STOICHIOMETRY - Engineers' Class...2013/08/11 · Stoichiometry • The study in chemistry dealing with calculations based on balanced chemical equations. • The branch of chemistry

STOICHIOMETRY

Engr. Yvonne Ligaya F. Musico 1

Stoichiometry

• The study in chemistry dealing with

calculations based on balanced chemical

equations.

• The branch of chemistry dealing with mass

relationships


Interrelationship between Mole,Molar

Mass and Number of Particles



Flowchart Atoms or

Molecules

Moles

Mass

(grams)

Divide by 6.023 X 1023

Multiply by 6.023X 1023 Multiply by

atomic/molar mass

from periodic table

Divide by

atomic/molar mass

from periodic table

Calculations

molar mass Avogadro’s number Grams Moles particles

Everything must go through

Moles!!!


Calculations in Stoichiometry

• Mole to mole

• Mole to mass

• Mass to mole

• Mass to mass

• Mass to volume


Chocolate Chip Cookies!! 1 cup butter

1/2 cup white sugar

1 cup packed brown sugar

1 teaspoon vanilla extract

2 eggs

2 1/2 cups all-purpose flour

1 teaspoon baking soda

1 teaspoon salt

2 cups semisweet chocolate chips

Makes 3 dozen


How many eggs are needed to make 3 dozen cookies?

How much butter is needed for the amount of chocolate chips used?

How many eggs would we need to make 9 dozen cookies?

How much brown sugar would I need if I had 1 ½ cups white sugar?

Cookies and Chemistry…Huh!?!?


• Just like chocolate chip cookies

have recipes, chemists have

recipes as well

• Instead of calling them recipes,

we call them reaction equations

• Furthermore, instead of using

cups and teaspoons, we use

moles

• Lastly, instead of eggs, butter,

sugar, etc. we use chemical

compounds as ingredients

Chemistry Recipes

• Looking at a reaction tells us how much of something you need to react with something else to get a product (like the cookie recipe)

• Be sure you have a balanced reaction before you start!

Example: 2 Na + Cl2 2 NaCl

• This reaction tells us that by mixing 2 moles of sodium with 1 mole of chlorine we will get 2 moles of sodium chlorine

• What if we wanted 4 moles of NaCl? 10 moles? 50 moles?


Practice

• For the balanced reaction for hydrogen gas reacting with

oxygen gas.

2 H2 + O2 2 H2O

– How many moles of reactants are needed?

– What if we wanted 4 moles of water?

– What if we had 3 moles of oxygen, how much hydrogen would

we need to react and how much water would we get?

– What if we had 50 moles of hydrogen, how much oxygen

would we need and how much water produced?


Mole – Mole Calculations


Mole Ratios

• These mole ratios can be used to calculate

the moles of one chemical from the given

amount of a different chemical


Example 1

• How many moles of chlorine is needed to react with 5 moles

of sodium (without any sodium left over)?

2 Na + Cl2 2 NaCl


5 moles Na 1 mol Cl2

2 mol Na = 2.5 moles Cl2

Example 2

• 20 moles of CO2 is the average amount exhaled by day. How many moles of LiOH will there have to be to complete this reaction?

CO2(g) + 2LiOH(s) Li2CO3(s) + H2O(l)


20 moles CO2 2 mol LiOH

1 mol CO2

= 40 moles LiOH

Practice Exercise

• How many moles of sodium chloride will be

produced if you react 2.6 moles of chlorine gas

with an excess (more than you need) of sodium

metal?


Mole-Mass Calculation


Mole-Mass Conversions

• Most of the time in chemistry, the amounts are

given in grams instead of moles

• We still go through moles and use the mole

ratio, but now we also use molar mass to get to

grams


Mole-Mass Conversions

• How many grams of chlorine are required to

react completely with 5.00 moles of sodium to

produce sodium chloride? 2 Na + Cl2 2 NaCl


5.00 moles Na 1 mol Cl2 70.90g Cl2

2 mol Na 1 mol Cl2 = 177g Cl2

Practice Exercise

• Calculate the mass in grams of Iodine required

to react completely with 0.50 moles of

aluminum.


