# Percentage yield and atom economy

##### Intros
###### Lessons
1. Why do chemists do chemical reactions?
2. Practical problems in chemical reactions.
3. Atom economy and percentage yield.
4. Worked example: Calculate atom economy.
5. Worked example: Calculate percentage yield.
##### Examples
###### Lessons
1. Find the atom economy and percentage yield of chemical reactions.
Water can be produced by reaction of hydrogen and oxygen gas according to this equation:

2 H$_2 +$O$_2 \,$$\,$2 H$_2$O

1. What is the atom economy of this reaction?
2. If the reaction was performed and 150 g was the theoretical yield of water and the actual yield was only 92 g, what is the percentage yield?
2. Find the atom economy and percentage yield of chemical reactions.
The reaction to make iron metal by reduction uses carbon. CO$_2$ is an unwanted side-product. The reaction is as follows:

2 Fe$_2$O$_3 +$3 C$\,$$\,$3 CO$_2 +$4 Fe

1. What is the atom economy of this reaction?
2. 280 g of Fe$_2$O$_3$ was used in this reaction and 104 g of Fe metal was collected. Calculate the theoretical yield and then the percentage yield of this reaction.
3. This reaction was done again and a percentage yield of 90% was achieved this time. If 250 g of Fe was collected this time, how much Fe$_2$O$_3$ was used up?
3. Compare the viability of two reactions by finding the atom economy and percentage yield.
Magnesium reacts with hydrochloric acid as shown in the following equation:

Mg$+$2 HCl$\,$$\,$MgCl$_2 +$H$_2$

1. If this reaction was performed to make hydrogen gas, what would be the atom economy?
2. 55 g of Mg metal was used in this reaction with HCl in excess. Only 4.21 g of H$_2$ gas was produced from this reaction. What is the percentage yield of this reaction?
3. Hydrogen gas could also be made by electrolysis of water, in the following reaction:
2 H$_2$O→2 H$_2 +$O$_2$

Find the atom economy of this reaction. Is it a better or worse way to make hydrogen than reacting magnesium with hydrochloric acid?