math with chemical formulas honors chemistry unit 5

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Math with Chemical Formulas Honors Chemistry Unit 5

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Page 1: Math with Chemical Formulas Honors Chemistry Unit 5

Math with Chemical Formulas

Honors Chemistry Unit 5

Page 2: Math with Chemical Formulas Honors Chemistry Unit 5

Unit ObjectivesBe able to perform math

functions with and without your calculator using correct scientific notation.

Be able to find molar/molecular/formula mass using the periodic table.

Page 3: Math with Chemical Formulas Honors Chemistry Unit 5

Unit Objectives, cont.Be able to calculate Molarity.Be able to calculate percent

composition.Be able to determine empirical

and molecular formulas using lab data.

Page 4: Math with Chemical Formulas Honors Chemistry Unit 5

Unit Objectives, cont.Understand the mole and

Avogadro’s number.Be able to convert to/from

atoms, ions, molecules, moles and grams

Page 5: Math with Chemical Formulas Honors Chemistry Unit 5

What is a “Mole” (mol)?

A mole is a counting unitjust like a dozenother examples…..

Page 6: Math with Chemical Formulas Honors Chemistry Unit 5

What is a Mole, cont.?“Official Definition”

the amount of a substance that contains as many particles as there are atoms in exactly 12g of carbon-12

Page 7: Math with Chemical Formulas Honors Chemistry Unit 5

Avogadro’s NumberConstantthe number of particles in

exactly one mole of a pure substance

6.02 X 1023

Memorize this NumberMemorize this Number

Page 8: Math with Chemical Formulas Honors Chemistry Unit 5

Conversion Factors – 1 mol

1 mol = 6.02 X1023 of anything

Page 9: Math with Chemical Formulas Honors Chemistry Unit 5

Conversion Factors – 1 mol Examples…

1 mol 6.02 X 1023 atoms

1 mol 6.02 X 1023 ions

1 mol 6.02 X 1023 molecules

Page 10: Math with Chemical Formulas Honors Chemistry Unit 5

Molar Mass

Mass in g of 1 mole of anything

For elements, the molar mass is equal to the atomic mass

Page 11: Math with Chemical Formulas Honors Chemistry Unit 5

Molar Mass Examples…..

1 mol atomic wt. (g)

1 mol C 12.01 g C

1 mol Li 6.94 g Li

Page 12: Math with Chemical Formulas Honors Chemistry Unit 5

You try……

1 mol Ca ? g Ca

1 mol Fe ? g Fe

Page 13: Math with Chemical Formulas Honors Chemistry Unit 5

Now you have Two conversion Factors for a mole…..

1 mol 6.02 X 1023 atoms, ions or molecules

andand

1 mol ______(g)

Page 14: Math with Chemical Formulas Honors Chemistry Unit 5

Example 1How many g in 2.0 mol of He?

2.0 mol He 4.0g He = 8.0g He 1.0 mol He

Page 15: Math with Chemical Formulas Honors Chemistry Unit 5

Oct. 16-22, 2005

Example 2How many moles in 3.01 X 1023

atoms Ag?

3.01 X1023 atoms Ag 1mol Ag =

6.02 X 1023 atoms Ag

0.500 mol Ag

Page 16: Math with Chemical Formulas Honors Chemistry Unit 5

Example 3 What is the mass of 1.20 X 108 atoms of Cu?

