x chemistry unit 8 the mole problem solving involving chemical compounds

X Chemistry Unit 8The Mole

Problem Solving involving Chemical Compounds

Vocab: Percent composition – mass of the part

divided by mass of the whole times 100

Empirical Formula – the simplest whole number ratio of atoms in a compound

Molecular Formula- the true whole number ratio of atoms in a compound

Vocab: Molar Mass- the mass of 1 mole of a substance

Equal to the atomic mass for elementsFor a compound, the sum of the atomic

masses for all the atoms in a compoundUnits = grams/mole = g/mol

Mole - the amount of substance that is equal to 6.02x1023 particles of that substance

Vocab: Representative particle- the smallest particle of

a substance; defined by the type of substance:Element = atomCovalent compound = moleculeIonic compound = formula unitCharged atom = ion

Vocab: Avogadro’s Number- the number of particles in 1

mole of any substanceAvogadro’s # = 6.02x1023

Molar Volume- the volume of 1 mole of ANY gas at STP (Standard Temperature and Pressure = 0°C and 1atm), 22.4 Liters

What is a Mole? Amedeo Avogadro (Italian Mathematician)

used Carbon as the basis for the masses of the other elements

devised a counting relationship between grams and moles

1 mole = 6.02x1023 atoms of any element1 mole of carbon = 12.0 g Carbon

(matches the atomic mass on the P.T.)

What is a Mole?

(REAL WORLD EXAMPLE)

Relationships:1 dozen = 12 donuts just like

1 mole = 6.02x1023 atoms

BUT:1 dozen feathers = 0.015 grams

and1 dozen bricks = 32,400 grams!

What is a Mole?The mass of 1 mole of any element will be

different than 1 mole of any other element Practice:

What are the masses of 1 mole of the following elements?

Magnesium = 24.31 g/mol

Nitrogen = 14.01 g/mol

One-mole Amounts

Determining Molar Mass:

Steps in Solving:Write the element symbols in the formula down

the left hand side of the paperMultiply the number of atoms by the molar

mass of that element according to the P.T. and rounded to the tenths place

Sum the products from step 2 the units on your final answer are g/mol (grams

per mole)

Determining Molar Mass: Examples:

CO2

(NH4)2SO4

Mole Conversions

Moles!

MassRepresentative

particles(atoms, molecules, formula

units, ions)

Mole Conversions

Use molar mass

Use Avogadro’s # (6.02 x 1023)

Converting between Moles and Grams: Mole-Mass Conversions

Dimensional Analysis!! Use molar mass as conversion factor!

Moles to grams: # of moles molar mass (g) = # of grams

1 mole

Grams to moles: # of grams 1 mole = # of moles

Molar mass (g)

Converting between Moles and Grams:

Examples: How many moles of CCl4 are their in

523.4g?

Converting between Moles and Grams:

Examples: (cont.) How many grams of Na are there in 12.3

moles of Na?

Conversions Between Moles & Particles: Use Avogadro’s Number!!

Examples:

How many atoms of carbon are contained in 0.230 moles of C?

Conversions Between Moles & Particles

Examples:

How many molecules of sodium chloride are contained in 0.145 moles of NaCl?

Conversions Between Moles & Particles

Challenge!!How many molecules of barium chloride are

contained in 1.07 grams of barium chloride?

Percent Composition Problems:

2 types:1. Using data measured in grams to find

percentage of element in some given sample (More specific)

2. Using a chemical formula to find percentage of each element in a compound (More general)

Percent Composition Problems: Type 1: (just like the separation lab!)

% of element in a specific sample

100%compound or sampleof mass total

elementof mass ncompositio %

Percent Composition Problems: Examples:If 20.55 g of sodium combines completely with 31.75 g of chlorine to form a compound, what is the % composition of each element in this compound?

Percent Composition Problems:

Type 2:Mass of element in 1 mole of a compound

100%compoundof mass molar

compound in elementof mass ncompositio %

Percent Composition Problems:

Example:

What is the percent composition of each element in C12H22O11?

Empirical Formula Calculations

Percent to massMass to molesDivide by Small

Multiply ‘til whole

% g moles empirical formulaDivide by smallest

mole value

Multiply ‘til Whole Hints

Common Possible Endings:

.33 x 3

.25 x 4

.67 x 3

.50 x 2

Determining the Empirical Formula of a Compound: Steps for Solving: Convert grams of each element to moles using molar mass

Inspect your answers and find the smallest number of moles of all the answers

Divide each number of moles for each element by the smallest determined in step 2

Your answers to step 3 have no units since it was moles/moles they cancel out!!

IF the ratios do not come out to a whole number, multiply all values by some whole number to get all whole numbers

These numbers are the subscripts for each of the elements in the empirical formula

Example:

Find the empirical formula for a compound composed of 78% iron and 22% oxygen.

Determining the Empirical Formula of a Compound:

Determining the Empirical Formula of a Compound:

A compound is 81.1 % boron and 18.9% hydrogen. What is the empirical formula of this compound?

Determining the Molecular Formula of a Compound:

Steps for Solving:Find the Empirical FormulaFind the molar mass of the Empirical FormulaDivide the Molecular molar mass by the

Empirical molar massThe answer to step 3 is the factor you multiply

each subscript in the Empirical Formula by to get the Molecular Formula

Determining the Molecular Formula of a Compound:

A compound of B and H is 81.10% B and 18.9% H. Its empirical formula is B2H5. What is its molecular formula if the molar mass of the compound is 53.3 g/mol?