Mass-Mole Calculation


Mass-Mole

• We can also start with mass and convert to

moles of product or another reactant

• We use molar mass and the mole ratio to get to

moles of the compound of interest


Example 1

Calculate the number of moles of ethane (C2H6) needed to

produce 10.0 g of water

– 2 C2H6 + 7 O2 4 CO2 + 6 H20


10.0 g H2O 1 mol H2O 2 mol C2H6

18.0 g H2O 6 mol H20

= 0.185

mol C2H6

Example 2

If 8.00 g of SO2 reacts,

a. How many moles of CS2 are formed?

b. How many moles of CO2 are formed?

3C (s) + 2SO2 (g) CS2 (l) + 2CO2 (g)


Solution (a)

3C (s) + 2SO2 (g) CS2 (l) + 2CO2 (g)


8.00 g SO2 1 mol SO2 1 mol CS2

64.1 g SO2 2 mol SO2

= 0.06 mol

CS2

Solution (b)

3C (s) + 2SO2 (g) CS2 (l) + 2CO2 (g)


8.00 g SO2 1 mol SO2 2 mol CO2

64.1 g SO2 2 mol SO2

= 0.125

mol CO2

Practice

• Calculate how many moles of oxygen are

required to make 10.0 g of aluminum oxide


Mass-Mass Calculation


Mass-Mass Conversions

• Most often we are given a starting mass and

want to find out the mass of a product we will

get (called theoretical yield) or how much of

another reactant we need to completely react

with it (no leftover ingredients!)

• Now we must go from grams to moles, mole

ratio, and back to grams of compound we are

interested in


Example 1

Calculate how many grams of ammonia are produced

when you react 2.00g of nitrogen with excess

hydrogen.

N2 + 3 H2 2 NH3


2.00g N2 1 mol N2 2 mol NH3 17.06g NH3

28.02g N2 1 mol N2 1 mol NH3

= 2.4 g NH3

Example 2

How many grams of Tin (II) fluoride are

produced when 30.0 grams of HF react?

Sn (s) + 2HF (g) SnF2 (s) + H2 (g)


30.0g HF 1 mol HF 1 mol SnF2 157g SnF2

20.10g HF 2mol HF 1 mol SnF2

= 117.5 g SnF2

Practice

• How many grams of calcium nitride are

produced when 2.00 g of calcium reacts with

an excess of nitrogen?


Mass Volume Calculation


Avogadro’s Hypothesis

It states that one mole of a gas occupies 22.4 liters volume at standard condition

At Standard Condition:

P = 760 mm Hg

= 1 atm

T = 273.15 K


Example 1

How many grams of zinc must react with sulfuric acid

to produce 1000 ml of hydrogen at STP?

Zn (s) + H2SO4 (aq) ZnSO4 (s) + H2 (g)


1.0 L H2 1 mol H2 1 mol Zn 65.4 g Zn

22.4 L H2 1 mol H2 1 mol Zn

= 3.0 g Zn

Example 2

How many liters of oxygen is needed to react with 10.0

g of carbon to form carbon monoxide at STP?

2C (s) + O2 (aq) 2CO (g)


10.0 g C 1 mol C 1 mol O2 22.4 L O2

12.0 g C 2 mol C 1 mol O2

= 9.33 L O2

Practice

1. What is the volume of oxygen that will react

with 30.0 grams of magnesium to form

magnesium oxide?

2. How many grams of calcium is needed to

react with hydrochloric acid to form calcium

chloride and 2.6 liters of hydrogen?


Limiting and Excess Reactant


Limiting Reactant: The Sandwich Equation

2 pieces of bread + 3 slices meat + 1 slice cheese sandwich

• Now assume you have the following quantities of ingredients:

a. 20 slices of bread

b. 24 slices of meat

c. 12 slices of cheese

• How many sandwiches can you make?


2 pieces of bread + 3 slices meat + 1 slice cheese sandwich

Number of Sandwich can be made:

a. For 20 slices of bread = can make 10 sandwiches

b. For 24 slices of meat = can make 8 sandwiches

c. For 12 slices of cheese = can make 12 sandwiches

• How many sandwiches can you make?

There 8 sandwiches that can be made out of the available

ingredients because the number of meat limits the number of

product or sandwich.



