Question

Anhydrous CaCl2 is used as a desiccant in desiccators, i.e. It removes water from the air within the desiccators and forms a hydrated salt. A 16.43g mass of anhydrous CaCl2 weighed 21.75g after being left in desiccators for several months. What is the formula of the hydrated CaCl2?

Answer 1

You need to calculate the number of moles of CaCl2 and the number of moles of water

Number of moles of Ca Cl2 = mass of Ca Cl2 / molecular mass of Ca Cl2

mass of Ca Cl2 = 16.43 g

molecular moss of Ca Cl2 = 40 g / mol + 2 * 35.5 g/mol = 40 g/mol + 71 g/mol = 111 g/mol

Number of moles of Ca Cl2 = 16.43 g / 111 g/mol = 0.148 moles

Number of moles of water = mass of water / molecular mass of water

mass of water = mass of compound after being left in desiccators - initial mass of Ca Cl2

mass of water = 21.75 g - 16.43 g = 5.32 g

number of moles of water = 5.32 g / 18 g/ mol = 0.296 moles

Ratio water to Ca Cl2 (in moles) = 0.296 / 0.148 = 2

The there are two moles of water per mol of Ca Cl2 and the formula for the hydrated Ca Cl2 is:


`CaCl_2.2H_2O`