lecture 6; Stiochiometeric calculations by Dr. Salma Amir

1. Lecture No. 6Course title:Fundamental Analytical Chemistry Course Instructor: Dr Salma Amir GFCW Peshawar

2. Stoichiometry and stoichiometric calculations • Stoichiometry is the study of the quantitative relationships in substances and their reactions or The stoichiometry of a reaction is the relationship among the number of moles of reactants and products as shown by a balance equation • Stoichiometric calculations. By combining a knowledge of balancing equations with the concept of the mole, it is possible to easily calculate the masses of all reactants required or products formed in any reaction. These calculations are called STOICHIOMETRIC CALCULATIONS. • It tells us how much of the matter is required to form a specific amount of another form of matter or if a particular amount of matter is present, how much of the product could possible be formed under the given conditions.

3. Stoichiometric Values in a Chemical Reaction • A chemical equation is avisual representation of a chemical reaction. In a typical chemical equation, an arrow separates the reactants on the left and the products on the right. • The coefficientsnext to the reactants and products are the stoichiometric values. They represent the number of moles of each compound that needs to react so that the reaction can go to completion. • On some occasions, it may be necessary to calculate the number of moles of a reagent or product under certain reaction conditions. To do this correctly, the reaction needs to be balanced. The law of conservation of matter states that the quantity of each element does not change in a chemical reaction. Therefore, a chemical equation is balanced when the number of each element in the equation is the same on both the left and right sides of the equation.

4. Stoichiometric calculations • We have established that a balanced chemical equation is balanced in terms of moles as well as atoms or molecules. • We have used balanced equations to set up ratios, now in terms of moles of materials, that we can use as conversion factors to answer stoichiometric questions, such as how many moles of substance A react with so many moles of reactant B. • We can extend this technique even further. Recall that we can relate a molar amount to a mass amount using molar mass. • We can use that ability to answer stoichiometry questions in terms of the masses of a particular substance, in addition to moles.

6. 1. Moles to moles conversion • From the moles of Substance A, we can calculate the moles of Substance B formed (if it’s a product) or used (if it’s a reactant) from a balance chemical equation. • Moles of A Stoichiometric ratioMoles of B

7. Example:

8. 2. Mass to moles conversion3. Moles to mass conversion • If we start with a known mass of one substance in a chemical reaction (instead of a known number of moles),we can calculate the corresponding moles of other substances in the reaction. or • If we start with a known moles of one substance in a chemical reaction , we can calculate the corresponding masses of other substances in the reaction.

9. Example: moles to mass conversion

10. Example: mass to moles conversion

11. 4. Mass to mass conversion If we start with a known mass of one substance in a chemical reaction (instead of a known number of moles), we can calculate the corresponding masses of other substances in the reaction. • The first step in this case is to convert the known mass into moles, using the substance’s molar mass as the conversion factor. • Then, use the balanced chemical equation to construct a conversion factor to convert that quantity to moles of another substance, which in turn can be converted to a corresponding mass.

12. Example: