A sample of copper sulphate pentahydrate contains 8.64 g of oxygen. How many grams of Cu is present in the sample ?

To solve this problem, we need to use the concept of mole ratios from the chemical formula and the molar mass of the elements involved.

The chemical formula for copper sulphate pentahydrate is CuSO4.5H2O.

Step 1: Calculate the molar mass of CuSO4.5H2O

• The molar mass of Cu is approximately 63.55 g/mol.
• The molar mass of S is approximately 32.07 g/mol.
• The molar mass of O is approximately 16.00 g/mol.
• The molar mass of H2O is approximately 18.02 g/mol.

So, the molar mass of CuSO4.5H2O = 63.55 g/mol (for Cu) + 32.07 g/mol (for S) + 64.00 g/mol (for 4 O in SO4) + 90.10 g/mol (for 5 H2O) = 249.72 g/mol.

Step 2: Calculate the mole ratio of O to Cu in CuSO4.5H2O

• There are 9 moles of O for every 1 mole of Cu in the compound.

Step 3: Calculate the number of moles of O in the sample

• The given mass of O is 8.64 g.
• The molar mass of O is 16.00 g/mol.
• So, the number of moles of O = 8.64 g / 16.00 g/mol = 0.54 moles.

Step 4: Calculate the number of moles of Cu in the sample

• Using the mole ratio from Step 2, the number of moles of Cu = 0.54 moles (of O) / 9 = 0.06 moles.

Step 5: Calculate the mass of Cu in the sample

• The molar mass of Cu is 63.55 g/mol.
• So, the mass of Cu = 0.06 moles * 63.55 g/mol = 3.81 g.

Therefore, the sample contains approximately 3.81 g of Cu.