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 (MgCl2) in water
to produce 250.0 mL of solution. Molecular weight of the MgCl2 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)
nCl- = 2 n MgCl2 = 2 (m MgCl2/ MW MgCl2) = 0.541 moles
MCl- = nCl- / volume = 0.541 moles / 0.25 L = 2.17 M
b. What is the concentration of the Cl- in ppm? (3points)
Cl- ppm = mass Cl- (mg) / volume = nCl- MWCl- / volume = 0.541 x 35.5 x 1000 / 0.25L =
76840 ppm = 7.68 x 104 ppm
c. Calculate the pCl- value for this solution. (3points)
pCl- = -log [2.17] = -0.34
2. A bottle of a concentrated aqueous sulfuric acid is labeled 98.0 wt % H2SO4 (Molecular
weight is 98.09 g/mol) has a concentration of 18.0 M.
a. How many milliliters of reagent should be diluted to 1.000 L to give 1.00 M
H2SO4? (5 points)
Vcon = Vdil x (Mdil / Mcon) = 1000 mL x ( 1.00 M / 18.0 M) = 55.6 M
b. Calculate the density of 98.0 wt % H2SO4 (5 points)
Mass of the 1 liter of H2SO4: (18 moles) (98.09) = 1.77 x 103 gr.
Mass of the 1 mL of H2SO4: 1.77 g
d = mass / weight % = 1.77 g / (0.98 g H2SO4 /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(NO3)2 if the reaction is:
Ba2+ + SO42-
BaSO4?
(5 points)
Molecular weights of BaSO4 is 233.0 g/mole and Ba(NO3)2 is 261.3 g/mol.
Mass Ba(NO3)2 is 0.232 x 4.35 = 1.01 g
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.
6. The following data was collected when performing a spectrophotometric analysis for
cobalt.
x
Analysis No
4
5
6
mg Co / liter
5.23
10.52
15.41
y
Absorbance
0.095
0.198
0.295
a. Using the least squares method of linear regression, generate the equation to define the
line for the absorbance vs. concentration. (10 points).
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) 0.155 (2 points)
ii) 0.265 (2 points)
7. Chloroform is an internal standard in the determination of the pesticide DDT in a
polarographic analysis. A mixture containing 0.500 mM chloroform and 0.800 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 100.0 mL volumetric flask and 10.2 μL of
chloroform (FW 119.39 g/mol, density = 1.484 g/mL) were added. After diluting to the
mark with solvent, polarographic signals of 29.4 and 8.7 μA were observed for the
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 250.0 mL of 0.150 molar silver ion (Ag+). To this beaker is added
250.0 mL of 0.300 molar bromide ion (Br-). What is the concentration of Ag+ in the final
solution? Ksp for the AgBr is 5.0 10-13? (5 points)
Final concentration of the Ag+ and Br-:
Br- ion is in excess: 0.150 – 0.075 = 0.075 M. [Ag+] = x and [Br-] = (x+0.075)
Assuming x << 0.075, we have
x = [Ag+] = 6.67 10-12.
9. Iron in the +2 oxidation state reacts with potassium dichromate to produce Fe3+ and Cr3+
according to the equation:
6 Fe2+ + Cr2O72- + 14 H+
6 Fe3+ + 2 Cr3+ + 7 H2O
How many milliliters of 0.1658 molar K2Cr2O7 are required to titrate 200.0 mL of 0.2500
molar Fe2+ solution? (5 points)
Therefore,
10. A mixture having a volume of 10.0 mL and containing 0.100 M Ag+ and 0.100 M Hg22+
was titrated with 0.100 M KCN to precipitate Hg2(CN)2 (Ksp = 510-40) and AgCN (Ksp =
2.210-16). Calculate the concentration of the CN- at each of the following volumes of
added KCN:
Hg22+ will precipitate first and the equivalence point is at 20.00 mL. And the second
equivalence point is at 30 mL. At 5 and 15 mL there is an excess of unreacted Hg22+.
a. 5.00 mL (5points)
b. 15.00 mL (5points)
c. 35.00 mL (5points)
At 35.00 mL, there are 5 mL excess of the [CN-]:
11. Calculate the concentration of Ag+ in saturated solutions of Ag2CO3
(Ksp= 8.110-12) in:
x
x
Corresponding activity coefficients are taken from table (see supplemental
information).
(a) 0.001 M KNO3 (5points)
(b) 0.01 M KNO3 (5points)
(c) 0.1 M KNO3 (5points)
Supplemental information