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**% Composition, Empirical and Molecular Formulas**

Chemical Quantities The Mole, % Composition, Empirical and Molecular Formulas

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**How you measure how much?**

You can measure mass, or volume, or you can count pieces. We measure mass in grams. We measure volume in liters. We count pieces in MOLES.

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**Moles Defined as the number of carbon atoms in exactly**

12 grams of carbon-12. 1 mole is 6.02 x particles. Treat it like a very large dozen 6.02 x is called Avagadro’s number.

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**Representative particles**

The smallest pieces of a substance. For a molecular compound it is a molecule. For an ionic compound it is a formula unit. For an element it is an atom.

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**Types of questions 3 12 3 2 5 How many oxygen atoms in the following?**

CaCO3 Al2(SO4)3 How many ions in the following? CaCl2 NaOH 3 12 3 2 5

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**Types of questions using the equality; 1 mole = 6.02 x 1023**

How many molecules of CO2 are the in 4.56 moles of CO2 ? 4.56 mole x x1023 mc = mole How many moles of water is 5.87 x molecules? 5.87 x mc x mole = x1023 mc 2.75x1024mc mole

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**Types of questions using the equality; 1 mole = 6.02 x 1023**

How many atoms of carbon are there in 1.23 moles of C6H12O6 ? 1.23 moles x x1023 mc x 6 atoms = mole mc How many moles is 7.78 x 1024 formula units of MgCl2? 7.78x1024 FU x 1 mole = mole x1024 FU 4.44x1024 atoms

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**Measuring Moles Remember relative atomic mass?**

The amu was one twelfth the mass of a carbon 12 atom. Since the mole is the number of atoms in 12 grams of carbon-12, the decimal number on the periodic table is also the mass of 1 mole of those atoms in grams.

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**Gram Atomic Mass The mass of 1 mole of an element in grams.**

12.01 grams of carbon has the same number of pieces as grams of hydrogen and grams of iron. We can right this as g C = 1 mole We can count things by weighing them.

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**Examples How much would 2.34 moles of carbon weigh?**

2.34 moles C x 12 g C mole How many moles of magnesium in g of Mg? 24.31 g Mg x mole = g Mg = g 1.013 mole

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**How many atoms of lithium in 1.00 g of Li?**

1.00 g Li x 1 mole x 6.02x1023 atoms g Li mole How much would 3.45 x 1022 atoms of U weigh? 3.45x1022 atoms U x 1 mole x g U x1023atoms 1 mole 8.60x1022 atoms 13.6 g

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What about compounds? in 1 mole of H2O molecules there are two moles of H atoms and 1 mole of O atoms To find the mass of one mole of a compound determine the moles of the elements they have Find out how much they would weigh add them up

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**What about compounds? What is the mass of one mole of CH4?**

1 mole of C = 12 g 4 mole of H x 1 g = 4g 1 mole CH4 = = 16g The Gram Molecular mass of CH4 is 16.05g The mass of one mole of a molecular compound.

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**Gram Formula Mass The mass of one mole of an ionic compound.**

Calculated the same way. What is the GFM of Fe2O3? 2 moles of Fe x 56 g = 112 g 3 moles of O x 16 g = 48 g The GFM = 112 g + 48 g = 160g

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**Molar Mass The generic term for the mass of one mole.**

The same as gram molecular mass, gram formula mass, and gram atomic mass.

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Examples Calculate the molar mass of the following and indicate what type it is. Na2S 2 (23) + 32 = N2O4 2(14) + 4(16) = C = 78g 1mole Na2S = 78g Gram Formula Mass = 92g 1 mole N2O4 = 92g Gram Molecular Mass 12g 1 mole C = 12 g Gram Atomic Mass

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**Molar Mass Cont. Ca(NO3)2 40 + 2(14) + 6(16) = 40 + 28 + 96 = 164g**

1 mole Ca(NO3)2 = 164g C6H12O6 6(12) + 12(1) + 6(16) = = 180g 1 mole C6H12)6 = 180g Gram Molecular Mass (NH4)3PO4 3(14) + 12(1) (16) = = 149g 1 mole (NH4)3PO4 = 149g Gram Formula Mass Gram Formula Mass

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**Finding moles of compounds Counting pieces by weighing**

Using Molar Mass Finding moles of compounds Counting pieces by weighing

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**Molar Mass The number of grams of 1 mole of atoms, ions, or molecules.**

We can make conversion factors from these. To change grams of a compound to moles of a compound.

