UNIT 5: Mathematics of Chemistry (Review Book Topic 3) How can we calculate gram formula mass? What is meant by the percent composition of a compound?

UNIT 5: Mathematics of Chemistry (Review Book

Topic 3)How can we calculate gram formula mass?

What is meant by the percent composition of a compound?

What is a mole?How can we solve mole problems?

How can we solve stoichiometry problems?How can we determine the limiting reagent and the

percentage yield of a chemical reactions?How can we calculate empirical and molecular

formulas?How can we determine molar mass and density?

The mass of 1 proton is 1 amuThe mass of 1 neutron is 1 amuThe mass of an electron is

negligible

AIM: How can we calculate gram formula mass?

The atomic mass of an element is found in each element box on the Periodic Table


The number of atoms of each element present in a compound is denoted by the subscripts on each element


Gram formula mass is the mass of the smallest unit of a compound from the sum of the atomic masses of all atoms present


GFM of K2CO3


Element atoms X Mass #

Multiplied result

Add them all up to get gfm

K

C

O

2

1

3

39.10

12.01

16

78.20

12.01

48

138.21 g/mol

Calculate the gram formula mass of the following: 1.N2H2

2.H2SO4

1.CuSO4 H2O

AIM: How can we calculate gram formula mass? – PRACTICE

30 g/mol

98 g/mol

178 g/mol

Percent Composition represents the composition as a percentage of each element compared with the total mass of the compound

AIM: What is meant by the percent composition of a

compound?

% composition =


compound?

mass of part

mass of wholex 100 TABLE ____

1. What is the percentage, by mass, of O in Fe2O3?

AIM: What is meant by the percent composition of a compound? –

Practice

Element

# atoms X Mass #

Multiplied

result


Fe

O

2

3

55.85

16.00

111.70

48.00

159.70 g/mol

48.00 X 100 = 159.70 g/mol

30%

1. What is the percentage, by mass, of O in CO2?


Practice

Element

# atoms X Mass #

Multiplied

result


C

O

1

2

12.01

16.00

12.01

32.00

44.01g/mol

32.00 X 100 = 44.01g/mol

73%

Hydrates: crystals that contain attached water molecules

To calculate the percentage of water in a hydrate, treat the water molecule as a single unit


compound?

What is the percentage, by mass, of water in sodium carbonate crystals, Na2CO3 10H2O?


Practice

Mole is a way to express a quantity 1 mole of any compound or elements = mass of that

compound or element 1 mole = 6.02 x 1023 particles (atoms or molecules) 1 mole = 22.4 L Formula is on Table T

AIM: What is a mole?

MOLE TRIANGLES

What is a mole?

How many moles are equivalent to 4.75 g of NaOH?

AIM: How can we solve mole problems? - PRACTICE

Element

# atoms X Mass #

Multiplied

result


Na

O

H

1

1

1

22.99

16.00

1.00

22.99

16.00

1.00

39.99 g/mol

g_ = _4.75g __ = 0.119 molgfm 39.99g/mol

How many grams are equivalent to 2.80 moles of NaOH?

AIM: How can we solve mole problems? - PRACTICE

Element

# atoms X Mass #

Multiplied

result


Na

O

H

1

1

1

22.99

16.00

1.00

22.99

16.00

1.00

39.99 g/mol

mol x gfm = 2.80 mol x 39.99 g/mol = 112 g

Like following a recipe Need a balanced chemical equation Use mole relationships The study of quantities of materials

consumed and produced in chemical reactions

How can we use mole relationships to solve

stoichiometry problems?

Directions: Use the pancake recipe below (Bisquick) in order to answer the following questions:

FAMILY SIZE RECIPE: 14 pancakes PREP TIME: 4 min COOK TIME: 3 min per batch 1 cup milk2 eggs2 cups Original Bisquick mix STIR milk and eggs until blended. Stir into Bisquick mix. POUR slightly less than ¼ cup onto hot greased griddle. COOK until edges are dry, Flip, cook until golden

How can we solve stoichiometry problems? –like following a recipe

1. How much of the following “starting materials” do we need in order to make 14 pancakes?

1. MILK =2. PANCAKE MIX = 3. EGGS =

2. How many cups of pancake mix would we need to use in order to make 28 pancakes?


3. How many cups of pancake mix would we need to use in order to make 7 pancakes? Use your knowledge of chemistry to fill in this incomplete chemical equation that describes the formation of 14 pancakes (products) from its starting materials (reactants)

1 cup of milk + __________________ + __________________ 14 pancakes

4. Are the units used for all 4 substances in the equation the same?


Example: Calculate the mass of oxygen that will completely react with 96.1g of propane? Calculate for liters of CO2….. Calculate how many water molecules are produced

1. Write the chemical equations 2.Calculate the molar masses and put in parenthesis above

the formulas – only really need to do this for the products and reactants you are interested in

3.Balance the equations – coefficients are the mole ratios 4.Re read problem and input the amount 5.Calculate the number of moles of something 6.Use the mole ratio to find moles of everything else Ex) 1

= 2.18 then multiply that by the coefficient in from of the product or reactant

7.Re read problem to determine which was asked for

How can we solve stoichiometry problems? – Template for solving


Shows the mole relationships in balanced equations1.__H2 + __ O2 __ H2O

If we had 5 moles of H2 how many moles of O2 are required?

2.__ Na + __ H2O __ NaOH + __ H2 If we have 23 g of Na how many moles of H2 are required?

