A 0.2056 g chocolate sample was digested (completely dissolved) in nitric acid a
ID: 486829 • Letter: A
Question
A 0.2056 g chocolate sample was digested (completely dissolved) in nitric acid and the resulting solution diluted to a total volume of 10.00 mL. A series of solutions were then prepared with total constant volumes of 5.00 mL, each containing 1.00 mL of the chocolate solution plus variable volumes of a 125 ppb chromium standard. The constant volume solutions were analyzed using ICP-MS to determine the chromium concentrations (results shown below). *See page 16 in textbook for ppb definition. What is the chromium concentration (in ppb) plusminus 95% confidence interval for the diluted (5 mL) sample? What is the chromium concentration (in ppb) for the original (solid) chocolate sample?Explanation / Answer
8. From the given data set
A. concentration of standard in solution when,
20 uL of it was added to 5 ml total volume
= 125 ppb x 0.02 ml/5 ml
= 0.5 ppb
let Cx be the concentration of chromium in diluted sample
Cx/(Cx + 0.5) = 9508/14062
14062Cx = 9508Cx + 4754
Cx = 1.044 ppb
B. Concentration of chromium in 1 ml sample = 1.044 ppb x 5 = 5.22 ppb
concentration in original 10 ml sample = 5.22 x 10 = 52.2 ppb
concentration of chromium in original chocolate sample - 52.2 ppb
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