2. A bottle of a concentrated aqueous sulfuric acid is labeled 98.0 wt % H 2 SO 4 (Molecular weight is g/mol) has a concentration of 18.0 M.

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1 Chem201, Winter 2006 Midterm N1 01/26/06 Name Answer key SID 1. A solution is prepared by dissolving 25.8 grams on magnesium chloride (MgCl 2 ) in water to produce ml of solution. Molecular weight of the MgCl 2 is 95.3 g/mol. Molecular weights of Mg and Cl are 24.3 g/mol and 35.5 g/mol, respectively. a. Calculate the molarity of the chloride ion in the solution. (3points) n Cl- = 2 n MgCl2 = 2 (m MgCl2/ MW MgCl2) = moles M Cl- = n Cl- / volume = moles / 0.25 L = 2.17 M b. What is the concentration of the Cl - in ppm? (3points) Cl - ppm = mass Cl - (mg) / volume = n Cl- MW Cl- / volume = x 35.5 x 1000 / 0.25L = ppm = 7.68 x 10 4 ppm c. Calculate the pcl - value for this solution. (3points) pcl - = -log [2.17] = A bottle of a concentrated aqueous sulfuric acid is labeled 98.0 wt % H 2 SO 4 (Molecular weight is g/mol) has a concentration of 18.0 M. a. How many milliliters of reagent should be diluted to L to give 1.00 M H 2 SO 4? (5 points) V con = V dil x (M dil / M con ) = 1000 ml x ( 1.00 M / 18.0 M) = 55.6 M b. Calculate the density of 98.0 wt % H 2 SO 4 (5 points) Mass of the 1 liter of H 2 SO 4 : (18 moles) (98.09) = 1.77 x 10 3 gr. Mass of the 1 ml of H 2 SO 4 : 1.77 g d = mass / weight % = 1.77 g / (0.98 g H 2 SO 4 /g solution) = 1.8 g/ml 3. How many milliliters of 3.00 M sulfuric acid are required to react with 4.35 g of solid containing 23.2 g wt % Ba(NO 3 ) 2 if the reaction is: Ba 2+ + SO 4 2- BaSO 4? (5 points) Molecular weights of BaSO 4 is g/mole and Ba(NO 3 ) 2 is g/mol. Mass Ba(NO 3 ) 2 is x 4.35 = 1.01 g

2 4. A sample is certified to contain 94.6 ppm of a contaminant. Your analysis gives values of 98.6, 98.4, 97.2, 94.6 and 96.2 ppm. Do you results differ from the expected result at following confidence levels: i) 95%, ii) 99% and iii) 99.9%. (9points) s = 1.65 = Significant difference No significant difference No, significant difference 5. Using the appropriate statistical test, decide whether the value 216 should be rejected from the set of result: 192, 216, 202, 195 and 204? (3 points) Gap = 12 Range = 24 Value to be retained.

3 6. The following data was collected when performing a spectrophotometric analysis for cobalt. x Analysis No mg Co / liter Absorbance a. Using the least squares method of linear regression, generate the equation to define the line for the absorbance vs. concentration. (10 points). y Thus, linear regression line is: b. Based on the equation you have generated, calculate the concentration of the Co in the sample if the absorbance is: i) (2 points) ii) (2 points) 7. Chloroform is an internal standard in the determination of the pesticide DDT in a polarographic analysis. A mixture containing mm chloroform and mm DDT gave signals of 15.3 μa for chloroform and 10.1 μa for DDT. An unknown solution (10.0 ml) containing DDT was placed in a ml volumetric flask and 10.2 μl of chloroform (FW g/mol, density = g/ml) were added. After diluting to the mark with solvent, polarographic signals of 29.4 and 8.7 μa were observed for the

4 chloroform and DDT, respectively. Find the concentration of DDT in unknown. (10 points) Chloroform is S, and DDT is X: Concentration of the chloroform in unknown: For the unknown mixture: DDT in unknown: 8. A beaker contains ml of molar silver ion (Ag + ). To this beaker is added ml of molar bromide ion (Br - ). What is the concentration of Ag + in the final solution? K sp for the AgBr is ? (5 points) Final concentration of the Ag + and Br - : Br - ion is in excess: = M. [Ag + ] = x and [Br - ] = (x+0.075) Assuming x << 0.075, we have x = [Ag + ] = Iron in the +2 oxidation state reacts with potassium dichromate to produce Fe 3+ and Cr 3+ according to the equation: 6 Fe 2+ + Cr 2 O H + 6 Fe Cr H 2 O How many milliliters of molar K 2 Cr 2 O 7 are required to titrate ml of molar Fe 2+ solution? (5 points)

5 Therefore, 10. A mixture having a volume of 10.0 ml and containing M Ag + and M Hg 2 2+ was titrated with M KCN to precipitate Hg 2 (CN) 2 (K sp = ) and AgCN (K sp = ). Calculate the concentration of the CN - at each of the following volumes of added KCN: Hg 2+ 2 will precipitate first and the equivalence point is at ml. And the second equivalence point is at 30 ml. At 5 and 15 ml there is an excess of unreacted Hg a ml (5points) b ml (5points) c ml (5points) At ml, there are 5 ml excess of the [CN - ]: 11. Calculate the concentration of Ag + in saturated solutions of Ag 2 CO 3 (K sp = ) in:

6 x x Corresponding activity coefficients are taken from table (see supplemental information). (a) M KNO 3 (5points) (b) 0.01 M KNO 3 (5points) (c) 0.1 M KNO 3 (5points)

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