The Formula for a Hydrate

The presence of water and various mineral impurities account for the variety of different colored opals. Further changes in color occur when opals are allowed to dry out. 

In the formula for a hydrate, the number of water molecules associated with each formula unit of the compound is written following a dot for example, Na2CO3T0H2O. This compound is called sodium carbonate decahydrate. In the word decahydrate, the prefix deca- means ten and the root word hydrate refers to water. Decahydrate means that ten molecules of water are associated with one formula unit of compound. The mass of water associated with a formula unit must be included in molar mass calculations. Hydrates are found with a variety of numbers of water molecules. Table 11-1 lists some common hydrates. 

Tetra- 4 FeP04 4H20 Iron(lll) phosphate tetrahydrate 

Penta- 5 CuS04 5H20 Copper(ll) sulfate pentahydrate 

Hexa- 6 CoCl2 6H20 Cobalt(ll) chloride hexahydrate 

Many compounds, particularly ionic compounds, incorporate a specific number of water molecules into their crystals when they crystallize from aqueous solution. Compounds that include water in their crystal structures are called hydrates. The formula for a hydrate is written by adding the formula for water to the formula for the compound. The number of water molecules included per formula unit is indicated by a coefficient. The formula for the blue crystals of copper(ll) sulfate is CuS04 5H20, indicating that the compound includes five water molecules per formula unit of CUSO4. 

A hydrate of aluminum bromide is composed of 71.16% AlBrg and 28.84% H2O. What is the formula for the hydrate  

when dealing with percent composition, assume you are working with a 100-g sample of the compound. A 100-g sample of this hydrate would consist of 71.16 g of AlBrg and 28.84 g of H2O. By determining the ratio of moles of AlBr3 to moles of H2O in s sample, you can write the formula for the hydrate. 

calculate the moles of AlBrj and the moles of H2O. Before starting, you must add the molar masses of the atoms in AlBrj and in H2O to determine the molar mass of each compound. 

110 Chemistry Matter and Change Solving Problems A Chemistry Handbook 

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Calculate the formula for a hydrate using the mass of the hydrated compound and the mass of the anhydrous compound. 

Determine the formula for a hydrate from laboratory data. 

Formula for a hydrate How can you determine the formula for a hydrate You must find the number of moles of water associated with one mole of the hydrate. Suppose you have a 5.00-g sample of a hydrate of barium chloride. You know that the formula is BaCl2-xH20. You must determine x, the coefficient of H2O in the hydrate formula that indicates the number of moles of water associated with one mole of BaCl2. To find x, you would heat the sample of the hydrate to drive off the water of hydration. After heating, the dried substance, which is anhydrous BaCl2, has a mass of 4.26 g. The mass of the water of hydration is the difference between the mass of the hydrate (5.00 g) and the mass of the anhydrous compound (4.26 g). 

For more practice calculating the formula for a hydrate, go to Supplemental Practice Problems in Appendix A. 

Describe the experimental procedure for determining the formula for a hydrate. Explain the reason for each step. 

The formula for a hydrate consists of the formula for the ionic compound and the number of water molecules associated with one formula unit. 

To write the formula for a hydrate, write the formula for the compound and then place a dot followed by the number of water molecules per formula unit of compormd. The dot in the formula represents a ratio of compound formula emits to water molecules. For example, CaS04 2H2O is the formula for a hydrate of calcium sulfate that contains two molecules of water for each formula unit of calciiun sulfate. This hydrate is used to make portland cement and plaster of paris. To name hydrates, follow the regular name for the compound with the word hydrate, to which a prefix has been added to indicate the number of water molecules present. Use Table 5.5 to find the correct prefix to use. The name of the compound with the formula CaS04 2H2O is calcium sulfate dihydrate. 

What is the formula for a hydrate that consists of 80.15 percent ZnSOj and 19.85 percent H2O ... 

In this second example, we examine simple systems near the water-hexane interface. Specifically, we calculate the difference in the free energy of hydrating a hard-sphere solute of radius a, considered as the reference state, and a model solute consisting of a point dipole p located at the center of a cavity [11]. We derive the formula for A A assuming that the solute is located at a fixed distance z from the interface, and subsequently we examine the dependence of the free energy on z. The geometry of the system is shown in Fig. 2.3. 

