Scientific Tutor

STOICHIOMETRY CONVERSION RATIOS (MOLE TO MOLE RATIOS):

Many students think stoichiometry is a scary and evil word, but it is just another type of conversion we see in chemistry. Stoichiometry is using the coefficients of a chemical equation to set up a conversion with them. Lets look at an example of what I am talking about with the chemical equation below. Ignore the states of the equation. They are not important in stoichiometry.

 

N_2(g) + 3 H_2(g) —-> 2 NH_3(g)

 

From this chemical equation we can state some of the stoichiometry ratios:

 

What is the ratio between the N₂ and the H₂?

1 N2

3 H2

or

3 H2

1 N2

 

What is the ratio between the N₂ and the NH₃?

1 N2

2 NH3

or

2 NH3

1 N2

 

What is the ratio between the H₂ and the NH₃?

3 H2

2 NH3

or

2 NH3

3 H2

 

VIDEO Stoichiometry Ratios Demonstrated Example 1: For the chemical equation below what is the ratio of oxygen to H₂?

H₂SO_4(aq) —> H_2(g) + S_(s) + 2 O_2(g)

 

How do we set up the ratio?

Answer: O₂ on top and H₂ on bottom

O2

H2

 

What numbers do we put in front of each?

Answer: The coefficients of each in the chemical equation

COMPLETE ANSWER:

2 O2

1 H2

 

VIDEO Stoichiometry Ratios Demonstrated Example 2: For the chemical equation below what is the ratio of H₂O to O₂?

2 C₅H₇OH_(l) + 13 O_2(g) ——> 10 CO_2(g) + 8 H₂O_(g)

 

How do we set up the ratio?

Answer: H₂O on top and O₂ on bottom

H2O

O2

 

What numbers do we put in front of each?

Answer: The coefficients of each in the chemical equation

COMPLETE ANSWER:

8 H2O

13 O2

 

PRACTICE PROBLEMS: Give the stoichiometry (coefficient) ratio for the questions below.

 

What is the ratio of Fe₂O₃ to FeP?

4 P + 2 Fe₂O₃ ——> 4 FeP + 3 O₂

2 Fe2O3

4 FeP

 

What is the ratio of Al₂S₃ to PbS₂?

3 PbS₂ + 4 AlPO₄ ——> 2 Al₂S₃ + Pb₃(PO₄)₄

2 Al2S3

3 PbS2

 

What is the ratio of MgO to KF?

MgO + 2 KF ——> MgF₂ + K₂O

1 MgO

2 KF

 

What is the ratio of LiNO₃ to Ba?

2 Li + Ba(NO₃)₂ ——> Ba + 2 LiNO₃

2 LiNO3

1 Ba