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This lab will teach us how to identify several different substances
that may be present in tap water and ocean water using different
chemical tests. It will also show us the presence of solids in both tap
water and ocean water.
Balanced Equations:
9. Test Tube #3: a) Ca + CO3 -> CaCO3 b) Mg + CO3 -> MgCO3
10. Test Tube #4: Ag+ + Cl- -> AgCl(s)
11. Test Tube #5: Ba + SO42- -> BaSO4
12. Test Tube #6: Pb + HCl -> PbCl2(s) + H+
Calculations:
Weight of Residue = (Original Weight) – (Weight after boiling)
151.734g – 151.726g = 0.008g
%Total solids in Tap Water = (Grams of residue) / (Grams of original solution)
0.008g/(223.238g-151.726g) = 0.008g/75.512g = 0.011%
ppm = (mg solute) / (Liters Solution)
8mg/(0.075L) = 100ppm
Discussion:
The purpose of part one of this experiment was to observe the
presence of solids in the local tap or ocean water. To do this we
boiled 75mL of tap water in a beaker. We compared the masses of the
beaker before water was added, and after it was evaporated to determine
the weight of the remaining residue. We obtained a light crusty brown
residue at the bottom of our beaker. It was found to weight only 8mg,
which means the local tap water has very little solids.
Still looking at tap water, part two was to determine the presence
of several different elements. We used a variety of different chemical
solutions and observed their reactions when added to tap water. The
water contained no nitrate ions because there was no reaction with iron
(II) sulfate and sulfuric acid. However, we did get to see what it
would look like if nitrate was present because in one sample we added
HNO3. Both calcium and magnesium were present because when added
ammonium carbonate to our sample it formed calcium carbonate and
magnesium carbonate as can be seen in the following balanced equations:
Ca + CO3 -> CaCO3, Mg + CO3 -> MgCO3. Chloride was also present
because when we added nitric acid and silver nitrate, silver chloride
was formed as can be seen in the balanced equation: Ag+ + Cl- ->
AgCl(s). Sulfate was also present because when we added barium nitrate,
barium sulfate was formed as can be seen in the balanced equation: Ba +
SO42- -> BaSO4. Lead was not present in our sample because when
hydrochloric acid was added, lead (II) chloride was not observed as it
would if lead was present as can be seen in the balanced equation: Pb +
HCl -> PbCl2(s) + H+.
I found the second part especially interesting. It is good to know
our tap water does not contain lead or nitrate, since both are toxic to
humans. I would assume that calcium, magnesium, chloride, and sulfate
were either added or allowed to remain by the local water authorities
because they are good for humans in small quantities.
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