How would you prepare 500.0 ml. of a 0.02500 M Cr solution from solid K_2Cr_2O_7

Question

How would you prepare 500.0 ml. of a 0.02500 M Cr solution from solid K_2Cr_2O_7 (molar mass 294.185)? Use appropriate significant figures. Assume the purity of the potassium dichromate is essentially 100 percentage. Calculate the solubility of CaSO_4, i.e., the [Ca^2] (Ksp^-2.4 times 10^-5) in distilled water, and in 0.025 M K_2SO_4 (include activity effects). The effective hydrated ionic diameter, a_5, for Ca^2+ and SO_4^2 are 0.6, respectively. A 0.1809 g piece of pure Fe wire was dissolved in acid, reduced to the 2+ state and used to standardize a Ce^4+ solution. It required 31.33 mL of the Ce^4+ solution to titrate all the Fe^2+. Given they react 1:1, calculate the molarity of the Ce^4+ solution. A blood sample is analyzed for Pb^2+ by spectrophotometry using a single spike standard addition method. A 1.00 ml, sample of blood is diluted to 5.00 mL, and the diluted sample gave an absorbance of 0.193. A second 1.00 mL, sample of the blood was spiked with 1.00 mu L of a 1560 ppb pb^2+ solution, and diluted to 5.00 mL. The spiked diluted sample had an absorbance of 0.419. Calculate the concentration of Pb^2+ in the blood. A pesticide has hexane-water partition coefficient of 615. Your want to extract a 1.0 L, water sample, and perform two extractions on each sample. What volume of hexane should be used to remove 99.9 percentage of the pesticide from the sample (i.e., [P]/[P]_s = 0.001). Write down the equation relating fluorescence intensity intensity F to concentration C. Give two reasons why fluorescence is inherently a more sensitive analytical technique than absorbance for compounds that fluoresce. List a type of spectroscopy associated with the X-ray, UV/Visible, Infrared, and Radio regions of the electromagnetic spectrum. Calculate the wavelength of photon with an energy of 7.02 times 10^-19 J. What region of the electromagnetic spectrum is this light in? Write down the van Deemter Equation, define each term, and explain why open tubular columns produce better resolution than packed columns. What are the assumptions about the data when using linear regression? And therefore, when using linear regression to produce a calibration curve, why is it not okay to plot the standard concentrations on the y-axis and the instrument response on the x-axis?

Explanation / Answer

1. To prepare a 500 ml of 0.025 M Cr solution

1 mole of K2Cr2O7 has 2 moles of Cr

mass of K2Cr2O7 = 0.025 x 0.5 x 294.185/2 = 1.84 g

So dissolve 1.84 g of K2Cr2O7 in 500 ml to get 0.025 M Cr solution

2. Solubility of CaSO4

(a) In distilled water

Ksp = [Ca2+][SO4^2-]

2.4 x 10^-5 = x^2

solubility = x = 4.90 x 10^-3 M

(b) In 0.025 M K2SO4

ionic strength (u) = 1/2(0.05 x 1 + 0.025 x 4) = 0.075

activity coefficient for,

logY[SO4^2-] = -0.51 x 2^2 x sq.rt.(0.075)/(1 + 3.3 x 0.4 x sq.rt.(0.075)

Y[SO4^2-] = 0.39

Ksp = [Ca2+][SO4^2-]

2.4 x 10^-5 = [Ca2+](0.39 x 0.025)

solubility = [Ca2+] = 2.46 x 10^-3 M

3. Fe2+ reacts with Ce4+ in a 1 : 1 ratio

moles of Fe = 0.1809 g/55.845 g/mol = 0.00324 mol

Molarity of Ce4+ solution = 0.00324 mol/0.03133 L = 0.103 M

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