Chemistry II Laboratory Exam 1
a 0.0352g sample of pure CaCO₃(100.09 g/mol) was dissolved in acid and titrated to
its endpoint with newly made EDTA titrant ( 0.010M). The starting buret volume was
✅✅
0.10 mL. The ending buret volume was 35.52 mL. Calculate the exact concentration
of the EDTA titrant - -CaCO₃+2HCl → CaCl₂+CO₂+H₂O
a 100.0 mL aliquot of an unknown water sample was titrated using EDTA from
question 1 and the procedure below. The starting volume was 0.25 mL and the
✅✅
ending volume was 20.53 mL. Calculate the concentration of Ca2+ in the tap water -
-known:
- 0.00993 M EDTA
- 0.02028 L EDTA
- 0.100 L unknown
M₁V₁ = M₂V₂
(0.00993M)(0.02028L) = M₂(0.100L)
M₂ = 0.00201 M Ca²⁺
a volumetric flask will be used to make your stock solution and dilutions in
experiment 1. Suppose you fill the flask with the desired substance and then
accidentally add distilled water to just above the mark.
a) will the concentration be higher or lower than desired? Explain
b) will you be able to read the new volume? - ✅✅ -a) the concentration will be lower
than desired because the distilled water will dilute the desired substance
b) no, I will not be able to read the new desired volume
an experiment requires you to make 500.00 mL of 0.00019 M Allura Red (MW
✅✅
496.42 g/mol). How many grams of Allura Red would you need to weight out? -
-M = mol/L
, 0.00019M = x/0.5L
x = 0.00019M * 0.5L
x = 0.000095 mol * 496.42g
x = 0.0472g
analyte - ✅✅-unknown in solution (in flask)
consider if the stock solution was made improperly in the proceduer, such that the
volumetric flask was filled to just above the line.
a) how would this affect the slope of the graph? Would it increase, decrease, or stay
the same? Explain.
b) how would this affect the concentration determined for your unknown sample?
✅✅
would the reported unknown concentration be incorrectly high, or incorectly low, or
stay the same? - -a) the slop would stay the same because absorbance is
directly proportional to concentration
b) the concentration would be incorrectly low because the distilled water would have
diluted the desired substance
during an actual run with the calorimeter, there are two important criteria which must
✅✅
be satisfied by a correctly positioned thermometer. What are these two criteria? -
-1. open calorimeter top move up and down
2. thermometer face you to read
endothermic reaction - ✅✅-(plus) heat is absorbed
endpoint - ✅✅ -when the titration is complete, usually determined by a visual
indicator (example: strong acid and strong base, the endpoint pH=7)
enthalpy of formation - ✅✅
-the energy absorbed or released when the elements
react to form the compound
enthalpy of reaction - ✅✅-the quantity of heat absorbed or released for a specified
reaction
equivalence point - ✅✅ -the volume (of added titrant from buret) at which the
number of moles of titrant is equal to the number of moles of analyte (unknown in
flask)
exothermic reaction - ✅✅-(minus) heat is released
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