elements of life 7 questions

Question 1

Some students place a small piece of calcium in water with a few drops of universal indicator solution. The indicator turns an alkaline colour. After a time, the solution goes cloudy.
The equation for the reaction is Ca + 2H2O → Ca(OH)2 + H2

• Use the equation to suggest two observations that the students made, other than the indicator turning an alkaline colour.
• A student says that the cloudiness is caused by insoluble calcium hydroxide being formed slowly. Comment on this statement.
• The students filter off the solid from the solution, dry it and find it weighs 4.05 g. They calculate that the percentage yield of their reaction is 55.0%. Calculate the mass of calcium they started with.
• State an assumption you made in order to calculate (3).

Answer 1

• Calcium dissolves / reacts away, + bubbling / fizzing of hydrogen gas
• Ca(OH)2 is sparingly soluble, so dissolves at first, then reaches its maximum solubility and forms a precipitate, causing cloudiness
• 100 x 4.05 / theoretical = 55 → theoretical = 100 x 4.05 / 55 = 7.364 g → mol Ca(OH)2 = mol Ca = 0.09937 mol → mass Ca = 0.09937 x 40.1 = 3.98 g
• No Ca(OH)2 was in solution