9. (25 pts.) Calcium was determined with atomic absorption spectroscopy through

Question

9. (25 pts.) Calcium was determined with atomic absorption spectroscopy through standard addition. Results are provided in the graph below. Samples were prepared by adding 5.00 mL of your unknown to each 50 mL volumetric flask, adding specific amounts of Ca standard and diluting to the mark. The concentration of Ca standard after dilution to the mark is plotted on the x-axis in g/mL. Calculate the concentration of Ca in your unknown sample both in terms of ppm and M. Run 5 Standard Addition Plot (plot 5) y-6.004E+07x1.016+02 Rz= 9.994E-01 50 0.000002 0.000001 0.000001 0.000002 0.0000030.000004 0.0oDooS 50 Concentration Ca spike (9/mL) 10.(15 pts.) Using activities calculate the pH of a solution containing 0.005 M

Explanation / Answer

The absolute x-intercept value is give the diluted concentration of the sample

So, absolute x-intercept value = 1.7×10^-6g/mol

5ml of sample is diluted to 50ml

dilution factor = 50/5 = 10

Concentration in original sample solution= 10×1.7×10^-6g/ml =1.7×10^-5g/ml = 1.7×10^-2mg/ml=17mg/L= 17ppm

Molar mass of Ca = 40.08g/mol

Mass of Calcium present in 1L = 0.017g

No of mole of Calcium = 0.017g/40.08(g/mol) =4.24×10^-4mol

Molarity of Ca = 4.24×10^-4mol/L = 4.24×10^-4M

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