Can you help me with #3 only? 2. When using a laboratory instrument to analyze a
ID: 1034882 • Letter: C
Question
Can you help me with #3 only?2. When using a laboratory instrument to analyze a chemical in a sample, often we must first "calibrate "the instrument using known concentrations of the chemical of interest and measuring the corresponding signal from the instrument for each concentration. Absorption spectroscopy is a common method of chemical analysis is to see how much light is absorbed by a substance at a particular wavelength. a) An analysis of lead in water was made using an atomic absorption instrument at a wavelength of 217 nm. The calibration data was obtained using known lead standards with the parts-per-billion concentrations given below. The resulting signal is listed beside each standard concentration measured. Known Lead concentration (ppb) 2.00 5.00 10.0 20.0 Signal (Absorbance) 0.067 0.114 0.239 0.463 101
Explanation / Answer
a) Plot signal (absorbance) vs known lead concentration (ppb) as below.
b) The linear equation for the calibration curve is given as
y = 0.0224x + 0.0134
c) The diluted solution of the unknown sample containing lead gave an absorbance of 0.301, i.e, y = 0.301. Put this in the linear equation and obtain x (concentration of lead in the diluted solution of the unknown).
0.301 = 0.0224x + 0.0134
=====> 0.0224x = 0.301 – 0.0134 = 0.2876
=====> x = 0.2876/0.0224 = 12.839 ? 12.84
The concentration of lead in the diluted sample of the unknown is 12.84 ppb (ans).
d) 10.00 mL of the unknown sample containing lead was taken and diluted to a final volume of 100.00 mL. The dilution factor is
DF = (final volume of solution)/(volume of unknown taken) = (100.00 mL)/(10.00 mL) = 10.00.
The concentration of lead in the unknown solution = (concentration in the diluted sample)*(DF) = (12.84 ppb)*(10.00) = 128.4 ppb (ans).
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