsenate/L. Seven replicate determinations gave 0.39, 0.40, 0.38,0.41, 0.36, 0.35, and 0.39 μg/L.15 Find the mean percent recovery of the spike. 4. Three replicate determinations of Stotal for a standard solution that is 10.0 ppm in analyte give values of 0.163, 0.157, and 0.161 (arbitrary units). The signal for the reagent blank is 0.002. Calculate the concentration of analyte in a sample with a signal of 0.118. 5. A 10.00-g sample containing an analyte is transferred to a 250-mL volumetric flask and diluted to volume. When a 10.00 mL aliquot of the resulting solution is diluted to 25.00 mL it gives signal of 0.235 (arbitrary units). A second 10.00-mL portion of the solution is spiked with 10.00 mL of a 1.00-ppm standard solution of the analyte and di- luted to 25.00 mL. The signal for the spiked sample is 0.502. Calculate the weight percent of analyte in the original sample. 6. Serum containing Na gave a signal of 4.27 mV in an atomic emission analysis. Then 5.00 mL of 2.08 M NaCl were added to 95.0 mL of serum. This spiked serum gave a signal of 7.98 mV. Find the original concentration of Na in the serum. 7. In a preliminary experiment, a solution containing 0.0837 M sample (X) and 0.066 6 M S gave peak areas of AX =423 and AS= 347. (Areas are measured in arbitrary units by the instrument’s computer.) To analyze the unknown, 10.0 mL of 0.146 M S were added to 10.0 mL of unknown, and the mixture was diluted to 25.0 mL in a volumetric flask. This mixture gave signal of AX =553 and AS=582. Find the concentration of X in the unknown. 8. To determine the concentration of analyte in a sample, a standard ad- ditions was performed. A 5.00-mL portion of sample was analyzed and then successive 0.10-mL spikes of a 600.0-mg/L standard of the analyte were added, analyzing after each spike. The following table shows the results of this analysis.

Vspike (mL)

0.00

0.10

0.20

0.30

Stotal (arbitrary units)

0.119

0.231

0.339

0.442

Construct an appropriate standard additions calibration curve and use a linear regression analysis to determine the concentration of analyte in the original sample. 9. Loconto and co-workers describe a method for determining trace levels of water in soil. The method takes advantage of the reaction of water with calcium carbide, CaC2, to produce acetylene gas, C2H2. By carrying out the reaction in a sealed vial, the amount of acetylene produced may be determined by sampling the headspace. In a typical analysis a sample of soil is placed in a sealed vial with CaC2. Analysis of the headspace gave a blank corrected signal of 2.70 × 105. A second sample is prepared in the same manner except that a standard addition of 5.0 mg H2O/g solid is added, giving a blank-corrected signal of 1.06 × 106. Determine the milligrams H2O/g soil in the soil sample. 10. Convert the following data to standard uncertainties a. The manufacturer’s specification for a 100 mL Class A volumetric flask is 100 ± 0.08 (Assume triangular distribution). b. The purity of a compound is given by the supplier as 99.9 ± 0.1%. (Assume rectangular distribution) c. The standard deviation of 4 set of repeat weighing of a 0.3 g check weight is 0.00021 g.(i. assume s=σ, b. s is not equal to σ). 11. You prepared a 0.250 M NH3 solution by diluting 8.45 (±0.04) mL of 28.0 (±0.5) wt% NH3 [density =0.899 (±0.003) g/mL] up to 500.0 (±0.2) mL. Find the uncertainty in 0.250 M. The molecular mass of NH3, 17.0305 g/mol, has negligible uncertainty relative to other uncertainties in this problem.