Calculating Moles of Crystal Water and Anhydrous Salt in Hydrated Salts
Calculating Moles of Crystal Water and Anhydrous Salt in Hydrated Salts
In the context of chemistry, understanding how to calculate the moles of crystal water and anhydrous salt in a hydrated salt sample is crucial. This is particularly so for compounds like CuSO4·xH2O. Here, we’ll outline the steps and provide an example to make the process clear and straightforward.
Step-by-step Guide to Calculating Moles
To start, it's important to understand the basic principles involved:
Determine the Molar Masses: Molar Mass of Copper(ii) Sulfate (CuSO4):Cu: 63.55 g/mol
S: 32.07 g/mol
O: 4 × 16.00 g/mol 64.00 g/mol
Total: 159.62 g/mol
Molar Mass of Water (H2O):H: 2 × 1.01 g/mol 2.02 g/mol
O: 16.00 g/mol
Total: 18.02 g/mol
Define the Molar Mass of the Hydrate:The molar mass of the hydrate CuSO4·xH2O is given by:
Mr(CuSO4·xH2O) Mr(CuSO4) x × Mr(H2O)
So, Mr(CuSO4·xH2O) 159.62 x × 18.02
Calculate Moles of the Hydrate:Given the mass of the hydrate is 1 g, the number of moles of the hydrate can be calculated using the formula:
Nhyd nhyd mhyd / Mr(CuSO4·xH2O) 1 g / (159.62 x × 18.02)
Calculate Moles of Anhydrous Salt and Water: Moles of Anhydrous Salt (CuSO4):NCuSO4 Nhyd 1 / (159.62 x × 18.02)
Moles of Water (H2O):NH2O x × Nhyd x × 1 / (159.62 x × 18.02)
Summary
To find the moles of crystal water and anhydrous salt in your 1 g sample of CuSO4·xH2O, you need to know the value of x. Once you have x, you can substitute it into the formulas above to get the moles of CuSO4 and H2O.
Example
Consider the case where the hydrate is CuSO4·5H2O where x 5:
Calculate the Molar Mass:Mr(CuSO4·5H2O) 159.62 5 × 18.02 159.62 90.10 249.72 g/mol
Calculate Moles of the Hydrate:Nhyd mhyd / Mr(CuSO4·5H2O) 1 g / 249.72 g/mol ≈ 0.00401 moles
Moles of Anhydrous Salt (CuSO4):NCuSO4 0.00401 moles
Moles of Water (H2O):NH2O 5 × 0.00401 ≈ 0.02005 moles
This method can be applied to any value of x to find the corresponding moles.