I Name _ ___ ______________ Date ___ ___ Class ______

Composition of Hydrates Lab

Text reference: Chapter 7, pp . 154-159

Pre-lab Discussion Hydrates are ioni c compounds (salts) that. have a definite amount of water (water o f hydration) as part of their s tructure. The water is chemically combined with the salt in a definite ratio . Rati os vary in different hydrates but are specific for any given hydra te.

The formula of a hydrate is represented in a specia) manner. The hydrate of copper sulfate in this experiment has the formula CuS04 • .xH20. The unit formula for the salt appears firs t, and the wa ter formula is last. The raised dot means that the water is loosely bonded to the salt. The coefficient.x stands for the number of molecules of water bonded to one unit o f salt. This special formula, like a ll other formulas, illus tra tes the law of definite compositi on.

When hydrates are heated, the "water of hydration" is rel eased as vapor. Th e remaining solid is known as the a.nhydrous salt. The general reaction for heating a hydrate is:

hydrate anhydrous salt + water

The percent of water in a hydrate can be found experimentally by accurately determining the mass o f the hydrate and the mass of the anhydrous salt. The difference in mass is due to the water lost by the hydrate . The percentage of wate r in the original hydrate can easily be calculated :

m ass H20 percent H20 = X 100

mass hydrate

In this experiment, as was menti oned, a hydrate of co pper sulfate will b e s tudied (CuS04 • .xH20) . The change from hydrate to anhydrous salt is accompani ed by a change in color:

CuS04 + xH20 white

This investigation should aid in the unders tanding of the formulas and compositio n of hydrates and the law of definite composition.

Purpose Determine the perce ntage of water in a hydrate .

Equipment evaporating dish, porce lain crucible tongs microsparul a laboratory bala nce ring stand

iron ring wire gauze laboratory burner safe ty goggles lab apron or coat

57

58

Materials copper sulfate hydrate, CUS04 • xH20

Safety Do not touch a hot evaporating dish with your hands. Tw bac k long hair and seCUIe loose clothing when working around an o pen Dame. Note the caution alert symbols here and beside certain s teps in the "Procedure." Refer to page xi to review the special precautions cL<;sociated with each symbol.

Be SUIe to wear a lab apron or coat and safety goggles when working in the lab.

Procedure 1. Prepare the setup shown in Figure ] 1 - ] .

Figure 11-1

2. Heat the dish with th e hottes t part of the Dame for 3 minutes.

3. Using crucible tongs, remove the evaporating dish from the apparatus. Place it on an insulated pad and allow it to cool for several minutes.

4. Find the mass of the evaporating dish to ::':0.0] g. Record the mass in the Observations and Data section.

5. With the evaporating dish on the balance , measure into it exac tly 2.00 g of copper sulfate hydrate . Record the data below.

6. Place the evaporating di sh + hydrate on the wire gauze. Gently heat the dish by moving the burner back and forth around the base. Increase the heat graduaJly. Avoid any popping and spattering. 7. Heat strongly for 5 minutes or until the b lue co lor has di sap­peared. During heating, a microspatula may be used to "spread" the solid and break up any "caked" portions of the hydrate . Be careful not to pick up any of the solid on the microspatula. If the edges of the solid appear to be turning brown, remove the heat mome ntarily and resume heating at a gentler rate.

Name ______________________________________________________________________ _

11 Composition of Hydrates (continued)

8. Allow the evaporating dish to cool for about a minute. im­mediately find the mass of the dish + anhydrous salt, and record the data below ,

Observations and Data

a. Mass of evaporating dish ---------- g

b. Mass of evaporating dish + hydrate ----- - g

c. Mass of evaporating dish + anhydrous s,alt - ----g

Calculations 1. Find the mass of the hydrate used (b - a).

2. Find the mass o f the water lost (b - c) ,

3. Find the percentage of water in the hydrate:

mass water percent H,O = x 100

- mass hydrate

Conclusions and Ques,tions 1. The true vaJue for the percentage of water in this hydrate is 36.0%. What is your experimental error?

2. Why must you allo\-v the evaporating dish to cool before measuring its mass?

3. \\lhy must you measure the mass of the anhydrous salt immediately upon cooling?

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c,) r J./. pO j

/. ~,pl. 7/ - J/;;, 11 • .." OOj

,;; • 'II/, 71 - -'"I~ tJO O. 7/.::;1

/. %~rror :­

36 -3.s~ __----- -- 1- IPO=

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59

Lj. Find the value of x in the formula CUS04 . xH 20.

Example L)4~

S ep I Find the mass of the anhydrous salt (c -a )

YYOO 9 - Y2 7 19= 1.2Cl 9 CuSOy

Step 2 Find how many moles of CuSOy are in I .2Cl 9

mol CuSOy = ? 12<1g CuSOy (given)

I mol CUSOy

12<19 CuSOy X ______ = 0.00806 mol CuSO'j

I 60 9 CuSOy (formula mass)

Slep 3 Find how many moles are in O} I 9 H20 (#2 in ca lculat ions)

18 9 H20 (formula mass)

Step Y Find the ratio of moles of H20 to each mole of CuSOy

O.OY 5

mol CuSOy 0008