• Most of the time in chemistry we have more of one

reactant than we need to completely use up other

reactant.

• That reactant is said to be in excess (there is too

much).

• The other reactant limits how much product we get.

Once it runs out, the reaction s. This is called

the limiting reactant.



• Limiting Reactant – the reactant that controls

the amount of product formed in chemical

reaction

• Excess Reactant – the substance that is not

used up completely in a reaction


Limiting Reactant

• To find the correct answer, we have to try all of the reactants. We have to calculate how much of a product we can get from each of the reactants to determine which reactant is the limiting one.

• The lower amount of a product is the correct answer.


Limiting Reactant

• The reactant that makes the least amount of product is the limiting reactant. Once you determine the limiting reactant, you should ALWAYS start with it!

• Be sure to pick a product! You can’t compare to see which is greater and which is lower unless the product is the same!


Limiting Reactant: Example

• 10.0g of aluminum reacts with 35.0 grams of chlorine gas to

produce aluminum chloride. Which reactant is limiting, which is in

excess, and how much product is produced?

2 Al + 3 Cl2 2 AlCl3

• Start with Al:

• Now Cl2:


10.0 g Al 1 mol Al 2 mol AlCl3 133.5 g AlCl3

27.0 g Al 2 mol Al 1 mol AlCl3 = 49.4g AlCl3

35.0g Cl2 1 mol Cl2 2 mol AlCl3 133.5 g AlCl3

71.0 g Cl2 3 mol Cl2 1 mol AlCl3 = 43.9g AlCl3

LR Example Continued

• We get 49.4g of aluminum chloride from the given

amount of aluminum, but only 43.9g of aluminum

chloride from the given amount of chlorine. Therefore,

chlorine is the limiting reactant. Once the 35.0g of

chlorine is used up, the reaction comes to a complete

.


Limiting Reactant Practice

• 15.0 g of potassium reacts with 15.0 g of

iodine. Calculate which reactant is limiting

and how much product is made.


Finding the Amount of Excess

• By calculating the amount of the excess

reactant needed to completely react with the

limiting reactant, we can subtract that amount

from the given amount to find the amount of

excess.

• Can we find the amount of excess potassium

in the previous problem?


Finding Excess Practice

• 15.0 g of potassium reacts with 15.0 g of iodine.

2 K + I2 2 KI

• We found that Iodine is the limiting reactant, and 19.6 g of potassium

iodide are produced.


15.0 g I2 1 mol I2 2 mol K 39.1 g K

254 g I2 1 mol I2 1 mol K = 4.62 g K

USED!

15.0 g K – 4.62 g K = 10.38 g K EXCESS

Given amount

of excess

reactant

Amount of

excess

reactant

actually used

Note that we started with the

limiting reactant! Once you

determine the LR, you should

only start with it!

Limiting Reactant: Recap

1. You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT.

2. Convert ALL of the reactants to the SAME product (pick any product you choose.)

3. The lowest answer is the correct answer. 4. The reactant that gave you the lowest answer is the

LIMITING REACTANT. 5. The other reactant(s) are in EXCESS. 6. To find the amount of excess, subtract the amount used from

the given amount. 7. If you have to find more than one product, be sure to start

with the limiting reactant. You don’t have to determine which is the LR over and over again!


Theoretical and Percent Yield


Theoretical yield – the predicted amount of product

Percentage Yield = ____Actual Yield___ x100

Theoretical Yield


Sample Problem

• Barium sulfide,BaS, which is used as a white pigment in paints, is prepared through the following reaction:

BaSO4 + 4C BaS + 4CO

Suppose you have 38 grams of BaSO4 and 5 grams of carbon. What is the theoretical yield? If the reaction actually produced 15.32 grams of BaS, what is the percentage yield?


Practice

Aluminum and chlorine react with each other to form aluminum chloride according to the following equation:

2Al + 3Cl2 2AlCl3

Suppose you had 20.0 grams Al and 30.0 grams Cl2. What would be the theoretical yield? If the actual yield is 31.8 grams AlCl3, what is the percentage yield?


Thanks for Listening


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STOICHIOMETRY - Engineers' Class...2013/08/11 · Stoichiometry • The study in chemistry dealing with calculations based on balanced chemical equations. • The branch of chemistry