1.20 X108 atoms Cu 1mol Cu 63.6g Cu =

6.02 X 1023 atoms Cu 1 mol Cu

1.27 X 10-14 g Cu

Page 17: Math with Chemical Formulas Honors Chemistry Unit 5

You try:Convert 11.5 g B to moles B

1.06 mol BConvert 8.0 X 1019 atoms of

Ag to g

0.014 g Ag

Page 18: Math with Chemical Formulas Honors Chemistry Unit 5

Formula Mass/Molecular Mass/Molar Mass

Sum of masses in a compoundMolar Mass of sodium chloride

NaClNa 1 mol X 22.99g/mol = 22.99g

Cl 1 mol X 35.45g/mol = 35.45g

Total: 58.44g

Page 19: Math with Chemical Formulas Honors Chemistry Unit 5

Example 2Molar Mass of magnesium chloride

MgCl2Mg 1 mol X 24.31g/mol = 24.31g

Cl 2 mol X 35.45g/mol = 70.90g

Total: 95.21g

Page 20: Math with Chemical Formulas Honors Chemistry Unit 5

Example 3 - Calcium Nitrate

Molar Mass of Ca(NO3)2

Ca 1 mol X 40.08 g/mol = 40.08g

N 2 mol X 14.01 g/mol = 28.02g

O 6 mol X 16.00 g/mol = 96.00g

Total: 164.10g

Page 21: Math with Chemical Formulas Honors Chemistry Unit 5

You try:Calculate the molar mass of

sodium phosphate

Na3PO4

Na 3 mol X 22.99 g/mol = 68.97g

P 1 mol X 30.97 g/mol = 30.97g

O 4 mol X 16.00 g/mol = 64.00g

Total: 163.94g

Page 22: Math with Chemical Formulas Honors Chemistry Unit 5

Conversion Factors using Formula Mass, Molecular Mass/Molar Mass

1 mol NaCl58.44g NaCl

1 mol MgCl295.21g MgCl2

1 mol Ca(NO3)2

164.1g Ca(NO3)2

can be used as a conversion factors

Page 23: Math with Chemical Formulas Honors Chemistry Unit 5

You try….How many mol in 127g barium

chloride?

(set up on board)

Answer: 0.610 mol BaCl2

Page 24: Math with Chemical Formulas Honors Chemistry Unit 5

Percent CompositionPercent composition is the percent by

mass of each element in a compound.

Percent composition is the same, regardless of the size of the sample.

Page 25: Math with Chemical Formulas Honors Chemistry Unit 5

% Composition Calculations

% comp = mass of element X 100% molar mass of cpd

= % element in the compound

Page 26: Math with Chemical Formulas Honors Chemistry Unit 5

ExamplesFind the % composition of Cu2SFirst, find the molar mass:

2 mol Cu = 2 X 63.55g/mol = 127.1g Cu

1 mol S = 1 X 32.06g/mol = 32.06g S

Total: 159.15g/mol

Page 27: Math with Chemical Formulas Honors Chemistry Unit 5

Example, cont. Next, find the % of each elementFor Cu:For Cu:% Cu = 127.1g X 100% = 79.86% Cu

159.15gFor S:For S:% S = 32.06g X 100% = 20.14% S

159.15gNext, check your work – do the %s add up to 100?

Page 28: Math with Chemical Formulas Honors Chemistry Unit 5

You try:barium choride

sodium phosphate

Page 29: Math with Chemical Formulas Honors Chemistry Unit 5

Answers:barium chloride

barium 65.90%

chloride 34.10%sodium phosphate

sodium 42.07%

phosphorus 18.89%

oxygen 39.04%

Back to Objectives

Page 30: Math with Chemical Formulas Honors Chemistry Unit 5

Determining Formulas

Empirical Formula = Simplest Formula

Page 31: Math with Chemical Formulas Honors Chemistry Unit 5

To find the empirical formula from data:

1. Assume 100% sample; change % to grams for each element

2. Find moles from the grams of each element

3. Find the smallest whole # ratio by dividing by the smallest number of moles

4. If necessary, multiply to get rid of fractions.

Page 32: Math with Chemical Formulas Honors Chemistry Unit 5

Example A compound is 78% B and 22% H. What is

the empirical formula?

FirstFirst, change % to grams and find moles:

78g B 1mol B = 7.22 mol10.81g B

22g H 1mol H = 21.78 mol1.01g H

Page 33: Math with Chemical Formulas Honors Chemistry Unit 5

Example, cont.NextNext, divide all mole numbers by the smallest

number of moles:

B: 7.22 mol = 17.22 mol

H: 21.78 mol = 3.02 = 37.22 mol

Page 34: Math with Chemical Formulas Honors Chemistry Unit 5

Example, cont. Finally, use these whole numbers as the

number of each individual element. They are the subscripts.

