Final answer:
To find the percentage of water in a hydrate, one must subtract the mass of anhydrous compound from the mass of the hydrate and then divide by the hydrate's mass, multiplying by 100%. In the provided example, this results in approximately 11.72% water by mass.
Step-by-step explanation:
Calculating the Percentage of Water in a Hydrate
To calculate the percentage of water in a hydrate, you subtract the mass of the anhydrous (without water) compound from the mass of the hydrate to find the mass of the water. Then, divide the mass of the water by the total mass of the hydrate and multiply by 100% to get the percent by mass of water.
For the given problem, the mass of the hydrated copper(II) sulfate is 2.815 g. The mass after heating (the mass of the anhydrous compound) is 2.485 g. The mass of the water is the difference between these two masses:
Mass of water = 2.815 g - 2.485 g = 0.330 g
The percentage of water in the hydrate is:
Percent H₂O = (0.330 g / 2.815 g) × 100% = 11.72%
To find the formula of the hydrate, divide the mass of the anhydrous compound by its molar mass to get the number of moles. Then, divide the mass of water by the molar mass of water (approximately 18.015 g/mol) to find the number of moles of water. The ratio of moles of water to moles of copper(II) sulfate will give you the value of x in the formula CuSO₄·xH₂O.