Some chemicals tend to absorb moisture out of the atmosphere. In some cases, the water molecules actually incorporate directly into the chemical structure of a solid. Chemists refer to this water as the "water of hydration" and they represent it with chemical formulas such as MgSO4.7H2O, known as magnesium sulfate heptahydrate, or Epsom salts. In this case, the 7H2O means that the compound absorbs seven water molecules per formula unit. Alternatively, a chemist would say that it absorbs seven moles of water per mole of magnesium sulfate. Experimentally, chemists determine the moles of hydration by weighing a sample of the material before and after heating it to dryness. The difference in weight represented the amount of water driven off, which allows the chemist to calculate the moles of water driven off.
Things You'll Need
- Water of hydration data, including mass of the sample before and after heating
Calculate the amount of water in the sample by subtracting the weight of the sample before and after it was heated. For example, a 1.045-gram sample of hydrated copper(II) sulfate, CuSO4.xH2O, that weighs 0.662 grams after it was heated lost 1.045 - 0.662 = 0.383 grams of water.
Determine the moles of water driven out of the sample by dividing the mass of water lost by water's molecular weight, which is 18.02. Continuing the previous example, 0.383 / 18.02 = 0.0213 moles of water.
Calculate the moles of the compound remaining after the water was driven off. This is determined by dividing the mass of the residue in grams by the compound's formula weight, as determined by adding together the atomic weights of all of the elements in its chemical formula. In the case of CuSO4.xH2O, the residue would contain everything except the water, or CuSO4. This formula contains one copper, one sulfur and four oxygen atoms, which possess atomic weights of 63.55, 32.07 and 16.00, respectively, as found on the periodic table of the elements. The formula weight of CuSO4 is therefore 159.62. The 0.662 grams of CuSO4; therefore, represents 0.662 / 159.62 = 0.00415 moles of CuSO4.
Find the moles of hydration by dividing the moles of water by the moles of residue. In this case, the hydrated compound contained 0.0213 moles of water and 0.00415 moles of CuSO4. The moles of hydration is therefore 0.0213 / 0.00415 = 5.13.
Round the moles of hydration to the nearest integer and write the hydrated formula of the compound. Continuing the previous example, 5.13 round to 5; therefore, the formula of the hydrated salt is CuSO4.5H2O (ref 4).
- Colorado State University, Center for Multiscale Modeling of Atmospheric Processes: Hygroscopic Particles
- Portland Community College: Experiment 10: Hydrates
- Clackamas University, Department of Chemistry: Formula Weights
- University of Texas at Austin, Department of Chemistry: Compound Stoichiometry - Mole Concept
What Is a Hydrate in Chemistry?
Hydrates are chemical compounds that contain water molecules. A hydrate usually contains a specific proportion of water in relation to its weight....
Hydrous Vs. Anhydrous
Hydrous and anhydrous are terms used in chemistry to describe substances that contain water and that are devoid of water, respectively. Chemical...
How to Calculate the Heat of Solution
The heat or enthalpy of solution is the amount of heat released or absorbed when a substance dissolves in a solvent (usually...
How to Calculate Molar Mass
General chemistry can be confusing and tedious, but if you break down some of the concepts, they are much easier to understand....
How to Measure Solubility for a Science Project
Solubility refers to highest amount of a dissolvable substance, also known as a solute, that can actually be dissolved in a given...
How to Calculate Body Hydration
Water plays an essential role in keeping your muscles and organs healthy by transporting nutrients, regulating body temperature and aiding in digestion....
How to Calculate Hydrates
Salts known as hydrates contain molecules of water incorporated into their crystal structures. If you heat a hydrated salt, you can cause...
How to Find How Many Moles Are in a Compound
The mole concept is a fundamental concept in chemistry, and most students who take high school chemistry will encounter it at some...
How to Calculate the Percent of Water of KAl(SO4)2 12 H2O
Aluminum potassium sulfate, or KAl(SO4)2, known also as potash alum, is an inorganic crystalline salt. This compound is commonly used as an...
How to Find Percent Composition of Hydrate
Hydrates are typically inorganic salts that contain water in the form of bound H2O molecules. An example of hydrates is copper sulfate...
How to Determine Molar Mass of Hydrates
Some metal compounds crystallize and trap water molecules in their crystalline lattice. These compounds are hydrates. The compounds crystallize and associate with...
What Is a Hydrated Salt?
A hydrated salt is a crystalline salt molecule that is loosely attached to a certain number of water molecules. Salt is created...
Science Experiment Ideas: Epsom Salts
“Epsom salts” is the common name of magnesium sulfate heptahydrate: MgSO4.7H2O. Its common name originated from the name of an English town...