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Constructed Response . Topic 6: Moles and Stoichiometry

Set I: Hydrates, percent composition, significant figures, data interpretation

Base your answers to questions 23 through 27 on the information below.

A hydrate is a compound that has water molecules within its crystal structure. The formula for the

hydrate CuSO4 • 5H2O(s) shows that there are five moles of water for every one mole of CuSO4(s). When

CuSO4 • 5H2O(s) is heated, the water within the crystals is released, as represented by the balanced

equation below.

CuSO4 • 5H2O(s) → CuSO4(s) + 5H2O(g)

A student first masses an empty crucible (a heat-resistant container). The student then masses the

crucible containing a sample of CuSO4 • 5H2 O(s). The student repeatedly heat and masses the crucible

and its contents until the mass is constant. The student’s recorded experimental data and calculations

are shown below.

Teacher Preview Copy
Teacher Preview Copy

Data and calculation before heating:

mass of CuSO4 • 5H2O(s) and crucible 21.37 g
– mass of crucible 19.24 g

_________________________________________________

mass of CuSO4 • 5H2O(s) 2.13 g

Data and calculation after heating to a constant mass:

mass of CuSO4(s) and crucible 20.61 g
– mass of crucible 19.24 g

____________________________________________________

mass of CuSO4(s) 1.37 g

Calculation to determine the mass of water:

mass of CuSO4 • 5H2O(s) 2.13 g

– mass of CuSO4(s) 1.37 g

____________________________________________________

mass of H2O(g) 0.76 g

23. Identify the total number of significant figures 23.

.
recorded in the calculated mass of CuSO4 5H2O(s).

24. In the space to the right, use the student’s data to 24.

show a correct numerical setup for calculating the

percent composition by mass in the hydrate.

25. Explain why the sample in the crucible must be 25.

heated until the constant mass is reached.

26. How many moles of water is represented by the 26.

mass of CuSO4 calculated by the student?

27. How many moles of water is represented by the 27.

mass of H2 O calculated by the student?

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