It s just like finding out your test score!!!
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2 It s just like finding out your test score!!! 97 correct out of 100 questions = 97 x 100 = 97% 100 Total of something x 100 = % TOTAL
3 The chemical composition can be expressed as the mass percent of each element in the compound.
4 Example: Determine the percent composition of C 3 H g % C g 81.7% 8.08g % H g 18.3%
5 Example: Determine the percent composition of iron(iii) sulfate. Iron(III) sulfate = Fe 2 (SO 4 ) g %Fe % g 96.21g %S % g g %O % g
6 Determining Empirical Formulas If % composition can be determined from the formula, then the formula can be determined from the % composition of the compound.
7 Rules for determining Empirical Formulas 1. Assume a 100 g sample, so % becomes g. 2. Convert grams to moles by dividing by the atomic mass of each element. 3. Divide each result (mole) by the smallest result present (mole ratio). 4. Look for whole number ratios. 5. The whole number ratios become the subscripts for the formula.
8 Multiples for use when ratio is not initially a whole number Decimal Multiplier 0.5 X X X X X 4
9 Hydrated Compounds Some compounds exist in a hydrated state. Some specific number of water molecules are present for each molecule of the compound.
10 What is a Hydrate? Any salt that has water chemically bonded to the ions in the crystal structure is a hydrate or hydrated crystal. Copper(II) sulfate is a hydrate. Hydrated copper(ii) sulfate is deep blue in color.
11 Example An opal is hydrated silicon dioxide (SiO 2 ) The presence of water and various mineral impurities accounts for the variety of colors.
12 Examples of Hydrates Anhydrous (without water) CoCl 2 The hydrate CoCl 2 6 H 2 O
13 FORMULA FOR THE HYDRATED SALT HYDRATE = ANHYDROUS SALT + WATER EXAMPLE: MgCl 2 6H 2 O
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15 Calculating the Formula Mass of Hydrates Na 2 CO 3 10 H 2 O Na: 2 x 23 = 46 C: 1 x 12 = 12 O: 3 x 16 = 48 H 2 O: 10 x 18 = 180 (H: 20 x 1 = 20; O: 10 x 16 = 160; = 180) Total = 286 g/mol **Reminder-the dot in the formula means to add the mass of the water (not multiply as in math)
16 Practice Problem Calculate the formula mass of the following: 1. CuSO 4 5 H 2 O Answer : g/mol 2. BaCl 2 2 H 2 O Answer : 244 g/mol
17 Determining the Formula of Hydrates from Experimental Data The composition of a hydrate is determined to contain 48.8% MgSO 4 and 51.2% H 2 O. What is the formula of the hydrate? 48.8/120 (mass of MgSO 4 ) = moles MgSO /18 (mass of H 2 O) = 2.84 moles H 2 O 0.407/0.407 = /0.407 = 7 Formula is MgSO 4 7 H 2 O
18 Let s Try Another A g sample of a sample of a hydrate of magnesium iodide (MgI 2 ) heated until its mass is reduced to g and all water has been removed. What is the formula of the hydrate? / 278 = moles MgI = g water / 18 = moles H 2 O / = / = 8 Formula: MgI 2 8 H 2 O
19 If you heat hydrates to very high temperatures, they dehydrate, meaning that the water is lost. Once all of the water is lost, these compounds are referred to as anhydrous.
20 Anhydrides A compound that is normally a hydrate and has lost its hydration water is said to be anhydrous and is called an anhydride. BaCl 2 2H 2 O BaCl 2 Barium Chloride Dihydrate Barium Chloride Anhydride -or- Anhydrous Barium Chloride
21 Some hydrates are a color different than their anhydrous salt Cobalt (II) chloride hexahydrate is pink Without water it is blue.
22 1. Heat the sample to drive off all water. 2. Determine the mass of the anhydrous compound. 3. mass of H 2 O = mass of hydrate mass of anhydrous salt 4. Convert these masses to moles. 5. Divide through by the smallest number of mol. 6. Write the formula 7. Name the compound
23 Example: A 7.0 g sample of hydrated calcium nitrate is heated. After cooling, the sample weighed 4.9 grams. Find the percent water in the hydrate and the formula for it.
24 4.9 grams remains after heating This is the anhydrous salt. How much water was driven off from the hydrate? 7.0 g sample of hydrated calcium nitrate g anhydrous salt = 2.1 grams of water
25 To write the formula Covert the mass of anhydrous salt to moles. Convert the mass of water to moles. Write the formula for the hydrate.
26 Hydration Number Some molecules attach themselves to water molecules. This is done in set numbers, depending on the molecule. For example, copper sulfate attaches to 5 water molecules. We say its hydration number is 5 CuSO 4 5H 2 O (copper (II) Sulfate Pentahydrate)
27 Finding the Hydration Number The hydration number can be conveniently found by heating the compound and measuring its mass loss. This mass loss is usually due to the hydration water molecules being driven off. For example A g sample of Strontium nitrate, Sr(NO 3 ) 2 nh 2 O, is heated to a constant mass of g. Calculate the hydration number.
28 Sample Data: Mass Hydrate 15.35g Mass Anhydride 11.45g Mass of Water (mass loss) 3.90g
29 Calculations Moles Anhydride 11.45g Sr(NO ) 2 Moles Water Molar ratio 1molSr(NO ) molsr(no ) g Sr(NO 3) g H O 1molH2O g H O molh O molsr(no ) H2O molsr(no ) molh O Hydration Number is 4, Sr(NO 3 ) 2 4H 2 O (Strontium Nitrate Tetrahydrate) 2 4
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