Percent Mass Equation

Question

A 0.956 g sample of a hydrate is heated, leaving 0.343 g of anhydrous compound. What is the percent mass of water in the hydrate? Please attach your work for full credit!

Answer 1

In general, the percent mass of a component is found by taking the mass of that component, dividing by the mass of the whole, and then multiplying by 100%.

% mass = (component mass) / (whole mass) * 100%

We already know the mass of the whole, when the water is still combined with the compound, is 0.956 g.

To determine the mass of just the water, we can subtract the mass of the anhydrous compound from the mass of the hydrated compound. We can do this because of the law of conservation of mass. Because mass cannot be created or destroyed, the mass that was lost when the hydrate was dried must belong to all the water. This means the mass of the water component is 0.956 g – 0.343 g = 0.613 g.

We can now plug our numbers into the equation and solve.

% mass water = (0.613 g) / (0.956) * 100% = 64.1%

Answer 2

When the hydrate is heated to a sufficiently high temperature, the water is driven off. The mass of water driven off would be 0.956 - 0.343 = 0.613 g of water.

The percent of water in the hydrate is 0.613g / 0.956g x 100 = 64.1%