Percent Composition, Empirical Formula, Molecular Formula, Hydrates

Essential Questions• How can one explain the structure, properties, and interactions of matter?• How do substances combine or react to make new substances?• How can we count items we cannot see?

Learning Objectives•Analyze quantitatively the composition of a substance (empirical formula, molecular formula, percent composition, and hydrates)•Determine percent composition of a given compound•Calculate empirical formula from mass or percent using experiment data•Calculate molecular formula from empirical formula using molecular weight•Perform calculations based on percent composition•Determine the composition of hydrates using experimental data

Name: __________________________________

© B. Mack - 2018

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Packet Contents• Notes 2 – 3 • Reinforcement Pages 4 – 10

Packet Does NOT Include:• In-Class Demonstrations• In-Class Activities• Lab

The Mole … Extension

Page 2© B. Mack - 2018

Percent Composition, Empirical Formula, Molecular Formula, Hydrates

Percent Composition

VS.

Empirical Formula Molecular Formula

C2H4

CHKI

H2O

AhydrateofNa2CO3 hasamassof4.31gbeforeheating.Afterheating,themassoftheanhydrouscompoundisfoundtobe3.22g.Determinetheformulaofthehydrateandthenwriteoutthenameofthehydrate.

Page 3© B. Mack - 2018

hydrates

Step 1

Naming

Finding the Formula for a hydrate

Step 2

Step 3

What is a hydrate?

A hydrate of Na2CO3 has a mass of 4.31 g before heating. After heating, the mass of the anhydrous compound is found to be 3.22 g. Determine the formula of the hydrate and then write out the name of the hydrate.

Name: _____________________________________ Date: ___________________ Period: __________

Worksheet: Percent Composition 1. What is the percent mass of Carbon and Oxygen in CO2?

2. What is the percent composition of hydrogen in phosphoric acid, H3PO4?

3. Which has a larger percent by mass of sulfur, H2SO3 or H2S2O8?

4. Determine the percent composition of each element in the following compounds a. Na2C2O4

b. C2H5OH

c. Aluminum oxide

d. Potassium sulfate

5. Calculate the mass percent of bromine in 50.0 grams of potassium bromide.

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Name: _____________________________________ Date: ___________________ Period: __________

6. Calculate the percentage of water in sodium carbonate decahydrate (Na2CO3�10H2O)

7. Expressthecompositionofeachcompoundasthemasspercentofitselements a. Sucrose(C12H22O11)

b. Magnetite(Fe3O4)

c. Aluminumsulfate

8. CalculatethepercentageofBariuminBaCl2�2H2O

9. What is the percent composition of sulfate in sodium sulfate?

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Name: _______________________________________________ Date: ____________ Period: _______

Worksheet: Empirical and Molecular Formulas Part I. Find the empirical formula for the following molecular compositions

a. 88.8% copper; 11.2% oxygen

b. 40% carbon; 6.7% hydrogen; 53.5% oxygen

c. 92.3% carbon; 7.7% hydrogen

d. 70.0% iron; 30.0% oxygen

e. 5.88% hydrogen; 94.12% oxygen

f. 38.7% chlorine; 61.3% fluorine

g. 7.19% phosphorus; 92.81% bromine

h. 30.4% nitrogen; 69.6% oxygen

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Name: _______________________________________________ Date: ____________ Period: _______ Part II.

a. Naphthalene is a carbon and hydrogen containing compound often used in moth balls. The empirical formula is C5H4 and its molar mass is 128.16 g/mol. Find the molecular formula.

b. A compound with the following composition has a molar mass of 60.10 g/mol. 39.97% carbon; 13.41% hydrogen; 46.62% nitrogen. Find the molecular formula.

c. Monosodium glutamate, known as MSG, is sometimes added to food to enhance flavor. Analysis determined this compound to be 35.5% C, 4.77% H, 8.29% N, 13.6% Na, and 37.9% O. What is the molecular formula?

d. A compound contains 18.8% sodium, 29.0% chlorine and 52.2% oxygen, by mass. If the molar mass of the compound is 122.44 g/mol, determine the empirical and molecular formulas.

e. A certain compound has an empirical formula of ClCH2 and has a molecular weight 98.96 g/mol. What is its molecular formula?

