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Measuring Water Uptake in Setting Plaster 4 questions
Measuring Water Uptake in Setting Plaster
Base your answers on the information provided and your knowledge of chemistry. Use the 2025 Chemistry Reference Tables as needed.
A sculpture class uses a model of plaster setting. The dry ingredient is calcium sulfate hemihydrate. As the plaster sets, that solid combines with water to form gypsum. Water in the formula of a solid hydrate is part of its crystal structure.
2(CaSO4·½H2O)(s) + 3H2O(l) → 2(CaSO4·2H2O)(s)
Question 1
According to the equation, how many moles of liquid water become part of gypsum for every mole of starting hemihydrate that reacts?
- 0.5 mol
- 1.0 mol
- 1.5 mol
- 2.0 mol
The class mixes 29.0 g of hemihydrate with 12.0 g of water. Use model molar masses of 145 g/mol for hemihydrate and 18.0 g/mol for water. Assume the hemihydrate reacts completely and that during drying only water outside the crystals evaporates.
Question 2
Calculate the mass of water that becomes part of the crystals. Show how the equation enters your calculation.
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Question 3
Predict the mass of the dry casting after the extra water has evaporated. Show your setup.
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The class measures 35.1 g on the balance. It assumes the mass reading is accurate and that the model reaction went to completion.
Question 4
Under those assumptions, is the measured casting completely dry? Use the predicted and measured masses to explain.
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