Plaster of Paris is the compound calcium sulfate hemihydrate, CaSO4*1/2H2O
What mass of water is liberated when a 2.763 gram sample is heated to constant mass?
Consider the reaction:
CaSO4*(1/2H2O) -> CaSO4 + 1/2H2O
1/2 mole of H2O will be released per mole of CaSO4*(1/2H2O)
Start by calculating how many moles of CaSO4*(1/2H2O) you start with.
The molar mass of CaSO4*(1/2H2O) is
40 + 32 + 64 + 9 = 145 g/mol
You start out with 0.01906 moles of the hemihydrate and end up with half as many moles of H2O.
To determine the mass of water liberated when a sample of Plaster of Paris is heated, we need to calculate the difference in mass before and after the heating process.
Step 1: Calculate the molar mass of the compound.
The molar mass of calcium sulfate hemihydrate (CaSO4*1/2H2O) can be calculated by adding the molar masses of its individual atoms:
- Calcium (Ca): 1 atom x atomic mass = 1 x 40.08 g/mol = 40.08 g/mol
- Sulfur (S): 1 atom x atomic mass = 1 x 32.07 g/mol = 32.07 g/mol
- Oxygen (O): 9 atoms x atomic mass = 9 x 16.00 g/mol = 144.00 g/mol
- Hydrogen (H): 2 atoms x atomic mass = 2 x 1.01 g/mol = 2.02 g/mol
Adding these masses together:
Molar mass of CaSO4*1/2H2O = 40.08 g/mol + 32.07 g/mol + 144.00 g/mol + 2.02 g/mol = 218.17 g/mol
Step 2: Calculate the number of moles of the sample.
Using the given mass of the sample (2.763 grams) and the molar mass calculated above:
Number of moles = Mass / Molar mass
Number of moles = 2.763 g / 218.17 g/mol
Step 3: Determine the molar ratio between water and the compound.
From the chemical formula, we can see that for every 1 mole of the compound, there are 1/2 moles of water (H2O).
Step 4: Calculate the moles of water.
To find the number of moles of water, we multiply the moles of the sample by the molar ratio:
Number of moles of water = (Number of moles of the sample) x (Molar ratio)
Number of moles of water = (2.763 g / 218.17 g/mol) x (1/2)
Step 5: Calculate the mass of water liberated.
To find the mass of water liberated, we need to convert the moles of water to grams by multiplying by the molar mass of water (H2O):
Mass of water liberated = (Number of moles of water) x (Molar mass of water)
Mass of water liberated = [(2.763 g / 218.17 g/mol) x (1/2)] x (18.02 g/mol)
By evaluating the above expression, you can find the mass of water liberated when the sample is heated to constant mass.