3.__N2 + __ H2 __ NH3 If we have 2 moles of N2 how many grams of H2 are needed?

How can we solve stoichiometry problems? -

Practice



How can we solve stoichiometry problems? –

Template for solving

Use PV = nRT to solve for the volume of one mole of gas at STP

That is the molar volume of a gas at STP = 22.4L Use the ideal gas law to convert quantities that are

not at STP

How can we solve stoichiometry problems? – Gas

Stoichiometry

How many liters of carbon dioxide gas will be produced form the complete combustion of 30.0L of ethane according to the following equation?

2C2H6 (g) + 7O2 (g) 4CO2 (g) + 6H2O (g)

How can we solve stoichiometry problems? – GAS

STOICH PRACTICE

A sample of nitrogen gas has a volume of 1.75L. How many moles of N2 are present?


Stoichiometry

Quicklime (CaO) is produced by the thermal decomposition of calcium carbonate (CaCO3). Calculate the volume of CO2 at STP produced from the decomposition of 152 g CaCO3 by the reaction:

____ CaCO3 (s) _____ CaO (s) + ____ CO2


Stoichiometry

Notice how hot dogs are sold in packages of 10 while the buns com in packages of 8?

The bun is the limiting reactant and limits the hot dog production to 8

The limiting reactant is the one consumed most entirely in the chemical reaction

How can we determine the limiting reagent and the

percentage yield?

If you are faced with two starting amounts of matter reacting, you have entered “The Land of Limiting Reactant”

Calculate the number of moles of everything you are given – set up the same table –two sets of moles now

Cover one set pretending the other exists – What if all these moles reacted? How many moles of the other reactants would you need to use

up all these moles?

Do the calculation of how many moles of the “other” amount you would need Do you have enough? If so, the reactant you began with IS the limiting reactant If not repeat this process with the “other” reactant amount

you were given

How can we determine the limiting reagent and the percentage yield?- STRATEGY

Suppose 25.0g of nitrogen reacts with 5.00g of hydrogen to form ammonia. What mass of ammonia can be produced? Which reactant is the limiting reactant? What is the mass of the reactant that is in excess?

How can we determine the limiting reagent and the percentage yield?-

PRACTICE

Nitrogen gas can be prepared by passing gaseous ammonia over solid copper (II) oxide at high temperature. The other products of the reaction are solid copper and water vapor. If a sample containing 18.1g of NH3 is reacted with 90.4g of CuO, which is the limiting reactant? How many grams of N2 will be formed?

How can we determine the limiting reagent and the percentage yield?-

PRACTICE

Theoretical yield: the amount of product formed when a limiting reactant is completely consumes. This assumes perfect conditions and gives maximum amount

Actual yield: that which is realistic Percent yield: the ratio of actual to theoretical yield

How can we determine the limiting reagent and the

percentage yield of a chemical reactions?

How can we determine the limiting reagent and the percentage yield of a chemical reactions? –

PRACTICE Methanol (CH3OH), also called methyl alcohol, is the simplest alcohol. It is used as a fuel in race cars and is a potential replacement for gasoline. Methanol can be manufactured by a combination of gaseous carbon monoxide and hydrogen. Supposed 68.6g of CO (g) is reacted with 8.60g of H2(g). Calculate the theoretical yield of methanol is 35.7 g CH3OH is actually produced, what is the percent yield of methanol?

1. convert % to grams2. divide each element by mass #3. Divide each quotient by smallest value

calculated Round your results to nearest whole number

4. Use whole #s to write empirical formula 5. Sequence of elements in the question is the

same as in the formula

AIM: How can we calculate empirical and molecular formulas? - EMPIRICAL

1. 50% Sulfur, 50% Oxygen; determine the empirical formula

AIM: How can we calculate empirical and molecular formulas? – EMPIRICAL –

PRACTICE

1. determine the gfm of the empirical formula 2. Divide the given molecular mass by the gfm

1. Might have to calculate empirical formula2. Given molecular mass

3. Multiply your whole number result by the empirical formula to determine molecular formula

AIM: How can we calculate empirical and molecular formulas? – MOLECUALR

1. Upon analysis the empirical formula is determined to be CH2O. The mass of the compound is 180amu. Determine the molecular formula

AIM: How can we calculate empirical and molecular

formulas? – MOLECULAR – PRACTICE

AIM: How can we determine molar mass and density? –

DENSITY - PRACTICE

1. Calculate the mass of a liquid with a density of 2.5 g/mL and a volume of 15 mL. 2. Calculate the volume of a liquid with a density of 5.45 g/mL and a mass of 65 g.

AIM: How can we determine molar mass and density? –

MOLAR MASS

Rearrange the ideal gas equation we can find the molar mass of an unknown gas

d = density of a gas g/L)R = universal gas constant 0.0821atm L/mol K)T = temp (K)P = pressure (atm)

AIM: How can we determine molar mass and density? – MOLAR MASS - PRACTICE

1. What is the density of CO2 at 77° C and a pressure of 2.00 atm? 2. Calculate the molar mass of a gas that has a density of 0.635 g/L at 37° C and 787 torr (760 torr = 1 atm) 3. The density of a gas was measured at 1.50 atm and 27° C and found to be 1.95 g/L. Calculate the molar mass of the gas.

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UNIT 5: Mathematics of Chemistry (Review Book Topic 3) How can we calculate gram formula mass? What is meant by the percent composition of a compound?