**Stoichiometry and chemical reaction**

The study of the quantitative relationships implied by chemical reactions is called stoichiometry. A chemical equation is both a quantitative and a qualitative statement of a chemical change. A balanced chemical equation, expressed a number of laws and facts. These laws are those of; (a) conservation of mass and (b) constant ( definite) composition.

**Note:** stoichiometry is a term used by chemists and chemical engineers.

A stoichiometric diagram of the combustion reaction of methane

Solution: www.en.m.wikipedia.org

**Mole ratios and mass **

The numerical coefficients of a balanced equation represent the numbers of moles of reactants and products. From these coefficients, we get the mole ratio of the reactants and products in a reaction. e.g

H_{2}SO_{4(aq)} > 2H^{+}_{(aq) }+ SO^{2-}_{4(aq)}

Numbers

Of moles. 1. 2 1

Mole ratio. 1. : 2 : 1

**Note** that one mole of any element contains the same number of atoms. This number, which has a value of 6.02 × 10^{23} is called the Avogadro number or constant. Chemists often express the concentration of a solution in moles per cubic decimeter (mol dm^{-3}). This concentration is called molar concentration (m). Number of moles = mass/molar mass

**Calculations**

**1.Calculation based on chemical equations**

Considering the equation

2NO_{(g)} + O_{2(g)} > 2NO_{2(g)}

The quantitative information provided is that, two moles of nitrogen (ii) oxide reacts with one mole of oxygen to produce two moles of nitrogen (IV) oxide. Using molar masses, 60g of nitrogen (ii) oxide, since the three substances are gases and at standard temperature and pressure, one mole of molecules of any gas occupies 22.4dm^{3}. Therefore, 44.8dm^{3} of nitrogen (ii) oxide reacts with 22.4dm^{3} of oxygen to yield 44.8dm^{3} of nitrogen (IV) oxide.

**2. Calculations involving gas volume**

4NH_{3(g)} + 5O_{2(g)} > 4NO_{(g)} + 6H_{2}O_{(g)} if 250cm^{3} of NH_{3} are burned completely, what volume of (a) oxygen is used up? (b) NO is produced?

Solution: equation of reaction

4NH_{3(g)} + 5O_{2(g)} > 4NO_{(g)} + 6H_{2}O_{(g)}

Volume. 4 : 5 : 4 : 6

Ratio. 1 5/4 1 6/4

250cm^{3} of NH_{3} will used up 5/4 × 250

= 312.5cm^{3} of oxygen

250cm^{3} of NH_{3} will produce 1×250 = 250cm^{3} of NO.

**3. Calculations involving liquid volume**

What volume of 1.0M HCL will be required to react completely with 3.25g of zinc.

Solution: equation of reaction

Zn + 2HCL > ZnCL_{2} + H_{2}

65g. 73g

From the equation

65g of Zn react with 73g of HCL :. 3.25g of Zn will react with 73 × 3.25g/65 of HCL

= 3.65g of HCL

Converting this mass of HCL to a volume of 1.0M solution 3.65g is contained in 1000cm^{3} of 1.0M solution. :. 3.65g of HCL will be contained in 100 × 3.65cm^{3}/3.65

= 100cm^{3} of 1.0m HCL

:. The volume of 1.0 HCL requires is 100cm^{3}

**4. Calculations involving masses**

Calculate the mass of carbon (IV) oxide produced on burning 105g of ethyne (C=12, O=16, H=1)

Solution: equation

2C_{2}H_{2(g)} + 5O_{2(g)} > 4CO_{2(g)} + 2H_{2}O_{(g)}

Molar

Mass: 26g 44g

Mole

Ratio: 2 : 4

Number of moles of C_{2}H_{2} reacts = 105/26

=4 moles

From the equation, 2moles of C_{2}H_{2} produces 4 moles of CO_{2 }:. 4moles of C_{2}H_{2} will produce 4×4 mole of CO_{2}

2 = 8moles

Reacting mass = No of moles X molar mass = 8×44=352g of CO_{2}

Thus, 352g of CO_{2} is produced by burning 104g of CH.

Source: www.slideplayer.com

**Volumetric Analysis**

Volumetric Analysis usually includes titrations of acid against base or trioxocarbonate (IV), oxidizing agent against reducing agent or one substance against another giving a precipitate.

**Indicators for acid-base titrations**

Acids and base | Indicator |

Strong acid and strong base | Any indicator |

Strong acid and weak base or trioxocarbonate IV | Methyl orange |

Weak acid and strong base | Phenolphthalein |

Weak acid and weak base | No suitable indicator |

**Definition of terms used in volumetric Analysis**

**1.Mass concentration:** the mass concentration of a solution is the amount of solute present in a given volume of the solution. It is expressed in g/dm^{3} or gdm^{-3} thus, **mass concentration= molar concentration × molar mass**

**2. Molar concentration: **it is defined as the concentration of a solution in moles per dm^{3}.

Number of mole of a substance

= Number of particles

6.02 × 10^{23}

Number of moles of a substance

= Mass of substance mol^{-1}/ Molar mass

**Note:** in volume per cm^{3} of solution, we have. Number of moles of solute

= Volume × molar concentration/ 1000

I.e = V/1000 × M mole

**3. Standard solution:** a standard solution is a solution of a known concentration.

**4. Molar solution:** a molar solution of a compound is one which contains one mole or the molar mass of the compound in one dm^{3} of the solution.

*Acid-base titration

Source: www.commons.wikimedia.org

**Past questions**

1. How many miles of (H+) are there in 1dm^{3} of 0.5 M solution of H_{2}SO_{4}? (Jamb 1992)

A. 2.0moles

B. 1.0mole

C. 0.5mole

D. 0.25mole

**Answer:** B

**Solution:** H_{2}SO_{4} > 2H^{+} + SO^{2-}_{4}

1mole of H_{2}SO_{4} contains 2 mole of H^{+} :. (0.5×1000/1000) mole of H_{2}SO_{4} will contain (2/1 × 0.5) of H^{+} = 1.0 mole

2. A balance chemical equation obeys the law of? (Jamb 1993)

A. Conversation of Mass

B. Definite proportions

C. Multiple proportions

D. Conservation of energy

**Answer:** A

3. What volume of carbon(ii)oxide is produced by reacting excess carbon with 10dm^{3} of oxygen? (Jamb 2013)

A. 15dm^{3}

B. 10dm^{3}

C. 5dm^{3}

D. 20dm^{3}

**Answer:** C

**Solution:** 2C_{(s)} + O2_{(g)} > 2CO_{(g)}

1vol O_{2} produced 2 vol CO

10dm^{3} will produce 2/1 × 10dm^{3}=5dm^{3}

4.what is the percentage of sulphur in sulphur (IV) oxide? (Jamb 2014)

A. 66%

B. 25%

C. 40%

D. 50% [S=32, O=16]

**Answer:** B

**Solution:** SO_{2} = 32 + (2×16)

=32 + 32 =64; 32/64 × 100% = 50%

5. Define molar solution?

**Answer:** a molar solution of a compound is one which contains one mole or the molar mass of the compound in one dm^{3} of the solution.

6. Mention the two laws that are required in a balanced equation?

**Answer: **conservation of mass and constant composition

7. Name the two careers/jobs that make use of stoichiometry?

**Answer:** chemists and chemical engineers

8. Defined stoichiometry?

**Answer:** it is defined as the study of quantitative relationships implied by chemical reactions.

## No Comments