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For example How many moles is 5.69 g of NaOH?

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For example How many moles is 5.69 g of NaOH?

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**For example How many moles is 5.69 g of NaOH?**

need to change grams to moles

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**For example How many moles is 5.69 g of NaOH?**

need to change grams to moles for NaOH

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**For example How many moles is 5.69 g of NaOH?**

need to change grams to moles for NaOH 1mole Na = 23g 1 mol O = g 1 mole of H = 1 g

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**For example How many moles is 5.69 g of NaOH?**

need to change grams to moles for NaOH 1mole Na = 23g 1 mol O = 16 g mole of H = 1 g 1 mole NaOH = 40 g

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**For example How many moles is 5.69 g of NaOH?**

need to change grams to moles for NaOH 1mole Na = 23g 1 mol O = 16 g 1 mole of H = 1 g 1 mole NaOH = 40 g

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**For example How many moles is 5.69 g of NaOH?**

need to change grams to moles for NaOH 1mole Na = 23g 1 mol O = 16 g mole of H = 1 g 1 mole NaOH = 40 g

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**Examples How many moles is 4.56 g of CO2? 4.56g CO2 x 1 mole 1 44gCO2**

How many grams is 9.87 moles of H2O? 9.87 moles x 18g H2O mole = moles = 178g

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**Examples How many molecules in 6.8 g of CH4?**

6.8g CH4 x 1 mole x 6.02x1023 mc g CH mole 49 molecules of C6H12O6 weighs how much? 49 mc x 1 mole x g C6H12O6 = x1023 mc mole 8820 g____= 1.5x10-20 g 6.02x1023 = 2.56x1023 mc

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Gases and the Mole

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**Gases Many of the chemicals we deal with are gases.**

They are difficult to weigh. Need to know how many moles of gas we have. Two things effect the volume of a gas Temperature and pressure Scientists compare gases at Standard Temperature and Pressure

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**Standard Temperature and Pressure**

0ºC and 1 atm pressure abbreviated STP At STP 1 mole of gas occupies 22.4 L Called the molar volume Avogadro’s Hypothesis - at the same temperature and pressure equal volumes of gas have the same number of particles.

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**Examples What is the volume of 4.59 mole of CO2 gas at STP?**

4.59mole x 22.4L mole How many moles is L of O2 at STP? 5.67L x 1 mole L What is the volume of 8.80g of CH4 gas at STP? 8.80g CH4 x 1mole x 22.4L g CH mole = = 103L = .523moles = = 12.3L

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**We have learned how to change moles to grams moles to atoms**

moles to formula units moles to molecules moles to liters molecules to atoms formula units to atoms formula units to ions

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Mass Periodic Table Moles

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Mass Volume Periodic Table Moles

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Mass Volume Periodic Table 22.4 L Moles

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Mass Volume Periodic Table 22.4 L Moles Representative Particles

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**Mass Volume Moles 6.02 x 1023 Representative Particles 22.4 L**

Periodic Table 22.4 L Moles 6.02 x 1023 Representative Particles

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**Mass Volume Moles 6.02 x 1023 Representative Particles Atoms 22.4 L**

Periodic Table Moles 6.02 x 1023 Representative Particles Atoms

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**Mass Volume Moles 6.02 x 1023 Representative Particles Ions Atoms**

Periodic Table Moles 6.02 x 1023 Representative Particles Ions Atoms

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**Percent Composition Like all percents Part x 100 % whole**

Find the mass of each component, divide by the total mass.

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Example Calculate the percent composition of each element in a compound that is 29.0 g of Ag with 4.30 g of S. Ag g S g 33.3g /33.3 = x 100 = 87.09% /33.3 = x 100 = 12.91%

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**Getting % from the formula**

If we know the formula, assume you have 1 mole. Then you know the pieces and the whole.

