Final answer:
To find the mass percent of water in a hydrate, calculate the total mass of water in one mole of the hydrate and divide it by the molar mass of the hydrate, then multiply the result by 100%.
Step-by-step explanation:
To calculate the mass percent of water in a hydrate, we need to use the formula percent H₂O = (mass of H₂O in 1 mole of the hydrate/molar mass of the hydrate) × 100%. For cobalt(II) chloride hexahydrate, CoCl₂ • 6H₂O, first, calculate the molar mass of the hydrate which is the sum of the molar masses of CoCl₂ and the six water molecules. Then, find the mass of the water by multiplying the coefficient (6) by the molar mass of H₂O. The molar mass of CoCl₂ is around 129.839 g/mol, and the molar mass of one water molecule is approximately 18.015 g/mol, thus for six molecules it's 6 × 18.015 = 108.09 g/mol. The total molar mass of the hydrate is, therefore, 129.839 + 108.09 = 237.929 g/mol.