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Name: _______________________________________________ Date: ____________ Period: _______

Worksheet: Hydrates

Part I. Name the following hydrates

a. FeCl3�6H2O ______________________________ b. CuSO4�5H2O ______________________________ c. MgCrO4�5H2O ______________________________ d. MnBr4�4H2O _______________________________ e. barium chloride dehydrate _______________________

f. magnesium sulfate heptahydrate _______________________

g. tin (IV) chloride pentahydrate _______________________

h. barium hydroxide octahydrate _______________________

i. mercury (I) nitrate monohydrate _______________________

Part II. Table 1 shows data from an experiment to determine the formulas of hydrated barium chloride. Determine the formula for the hydrate and its name

Data for BaCl2 �xH2O Mass of empty crucible 21.30 g

Mass of hydrate + crucible 31.35 g Initial mass of hydrate

Mass after heating 5 min 29.87 g Mass of anhydrous solid

Part III. The hydrated form of nickel (II) chloride is a green compound commonly found in the chemistry laboratory. If a 5.00 gram sample of hydrated nickel (II) chloride loses 2.27 grams of water when heated, what is the formula of the hydrate? Name the hydrate. Page 8

Name: _______________________________________________ Date: ____________ Period: _______ An 11.75 gram sample of common hydrate of cobalt (II) chloride is heated. After heating, 0.0712 mole of anhydrous cobalt chloride remains. What is the formula and the name of this hydrate. Chromium (III) nitrate forms a hydrate that is 40.50% water by mass. What is its chemical formula? If 125 grams of magnesium sulfate heptahydrate is completely dehydrated, how many grams of anhydrous magnesium sulfate will remain? A calcium chloride hydrate has a mass of 4.72 grams. After heating for several minutes, the mass of the anhydride is found to be 3.56 grams. Use this information to determine the formula for the hydrate. A sample of copper (II) sulfate hydrate has a mass of 3.97 grams. After hearing, the copper (II) sulfate that remains has a mass of 2.54 grams. Determine the correct formula and name the hydrate. A 1.628 gram sample of a hydrate of magnesium iodide is heated until its mass is reduced to 1.072 grams and all water has been removed? What is the formula of the hydrate?

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Determining the Formula of a Hydrate Chem Worksheet 11-6

Name

A hydrate is an ionic compound that contains water molecules in its structure. To determine the formula of a hydrate experimentally, we must calculate the mole: mole ratio of the water portion compared to the anhydrate portion. An anhydrate is the substance that remains after the water is removed from a hydrate. When a hydrate is heated the water molecules are driven off as steam, leaving behind the water-free anhydrate.

The first step to finding the formula for a hydrate is to record the mass of the hydrate. After heating the hydrate, the mass is determined for the anhydrate that remains. The mass of the water that was present is calculated by finding the difference between the mass of the hydrate and the mass of the anhydrate. The mass of the water and the mass of the anhydrate are each converted to moles using their respective molar masses. From this a whole number ratio can be determined (see example).

Data Table Mass of hydrate (CaCl2 · xH2O) 4.72 g Mass of anhydrate (CaCl2) 3.56 g Mass of water 1.18 g

Solve the following problems. Show work to support your answer.

1. A hydrate of magnesium sulfate has a mass of 13.52 g. This sample is heated until no water remains. The magnesium sulfate anhydrate has a mass of 6.60 g. Find the formula and name of the hydrate.

2. A sample of copper (II) sulfate hydrate has a mass of 3.97 g. After heating, the copper (II) sulfate that remains has a mass of 2.54 g. Determine the correct formula and name of the hydrate.

3. When 5.00 g of iron (III) chloride hydrate, 2.00 g of H2O are driven off. Find the chemical formula and the name of the hydrate.

4. A sample of the hydrate of sodium carbonate has a mass of 8.85 g. It loses 1.28 g when heated. Find the formula and the name of the hydrate.

5. A 16.4 g sample of hydrated calcium sulfate is heated until all the water is driven off. The calcium sulfate that remains has a mass of 13.0 g. Find the formula and the chemical name of the hydrate.

6. When 8.00 g of lead (II) acetate hydrate is heated, 1.14 g of H2O are driven off. Find the chemical formula and the name of the hydrate.

7. A hydrate is determined to be 45.43% water and 54.57% Cobalt (II) chloride. Find the chemical formula and name for this hydrate.

Example

A calcium chloride hydrate has a mass of 4.72 g. After heating for several minutes the mass of the anhydrate is found to be 3.56 g. Use this information to determine the formula for the hydrate.

- find the mass of the water driven off: mass of hydrate – mass of anhydrate = mass of water 4.72 g – 3.56 g = 1.18 g

- convert the mass of anhydrate to moles: 3.56 g CaCl2 ´ 1 mol CaCl2 = 0.0321 mol CaCl 110.98 g CaCl

- convert the mass of water to moles: 1.18 g H2O ´ 1 mol H2O = 0.0655 mol H O

18.02 g H O

- find the mole H2O to mole CaCl2 ratio: 0.0321 mol CaCl2 1 mol CaCl2

0.0655 mol H2O 2 mol H2O

Since the compound contains 2 moles of water for every 1 mole of anhydrate the formula is CaCl2 · 2 H2O

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