GRAVIMETRIC ANALYSIS
The concentration of lead ions Pb[2+] in a sample
of polluted water that also contains nitrate ions NO3[-] is determined by
adding sodium sulfate Na2SO4 to exactly 0.5 liters of water. The unbalanced
equation is:
Pb(NO3)2 + Na2SO4 => NaNO3 + PbSO4
Calculate the
molar concentration of Pb[2+] if 0.450 grams of Na2SO4 was needed for the
complete precipitation of Pb[2+] ions as PbSO4.
Solution:
-The
balanced equation is:
Pb(NO3)2 + Na2SO4 => 2NaNO3 + PbSO4
-For
every mole of PbSO4 which percipitates we have one mole of Pb[2+] ions which were in the water
before Na2SO4 was added.
-Submitting the equation and entering 0.450 grams for Na2SO4, we
find we have 0.0031678 moles of PbSO4 and thus that many Pb[2+] ions.
-We
divide this by hand, 0.5 liters of water, to find the concentration. (
0.00633 moles/liter )
Note that an external operation is
needed because H2O does not occur in the formula or spreadsheet.
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