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?

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:

 

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