Compounds - Mark Rosengarten

Determining the % Of Water In A Hydrate Experimentally:
(given mass of water in hydrate / given mass of hydrate) X 100%
To determine the mass of the water that was driven out of the hydrate:
Take the mass of the original hydrate and subtract from it the mass of the anhydride.
To determine the percent of water in the hydrate:
Divide the mass of the water driven off from the hydrate by the total mass of the hydrate and multiply by 100.
2) A 10.40 gram sample of hydrated crystal is heated to a constant mass of 8.72 grams. This means all of the water
has been driven out by the heat.
a) Calculate the mass of water that was driven out:
(mass of hydrate – mass of anhydride) = 10.40 grams – 8.72 grams = 1.68 grams of water driven off by the heat
b) Calculate the percent, by mass, of water in this hydrate:
(given mass of water in hydrate / given mass of hydrate) X 100 = (1.68 grams / 10.40 grams) X 100 = 0.162 X 100 =
16.2% water by mass
3) An empty evaporating dish is found to have a mass of 29.993 grams. A sample of hydrated crystal is placed into
the evaporating dish, the combined mass is 39.486 grams. The evaporating dish is placed on a ringstand and heated
gently over a Bunsen burner flame, stirring constantly so that the water does not cause the drying crystal to clump
together. The dish is removed from the flame, and weighed, then reheated, then weighed again, until the mass
remains a constant 38.378 grams.
a) What was the mass of hydrated crystal that was put into the dish?
Mass of hydrated crystal = (mass of crystal + dish) – (mass of dish) = 39.486 g – 29.993 g = 9.493 grams of hydrate
b) What was the mass of water that was driven out of the hydrate?
The total mass was 39.486 grams before heating, and 38.387 g after heating. The difference is the mass of water:
39.486 g – 38.387 g = 1.099 grams of water driven off by the heat
c) Calculate the percent, by mass, of water in the hydrate:
(given mass of water in hydrate / given mass of hydrate) X 100 = (1.099 g / 9.493 g) X 100 = 0.116 X 100 =
11.6% water by mass
© 2011, Mark Rosengarten AE 18