Percent Calc, EF and MF February 02, 2015

WARM UP: Feb. 2

Convert the number of moles or grams to the number of particles using Avogadro’s number:

1. 2.3 moles

2. 88.1 moles

3. 11.9 grams of Ca(OH)2 (HINT: TWO STEPS) MM Ca(OH)2

Ca: 40 g/M

O: 16 x 2 g/M

H: 1 x 2 g/M

MM: 74 g/M

Convert to Moles first: 11.9 grams 74 grams/mole = 0.16 moles

Convert to Molecules second: 0.16 moles x (6.02 x 1023 mol./moles) = 9.6 x 1022 molecules

Percent Calc, EF and MF February 02, 2015

WARM UP: Feb. 2

4. Find the Molar Mass of Al(NO3)3?

Al:

N:

O:

Percent Calc, EF and MF February 02, 2015

February 2, 2015

Calculating Percent Composition, Empirical Formula and Molecular

Formulas

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Percent Calc, EF and MF February 02, 2015

Percent Composition

• Sometimes it’s useful to know the composition of a compound in terms of what percentage of the total is each element

• Percent> “Parts per 100”> The number of specific items per a group of 100

items> 50% of $100 is $50 (50 items/100 total items)

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Percent Calc, EF and MF February 02, 2015

Percent Composition

• Each compound has a different percent composition

3

Percent Calc, EF and MF February 02, 2015

Percent Example

• You have 4 oranges and 5 apples. What percent of the total is oranges?

• In “parts per 100”

4

Percent Calc, EF and MF February 02, 2015

Percent Composition• It is the percent by mass of each element in a

compound• Can be determined

> By its chemical formula> Molar masses of the elements that compose the

compound • The percent of each element contributes to

the mass of the compound

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Percent Calc, EF and MF February 02, 2015

Calculating Percent Composition Example #1

• What is the percent composition of each element in NH4OH?

Determine the contribution of each element

Molar mass

6

CALCULATE MOLAR MASS OF NH4OH

Percent Calc, EF and MF February 02, 2015

Calculating Percent Composition Example # 2

• Calculate the percent composition of CuBr2?

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Percent Calc, EF and MF February 02, 2015

Empirical Formulas

• The simplest ratio of elements in a compound

• It uses the smallest possible whole number ratio of atoms present in a formula unit of a compound

• If the percent composition is known, an empirical formula can be calculated

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Percent Calc, EF and MF February 02, 2015

Empirical Formulas• To Determine the empirical formula:• Calculate the moles of each element

> Use molar mass (sum of atomic masses)• Calculate the ratios of the elements to each

other• Find the lowest whole number ratio

> Divide each number of moles by the smallest number of moles present

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Percent Calc, EF and MF February 02, 2015

Empirical Formula: Converting Decimal Numbers to Whole Numbers

• The subscripts in a formula are expressed as whole numbers, not as decimals

• The resulting numbers from a calculation represent each element’s subscript

• If the number(s) are NOT whole numbers, multiply each number by the same small integer (2, 3, 4, 5, or 6) until a whole number is obtained

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Percent Calc, EF and MF February 02, 2015

Relating Empirical and Molecular Formulas

• n represents a whole number multiplier from 1 to as large as necessary

• Calculate the empirical formula and the mass of the empirical formula

• Divide the given molecular mass by the calculated empirical mass> Answer is a whole number multiplier 10

Percent Calc, EF and MF February 02, 2015

Relating Empirical and Molecular Formulas

• Multiply each subscript in the empirical formula by the whole number multiplier to get the molecular formula

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Percent Calc, EF and MF February 02, 2015

Calculate Empirical Formula from Percent Composition

• Lactic acid has a molar mass of 90.08 g and has this percent composition:

40.0% C, 6.71% H, 53.3% O• What are the empirical and molecular formulas of

lactic acid?• Assume a 100.0 g sample size

> Convert percent numbers to grams> Ex: 40.0% C is 40.0 g

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Percent Calc, EF and MF February 02, 2015

Calculate Empirical Formula from Percent Composition

• Convert mass of each element to moles• Divide each mole quantity by the smallest number of moles

Empirical formula is

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Percent Calc, EF and MF February 02, 2015

Determination of the Molecular Formula for Lactic Acid

• Obtain the value of n (whole number multiplier)• Multiply the empirical formula CH2O by the

multiplier

Molecular formula = n х empirical formula

Molecular formula = 3 (CH2O) = 14

Percent Calc, EF and MF February 02, 2015

Formulas for Compounds• Empirical Formula (EF)

> Smallest possible set of subscript numbers > Smallest whole number ratio> All ionic compounds are given as empirical formulas

• Molecular Formulas (MF)> The actual formulas of molecules> It shows all of the atoms present in a molecule> It may be the same as the Empirical Formula or a

whole-number multiple of its EF

Molecular formula = n х Empirical formula15

Percent Calc, EF and MF February 02, 2015

A 5.438 g sample of ludlamite, a greyish-blue mineral used in ceramics was found to contain 2.549 g of iron, 1.947 g of oxygen and 0.9424 g of phosphorous. What is the EF?Convert to moles: Mole Ratio:

Percent Calc, EF and MF February 02, 2015