Final answer:
To find the percent of water in a hydrate, the mass of water is calculated by subtracting the mass of the anhydrous product from the original hydrate mass, then the percentage is determined by dividing this by the original mass and multiplying by 100%. In this question, the hydrate contained 20% water by mass.
Step-by-step explanation:
To calculate the percent of water in the hydrate, you subtract the mass of the anhydrous compound from the original mass of the hydrate. Then, divide this difference by the original mass of the hydrate and multiply by 100% to get the percent by mass of water in the hydrate.
In this case, the original mass of the hydrate is 10.0 g and the mass of the anhydrous compound left after heating is 8.00 g. Subtracting these gives us the mass of the water driven off, which is 2.00 g.
The formula to calculate the percent of water is:
Percent H₂O = (mass of H₂O / mass of hydrate) × 100%
Substituting the given values:
Percent H₂O = (2.00 g / 10.0 g) × 100% = 20%
Therefore, the hydrate contained 20% water by mass.