Chapter 12

Stoichiometry

Goal Calculate quantities of reactants and

products needed in chemical rxns using balanced chemical equations.

Using an Equation as a Recipe

Ingredients: Frame, Seat, Wheels, Handlebar, Pedals

Balanced equation:

1F + 1S + 2W + 1H + 2P 1 FSW2HP2

How many pedals are needed to make 128 bicycles?

= 256 pedals128 FSW2HP2

1 FSW2HP2

2P

1F + 1S + 2W + 1H + 2P 1 FSW2HP2

Examining a Balanced Chemical Equation

2H2S(g) + 3O2(g) 2SO2(g) + 2H2O(g) 2 moles of H2S reacts with 3 moles of O2 to

produce 2 moles of SO2 and 2 moles of H2O.

4 moles of H2S reacts with ___moles of O2

____ moles of O2 are needed to produce 7 moles of H2O

How to solve these is on the next slide.

610.5

= 6 mol O2

4 mol H2S

2 mol H2S

3 mole O2

Mole-Mole Ratio Problems

= 10.5 mol O2

7mol H2O

2 mol H2O

3 mole O2

The mol-mol ratio comes from the coefficients in the balanced equation. In the balance equation, O2 has a coefficient of 3, H2O has a coefficient of 2, and H2S has a coefficient of 2.

Mass-Mass Calculation

Write the balanced chemical equationLabel above the equation what you are

given and what you are trying to find.Convert grams of your given to moles using

molar massMultiply by the mole-mole ratio from the

balanced chemical equationConvert moles of unknown to grams

Bringing It All Together

How many grams of ammonia ( ) should be produced when 6.00g nitrogen react with excess hydrogen?

Solve: Write the balanced equation with the information labeled above it.

6.00 g excess ? g

N2 + 3H2 2 NH3

NH3

Bringing It All Together

Convert given to moles:

Multiply by the mole ratio:

= 0.428 mol NH3

= 0.214 mol N2

6.00g N2

28 g N2

1 mole N2

0.214 mol N2

1 mol N2

2 mol NH3

Wrapping It Up

Convert moles of unknown to grams:

= 7.28g NH3

Summary:When 6.00g of nitrogen(N2) reacts with an excess of Hydrogen (H2), 7.28g of ammonia (NH3)should be produced.

0.428 mol NH3

1 mol NH3

17g NH3

Now You Try!

What mass of aluminum is required to

completely react with 7.80g oxygen in a

synthesis reaction?

Write the balanced equation

4Al + 3O2 2Al2O3

Label your given and unknown information

4Al + 3O2 2Al2O3

7.80 g

Since you were not given info aboutthis compound,don’t worry about itand don’t use it.

NEXT?...

Convert given to moles:

Multiply by the mole ratio:

7.80g O2

32g O2

1 mol O2 = 0.244mol O2

0.244mol O2

3 mol O2

4 mol Al= 0.325 mol Al

AND FINALLY!!

Convert moles of unknown to grams:

0.325 mol Al

1 mol Al

27g Al= 8.78g Al

8.78g of aluminum (Al) should be required to react completely with7.80g of oxygen (O2) in the synthesis of aluminum oxide (Al2O3).

Mass-Volume Calculation Write the balanced chemical equation Label above the equation what you are given and

what you are trying to find. Convert grams of your given to moles using molar

mass Multiply by the mole-mole ratio from the balanced

chemical equation Convert moles of unknown to liters (a.k.a dm3)

using molar volume (22.4L/ 1mol)

Bringing It All Together

How many liters of oxygen are necessary for the combustion of 134g of magnesium, assuming the reactions occurs at STP?

Solve: Write the balanced equation with the information labeled above it.

134 g ? L

2Mg + O2 2 MgO

Bringing It All Together

Convert given to moles:

Multiply by the mole ratio:

= 5.514 mol Mg

= 2.757 mol O2

134g Mg

24.3 g Mg

1 mole Mg

5.514 mol Mg

2 mol Mg

1 mol O2

Wrapping It Up

Convert moles of unknown to liters:

= 61.8 L O2

61.8 L of O2 are required to completely combust 134g Mg in the synthesis of MgO.

2.757 molO2

1 mol O2

22.4 LO2

Volume-Mass Calculation

Write the balanced chemical equation Label above the equation what you are

given and what you are trying to find. Convert liters of your given to moles using

molar volume Multiply by the mole-mole ratio from the

balanced chemical equation Convert moles of unknown to grams using

molar mass

Bringing It All Together

Find the mass of sulfur (S8) required to react with oxygen to produce 2.47L of sulfur dioxide gas at STP.

Solve: Write the balanced equation with the information labeled above it.

? g 2.47L

S8 + 8 O2 8 SO2

Bringing It All Together

Convert given to moles:

Multiply by the mole ratio:

= 0.1103 mol SO2

= 0.01378 mol S8

2.47L SO2

22.4 L SO2

1 mole SO2

0.1103 mol SO2

8 mol SO2

1 mol S8

Wrapping It Up

Convert moles of unknown to grams:

= 3.53 g S8

3.53g of sulfur (S8) are required to produce2.47 L of sulfur dioxide (SO2).

0.01378 mol S8

1 mol S8

256g S8

Volume-Volume Calculations Write the balanced chemical equation Label above the equation what you are

given and what you are trying to find. Convert liters of your given to moles using

molar volume (22.4L/1mol) Multiply by the mole-mole ratio from the

balanced chemical equation Convert moles of unknown to liters using

molar volume (22.4L/1 mol)

Bringing It All Together

What volume of H2S gas is needed to produce 14.2 L of water at STP?

Solve: Write the balanced equation with the information labeled above it.

?L 14.2L

2H2S + 3O2 2 SO2 + 2 H2O

Bringing It All Together

Convert given to moles:

Multiply by the mole ratio:

= 0.6339 mol H2O

= 0.6339 mol H2S

14.2 L H2O

22.4 L H2O

1 mole H2O

0.6339 mol H2O

2mol H2O

2 mol H2S

Wrapping It Up

Convert moles of unknown to liters:

= 14.2 L H2S

14.2 L of H2S are required to produce 14.2 L H2O.

0.6993 mol H2S

1 mol H2S

22.4 LH2S