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**Examples Calculate the percent composition of C2H4? C 2(12g)=24**

H 4(1g) = +4 28g /28 = x 100 = 85.71% /28 = x 100 = 14.29%

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**Example Calculate the percent composition of Aluminum carbonate.**

Al2(CO3)3 Al 2(27g)= 54 C 3(12g)= 36 O 9(16)= 144 234g /234 = x 100 = 23.08% /234 = x 100 = 15.38% /234 = x 100 = 61.54%

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**• Step 2: Multiply the elements %, by the mass of the compound given.**

You can also calculate the mass of an element in a given amount of a compound using % composition. • Step 1: calculate the % comp. only of the element you want to find the mass of. • Step 2: Multiply the elements %, by the mass of the compound given. Example: Calculate the mass of sulfur in 3.54g of H2S. MM of H2S = H 2 (1) = S 1(32) = +32 34g H2S % S = 32/34 x 100 = 94.1% S 94.1% x 3.54g = 3.33g S

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**Calculate the mass of nitrogen in 25g of (NH4)2CO3.**

H 8(1g) = C 1(12g) = O 8(16g) = +128 176g (NH4)2CO3 %N = 28/176 x 100 = 15.91% 15.91% x 25g = 4.0g N

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**Calculate the mass of magnesium in 97.4g of Mg(OH)2.**

Mg 1 (24g) = 24 O 2(16g) = 32 H 2 ( 1g) = +2 58g Mg(OH)2 %Mg = 24/58 x 100 = 41.38% 41.38% x 97.4g = 40.3g Mg

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**From percentage to formula**

Empirical Formula From percentage to formula

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The Empirical Formula The lowest whole number ratio of elements in a compound. The molecular formula the actual ration of elements in a compound. The two can be the same. CH2 empirical formula C2H4 molecular formula C3H6 molecular formula H2O both

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**Calculating Empirical**

Just find the lowest whole number ratio C6H12O6 CH4N It is not just the ratio of atoms, it is also the ratio of moles of atoms. In 1 mole of CO2 there is 1 mole of carbon and 2 moles of oxygen. In one molecule of CO2 there is 1 atom of C and 2 atoms of O.

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**Calculating Empirical**

Means we can get ratio from percent composition. Assume you have a 100 g. The percentages become grams. Can turn grams to moles. Find lowest whole number ratio by dividing by the smallest moles.

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Example Calculate the empirical formula of a compound composed of % C, % H, and %N. Assume 100 g so 38.67 g C x 1mol C = mole C g C 16.22 g H x 1mol H = mole H g H 45.11 g N x 1mol N = mole N g N

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**Example The ratio is 3.223 mol C = 1 mol C 3.222 mol N 1 mol N**

The ratio is mol H = 5 mol H mol N mol N C1H5N1

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**A compound is 43.64 % P and 56.36 % O. What is the empirical formula?**

43.64 g P x 1mol P = mole P g P 56.36 g O x 1mol O = mole O g O The ratio is mol O = 2.5 mol O mol P mol P Can not have 2.5 atoms! Double P2O5

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**Caffeine is 49. 48% C, 5. 15% H, 28. 87% N and 16. 49% O**

Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula? 49.48 g C x 1mol C = mole C g C 5.15 g H x 1mol H = 5.15 mole H g H 28.87 g N x 1mol N = mole N g N 16.49 g O x 1mol O = mole O g O

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**The ratio is 4.123 mol C = 4 mol C 1.031 mol O 1 mol O**

The ratio is 5.15 mol H = 5 mol H mol O mol O The ratio is mol N = 2 mol N mol O mol O C4H5N2O1

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**Empirical to molecular**

Since the empirical formula is the lowest ratio the actual molecule would weigh more. By a whole number multiple. Divide the actual molar mass by the the mass of one mole of the empirical formula. Caffeine has a molar mass of 194 g. what is its molecular formula? C4H5N2O1 = 97g 194/97 = 2 Molecular Formula = C8H10N4O2

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**Example 71.65 g Cl x 1mol Cl = 2.047 mole Cl 35 g Cl**

A compound is known to be composed of % Cl, 24.27% C and 4.07% H. Its molar mas is known (from gas density) is known to be g. What is its molecular formula? 71.65 g Cl x 1mol Cl = mole Cl g Cl 24.27 g C x 1mol C = mole C g C 4.07 g H x 1mol H = 4.07 mole H g H

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**The ratio is 2.047 mol Cl = 1 mol Cl 2.023 mol C 1 mol C**

The ratio is 4.07 mol H = 4 mol H mol C mol C Empirical Formula = CClH4 = 51g 98.96/51 = 2 Molecular Formula = C2Cl2H8

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