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USA Physics and Chemistry Contests
National Chemistry Olympiad
2010 National Chemistry Olympiad
5
5
Part of
2010 National Chemistry Olympiad
Problems
(1)
2010 USNCO Local #5: Explanation for Error
Source:
11/21/2012
A student performed an experiment to determine the ratio of
H
2
O
\text{H}_2\text{O}
H
2
O
to
CuSO
4
\text{CuSO}_4
CuSO
4
in a sample of hydrated copper(II) sulfate by heating it to drive off the water and weighing the solid before and after heating. The formula obtained experimentally was
CuSO
4
∙
5.5
H
2
O
\text{CuSO}_4 \bullet 5.5\text{H}_2\text{O}
CuSO
4
∙
5.5
H
2
O
but the accepted formula is
CuSO
4
∙
5
H
2
O
\text{CuSO}_4 \bullet 5 \text{H}_2\text{O}
CuSO
4
∙
5
H
2
O
. Which error best accounts for the difference in results?
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During heating some of the hydrated copper(II) sulfate was lost
<span class='latex-bold'>(A)</span>\ \text{During heating some of the hydrated copper(II) sulfate was lost} \qquad
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During heating some of the hydrated copper(II) sulfate was lost
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B
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The hydrated sample was not heated long enough to drive off all the water
<span class='latex-bold'>(B)</span>\ \text{The hydrated sample was not heated long enough to drive off all the water}\qquad
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B
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The hydrated sample was not heated long enough to drive off all the water
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C
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The student weighed out too much sample initially.
<span class='latex-bold'>(C)</span>\ \text{The student weighed out too much sample initially.} \qquad
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C
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The student weighed out too much sample initially.
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D
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The student used a balance that gave weights that were consistently too high by 0.10 g
<span class='latex-bold'>(D)</span>\ \text{The student used a balance that gave weights that were consistently too high by 0.10 g }\qquad
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The student used a balance that gave weights that were consistently too high by 0.10 g
ratio