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# Chemistry STOICHIOMETRY

Stoichiometry deals with the quantitative relationships between reactants and products in chemical reactions. It involves the use of balanced chemical equations to make predictions about the amounts of substances consumed and produced in a chemical reaction. The term "stoichiometry" is derived from the Greek words "stoicheion," meaning element, and "metron," meaning measure. In simpler terms it allows chemists to use a balanced chemical equation as a “recipe” for a reaction, which gives the ratios of reactants needed and products formed.

HELP!  EVEN THE WORD STOICHIOMETRY SCARES ME...

## What is a Mole

A mole is just a group of things.  The “things” can be anything, atoms, molecules, electrons, ions, etc.  Just like a dozen means there are 12 things in a group, a mole means there are 6.022 x 10^23 things in a group.  This number is known as Avogadro's number.  The molar mass of a substance is the mass of one mole of that substance, expressed in grams per mole. Remember, all masses on the periodic table are the mass of a group of 6.022 x 10^23 elements.

## What is a Mole Ratio

A mole ratio is a ratio between the number of moles of any two chemical entitites or “things”.  For example, one molecule of water contains 2 hydrogen atoms atom and 1 oxygen atom. This means that if there is one mole of water, there are 6.022 x 10^23 water molecules, 1.204 x 10^24 hydrogen atoms, and 6.022 x 10^23 oxygen atoms.  The ratio of H:O in a mole of water molecules is 2:1, and subscripts are used in its molecular formula to express this.  Mole ratios can also be expressed as coefficients in a balanced chemical equation to show the number of things participating in the reaction.   In this case the ratio is not between elements in a compound, but rather between separate reactants and products.  Mole ratios are crucial in stoichiometry, because they allow us to predict the amounts of reactants consumed and products formed in a chemical reaction.

## The Law of Conservation of Matter

The Law of Conservation of matter states that matter cannot be created or destroyed in a chemical reaction. In other words, the total mass of the reactants in a chemical reaction is equal to the total mass of the products. This principle emphasizes the idea that atoms are neither created nor destroyed during a chemical reaction; they are merely rearranged to form new compounds.  In the context of balancing chemical equations, the law of conservation of matter ensures that the number of atoms of each element on the reactant side is equal to the number of atoms of the same element on the product side

## Gram to Mole Conversions

Converting between grams and moles involves using the molar mass of a substance. The molar mass is the mass of one mole of a substance.  It is found on the periodic table and is expressed in grams per mole (g/mol).  For elements, it is the atomic mass in grams/mol.  For compounds, it is the sum of the atomic masses of all the elements in one mole of the compound.  To determine the mass of some number of (or fraction of) moles, set up a conversion factor with the molar mass so that units cancel out appropriately. This conversion factor will have moles in the denominator and grams in the numerator.  This same conversion factor can be used to convert from grams to moles of the same substance.  This time though, the conversion factor will have grams in the denominator and moles in the numerator.

## Limiting Reagents Explained

In chemistry, the limiting reagent is the reactant that is entirely consumed in a reaction, thus limiting the amount of product that can be formed. To identify the limiting reagent, one must compare the stoichiometric ratios of the reactants based on the coefficients in the balanced chemical equation. Whichever reactant is not present in sufficient amounts to satisfy the ratio is limiting. Please note that an amount that doesn’t satisfy the ratio may not necessarily be the smallest amount present.  Once the limiting reagent is determined, it allows for the calculation of the theoretical yield, which is the maximum amount of product that can be obtained under ideal conditions.

## Determining Percent Yield

The percent yield of a chemical reaction is a measure of how efficiently a reaction produces the desired product compared to the maximum possible yield predicted by stoichiometry. Once the limiting reagent is determined, it allows for the calculation of the theoretical yield, which is the maximum amount of product that can be obtained under ideal conditions.   The theoretical yield is always a calculated amount. Percent yield is calculated by dividing the actual measured yield by  the theoretical yield and then multiplying by 100.  The goal in experimental chemistry is often to optimize conditions to maximize the percent yield and improve the efficiency of reactions.

## The Law of Constant Composition

The Law of Constant Composition, also known as the Law of Definite Proportions, states that a given chemical compound always contains the same elements in the same proportion by mass, regardless of the source or method of preparation. This means that the ratio of the masses of the constituent elements in a compound is constant.  For example, consider water (H₂O). According to the law of constant composition, regardless of whether water is obtained from a natural source, synthesized in a laboratory, or extracted from different sources, it will always consist of hydrogen and oxygen in a fixed mass ratio.

## Percent Composition

Percent composition in chemistry refers to the percentage by mass of each element in a compound, compared to the compound's total mass. It is a way of expressing the relative amount of each element in a chemical compound.  To find the % composition of any element within a compound, divide the individual mass of the element in the compound by the molar mass of the entire compound, and then multiply by 100.

## Empirical Formula

Determining the empirical formula of a compound involves finding the simplest whole-number ratio of the different elements present in the compound. This ratio represents the smallest possible integer subscripts in the chemical formula.  Starting with the mass data of the compound, convert the grams to moles.  To determine the smallest whole number ratio of moles of each element, divide the number of moles of each element by the smallest number of moles obtained.  This step ensures that you have the simplest whole-number ratio.  It's important to note that the empirical formula may not necessarily be the same as the molecular formula.  This is true for molecular compounds with multiple units in their chemical formula. The molecular formula may be a multiple of the empirical formula.

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