Question

Chem Lab prelab
1. In an experiment, a student dehydrated an unknown hydrate and found that the mass of the hydrate was initially 1.2067g. After heating, mass of residue was 0.7592
a) calculate mass of water in hydrate
b) calculate mass percent of water in the hydrate
2. What is the formula of copper (II) nitrate pentahydrate? what is the percentage by mass of copper in copper (II) nitrate pentahydrate?
3. A student takes 1.1705g of an unknown copper compound. Following isolation by displacement by Mg, 0.6382g of copper metal was recovered.
a) percentage by mass of copper?
b) molar mass of the compound given that there is only 1 mole of copper of the compound

Answer 1

1a) To calculate the mass of water in the hydrate, we can subtract the mass of the residue after heating from the initial mass of the hydrate:

Mass of water = Initial mass of the hydrate - Mass of residue
Mass of water = 1.2067g - 0.7592g
Mass of water = 0.4475g

1b) To find the mass percent of water in the hydrate, we'll use the following formula:

Mass percent = (Mass of water / Initial mass of the hydrate) * 100%
Mass percent = (0.4475g / 1.2067g) * 100%
Mass percent ≈ 37.12%

2) The formula of copper (II) nitrate pentahydrate is Cu(NO3)2·5H2O. To find the percentage by mass of copper in copper (II) nitrate pentahydrate, we can calculate the molar mass of copper and then use it to find the percentage:

Molar mass of Cu = 63.55 g/mol
Molar mass of Cu(NO3)2·5H2O = 63.55g/mol + 2(14.01g/mol + 3(16.00g/mol)) + 5(18.02g/mol)
Molar mass of Cu(NO3)2·5H2O = 63.55g/mol + 2(14.01g/mol + 3(16.00g/mol)) + 5(18.02g/mol)
Molar mass of Cu(NO3)2·5H2O = 63.55g/mol + 2(14.01g/mol + 3(16.00g/mol)) + 5(18.02g/mol)
Molar mass of Cu(NO3)2·5H2O = 63.55g/mol + 2(14.01g/mol + 48.00g/mol) + 5(18.02g/mol)
Molar mass of Cu(NO3)2·5H2O = 63.55g/mol + 2(62.01g/mol) + 90.10g/mol
Molar mass of Cu(NO3)2·5H2O ≈ 248.63g/mol

Percentage by mass of copper = (Molar mass of Cu / Molar mass of Cu(NO3)2·5H2O) * 100%
Percentage by mass of copper = (63.55g/mol / 248.63g/mol) * 100%
Percentage by mass of copper ≈ 25.54%

3a) The percentage by mass of copper is calculated by dividing the mass of copper recovered by the initial mass of the unknown copper compound:
Percentage by mass of copper = (Mass of copper recovered / Mass of unknown compound) * 100%
Percentage by mass of copper = (0.6382g / 1.1705g) * 100%
Percentage by mass of copper ≈ 54.50%

3b) Given that there is only 1 mole of copper in the compound, the molar mass of the compound can be calculated as follows:
Molar mass of unknown compound = Mass of unknown compound / Moles of copper
Molar mass of unknown compound = 1.1705g / 0.6382mol
Molar mass of unknown compound ≈ 1.834g/mol