CHEMISTRY 311 - ASSIGNMENT 1
Hand-in your answers in a neat organized format to the appropriate number significant figures,
showing chemical equations and calculations where appropriate.
Due date: Tuesday, Sept. 25th, 2007
1. A student prepares a combined standard solution by weighing 268 mg of sodium
hydrogen phosphate heptahydrate and 226 mg of anhydrous potassium nitrate into a 250
mL volumetric flask, adding ~100 mL of deionized water, mixing to dissolve and diluting
to the mark. After mixing thoroughly, an aliquot was transferred into a 500 mL
volumetric flask using a 10 mL transfer pipet and diluted to the mark. Report the final
concentration in μM and ppm of P-PO43- and N-NO3-.
2. A mineral is analyzed for it’s calcium content as follows. A 1.032 g finely ground
sample was dissolved in 25 mL of hot 4 M HCl and diluted with 175 mL of H2O. This
solution was heated to a boil and 50 mL of a hot solution containing 2.0 g of (NH4)2C2O4
was slowly added to precipitate CaC2O4(s) as the monohydrate. The solution was
neutralized by adding 6 M NH3 and slowly cooled. After decanting the supernatant
solution, the solid was transferred to a a filter crucible and washed with several portions
of cold 0.1% (NH4)2C2O4 . The crucible was dried at 105oC for 1 hr and then heated at
500oC in a furnace for an additional 2 hr. The mass of the precipitate was recorded to be
0.504 g.
105oC
500oC
Ca2+ + C2O42Æ
CaC2O4 . H2O(s)
Æ
CaCO3(s)
a) Determine the wt % of Calcuim in the mineral sample.
b) Provideive three desired properties of the precipitate in gravimetric analysis.
3. A solution of sodium thiosulfate (Na2S2O3) was standardized by titration of
10.00±0.05mL of an aqueous solution of IO3- prepared by weighing 326.1 ± 0.4 mg of
anhydrous potassium iodate into a 250 mL volumetric flask. Prior to the titration, the
iodate solution is reacted with excess potassium iodide and acid. The reactions are shown
below.
IO3- + 8 I- + 6 H+ Æ 3 I3- + 3 H2O
I3- + 2 S2O32- Æ 3 I- + S4O62A volume of 7.32 ±0.04 mL of Na2S2O3 solution was required to reach the end-point.
Due to time limitations, the student was only able to carry out the standardization titration
once and has therefore been asked to estimate the uncertainty in the final result by
propagating the individual reading errors associated with each measured value. Determine
the molarity of the thiosulfate solution and report the absolute uncertainty.
4. Sodium sulfite (Na2SO3) is sometimes used to remove excess chlorine from
disinfected waters, especially if they are going to be used for fish rearing. Calculate the
mass of sodium sulfite required to react with a 0.85 mg/L Cl2 residual chlorine in a 1000.
L tank?
CHEM 311 assign 1 2007
5. Determine the Total Alkalinity expressed as mg/L of CaCO3 of a groundwater sample
if 36.40 mL of a 0.0160 N H2SO4 titrant is required to titrate a 100.0 mL sample.
6. The concentration of cyanide (CN-) in a copper electroplating bath can be determined
by a complexation titration with Ag+, forming the soluble Ag(CN)2- complex. In a typical
analysis a 5.00 mL sample from an electroplating bath is transferred to a 250 mL
Erlenmeyer flask and treated with 100 mL of H2O, 5 mL of 20% w/v NaOH and 5 mL of
10%w/v KI. The sample is titrated with 0.2012 M AgNO3, requiring 17.36 mL to reach
the end-point as signaled by the appearance of a yellow precipitate of AgI.
a) What is the molarity of a 20% w/v NaOH solution?
b) Why is a 10% w/v solution of KI added in this determination?
c) Report the concentration of cyanide in the electroplating bath as parts per million of
NaCN.
7. Organic pollutants in wastewaters can be quantitfied by oxidizing them in refluxing
acidic dichromate (Cr2O72-) and back titrating the excess dichromate ion with a
standardized solution of ferrous ammonium sulfate Fe(NH4)2(SO4)2 according to the
reaction below:
6 Fe2+ + Cr2O72- + 14 H+ Æ 6 Fe3+ + 2 Cr3+ + 7 H2O
The Chemical Oxygen Demand (COD) is defined as the amount of O2 that is chemically
equivalent to the Cr2O72- consumed in this process and is reported as mg/L of O2.
A 100.0 mL water sample was treated with 25.00 mL of 2.00 x 10-2 N K2Cr2O7 and heated
with excess sulfuric acid (H2SO4). After 30 minutes, the resulting solution was titrated with
2.50 x 10-3 N Fe2+ requiring 9.52 mL to reach the end point.
a) Determine the number of equivalents of dichromate consumed by the sample.
b) Report the COD in mg/L of O2.
8. The following data was obtained in a series of volumetric analysis using EDTA. When
samples are buffered at pH 10, the EDTA complexes all of the Ca2+ and Mg2+ ions for
Total Hardness analysis. At pH 12, Mg(OH)2 precipitates and the EDTA will only
complex the remaining Ca2+ ions, thus measuring the Calcium Hardness. Calculate the
Total and Calcium Hardness as ppm CaCO3 and report the absolute uncertainties in
each.
Data Table for the Total and Calcium Hardness of Groundwater Sample
[EDTA] (M)
VEDTA delivered at
pH = 10 (mL)
Experimental Mean
(1.024 ± 0.008) x 10-3
23.34 ± 0.22
Value and Error
# Replicates in
3
4
Determing the Mean
End-points determined using Calmagite indictor.
Sample volumes of 25.00 mL used for each titration.
Absolute errors quoted as experimental standard deviations.
VEDTA delivered at
pH = 12 (mL)
18.35 ± 0.32
5
CHEM 311 assign 1 2007