When substances like salts and water form compounds the force of attraction is of a subordinate character, and the formula of a hydrate is usually written with a period between the formula of the salt and that of the water, for example, CuS04-5H20. This indicates that the primary valence of the element is satisfied... 

Sodium Phosphate, Tribasic, occurs as white crystals or granules or as a crystalline material. It may be anhydrous or contain 1 to 12 molecules of water of hydration. The formula for a crystalline material is approximately 4(Na3P0412H20)-NaOH. It is freely soluble in water, but is insoluble in alcohol. The pH of a 1 100 aqueous solution is between 11.5 and 12.5. 

The ratio of moles of H2O to moles of BaCl2 is 2 1, so two moles of water are associated with one mole of barium chloride. The value of the coefficient X is 2 and the formula for the hydrate is BaCl2 2H20. What is the name of the hydrate The CHEMLAB at the end of this chapter will give you experience determining the formula of a hydrate. 

The physical appearance of one mole of each of some compounds is illustrated in Figure 2-11. Two different forms of oxalic acid are shown. The formula unit (molecule) of oxalic acid is (COOH)2 (FW = 90.0 amu molar mass = 90.0 g/mol). When oxalic acid is obtained by crystallization from a water solution, however, two molecules of water are present for each molecule of oxalic acid, even though it appears dry. The formula of this hydrate is (COOH)2 2H2O (FW = 126.1 amu molar mass = 126.1 g/mol). The dot shows that the crystals contain two H2O molecules per (COOH)2 molecule. The water can be driven out of the crystals by heating to leave anhydrous oxalic acid, (COOH)2. Anhydrous means without water. Copper(II) sulfate, an ionic compound, shows similar behavior. Anhydrous copper(II) sulfate (CuSO FW = 159.6 amu molar mass = 159.6 g/mol) is almost white. Hydrated copper(II) sulfate (CuSO 5H2O FW = 249.7 amu molar mass = 249.7 g/mol) is deep blue. The following example illustrates how we might find and use the formula of a hydrate. 

Write the formula of a hydrated hydrogen ion that contains only one water of hydration. Give another name for the hydrated hydrogen ion. 

Write the formula for a potential hydrate isomer of each of the following compounds. Name each one. 

Determining the Formula of a Hydrate A mass of 2.50 g of blue, hydrated copper sulfate (CuS04 xH20) is placed in a crucible and heated. After heating, 1.59 g of white anhydrous copper sulfate (CUSO4) remains. What is the formula for the hydrate Name the hydrate. 

Epsom salts are hydrates of magnesium sulfate. The formula for Epsom salts is MgS04 - 7H20. A 7.834-g sample is heated until a constant mass is obtained indicating that all foe water has been evaporated off. What is foe mass of foe anhydrous magnesium sulfate What percentage of foe hydrate is water ... 

Many of the d-block elements form characteristically colored solutions in water. For example, although solid copper(II) chloride is brown and copper(II) bromide is black, their aqueous solutions are both light blue. The blue color is due to the hydrated copper(II) ions, [Cu(H20)fJ2+, that form when the solids dissolve. As the formula suggests, these hydrated ions have a specific composition they also have definite shapes and properties. They can be regarded as the outcome of a reaction in which the water molecules act as Lewis bases (electron pair donors, Section 10.2) and the Cu2+ ion acts as a Lewis acid (an electron pair acceptor). This type of Lewis acid-base reaction is characteristic of many cations of d-block elements. 

A student measures 10.0 g of hydrated sodium carbonate (Na2C03 xH20) and places it m a crucible. After heating, 3.7 g of anhydrous sodium carbonate (Na2C03) remains. What is the formula for the hydrate ... 

In examining choice A, the original mass of the hydrate would not change however, because rust will not evaporate, the final mass of the anhydride would be higher than expected — let s say 8 g. Substituting this value into the formula for % water would give... 

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