Empirical Formula = BH3

Page 35: Math with Chemical Formulas Honors Chemistry Unit 5

Example 2 Analysis shows a compound to contain

26.56% K, 35.41% Cr, and 38.03% O. Find the empirical formula of this compound:

FirstFirst (always!) assume 100g sample, convert % to g and then find moles of each element

Page 36: Math with Chemical Formulas Honors Chemistry Unit 5

Example 2 cont. Next,Next, Conversion to moles:

26.56g K 1mol K = 0.6793 mol K39.10g K

35.41g Cr 1mol Cr = 0.6810 mol Cr52.00g Cr

38.03g O 1mol O = 2.377 mol O16.00g O

Page 37: Math with Chemical Formulas Honors Chemistry Unit 5

Next,Next, divide all numbers by the smallest whole number to find the smallest whole number ratios:

0.6793 mol K = 1.00 mol K0.6793

0.6810 mol Cr = 1.003 mol Cr ~ 1.00 mol Cr0.6793

2.377 mol O = 3.499 mol O – can’t be rounded

0.6793

Page 38: Math with Chemical Formulas Honors Chemistry Unit 5

So, if you have: multiply all by:

.25 or .75 4

.33 or .66 3

.50 2

Page 39: Math with Chemical Formulas Honors Chemistry Unit 5

For our example:

2 X 1.00 mol K = 2 mol K

2 X 1.00 mol Cr = 2 mol Cr

2 X 3.499 mol O= 7 mol O

Empirical Formula = K2Cr2O7

Page 40: Math with Chemical Formulas Honors Chemistry Unit 5

You try:What is the empirical formula if we have

a sample containing 66.0% Ca and 34.0% P?

Answer: Ca3P2

Page 41: Math with Chemical Formulas Honors Chemistry Unit 5

You try:Find the empirical formula of a

compound with 32.38% Na; 22.65% S; and 44.99% O.

Answer: Na2SO4

Page 42: Math with Chemical Formulas Honors Chemistry Unit 5

Molecular FormulaMolecular Formula = Actual Formula

Example:

C2H6 CH3

molecular empirical

MF = (EF)x where X = Molecular mass Empirical mass

Page 43: Math with Chemical Formulas Honors Chemistry Unit 5

Example The empirical formula of a compound was

found to be P2O5. Experimentation shows that the molar mass of this compound is 283.89 g/mol. What is the compound’s molecular formula?

Back to Objectives

Page 44: Math with Chemical Formulas Honors Chemistry Unit 5

Moles in Solution Molarity is the term used for moles dissolved

in solution Symbol for Molarity = M Definition – moles of solute per liter of

solution Formula

M = moles solute (mol)liter solution (L)

Page 45: Math with Chemical Formulas Honors Chemistry Unit 5

Example What is the molarity of a 0.5L solution

containing 2 moles of NaCl?

M = 2moles NaCl = 4 M NaCl0.5 L

Page 46: Math with Chemical Formulas Honors Chemistry Unit 5

Example 2 What is the molarity of a 250 mL solution

containing 12.7 g of lithium bromide?

M = moles = 12.7g LiBr 1mol LiBr 1000mL

L 86.8g LiBr 250mL 1L

= 0.59 M LiBr

Page 47: Math with Chemical Formulas Honors Chemistry Unit 5

Example 3 How would you make 500mL of a 0.32M

solution of LiBr and water? Given: M=0.32M, L=0.500 M= #moles/liters .32M= #moles .500L #moles= 0.16 molesLiBr

0.16 molesLiBr X 86.841g = 13.894gLiBr in .500L of Water 1 mole

Page 48: Math with Chemical Formulas Honors Chemistry Unit 5

You try: Calculate the M of a 700. mL solution of

23.2g calcium chloride

How would you make a 0.2 L solution of 0.50 M CaCl2 solution?

Back to Objectives