% Comp, Stoichiometry, Empirical Formula, and Limiting Reactants

Percent Composition:

To find the purity of a compound or the amount of all the different elements, by percentage, scientists calculate the percent composition by mass.

Percent Composition by mass of a compound represents the percent that each element in a compound contributes to the total mass of the compound.

A chemist often compares the percent composition of an unknown compound with the percent composition calculated from the formula of a known compound. If the percentages agree, it may confirm the identity of the unknown.

How to find the percent composition:

• Find molar mass

• Divide each element’s total mass by the molar mass and  multiply by 100 to get the percentage of that element

Sometimes scientists need to know the percentage of a single element that is found within the whole compound. The mass percentage of an element in a compound is always calculated by the total molar mass of the element in the compound divided by the molar mass of the compound times 100.

Empirical Formula:

The empirical formula of a compound is determined by the simplest whole-number ratio of every atom in the compound. It’s a formula showing the proportions of an element present in a compound.

For example, CH2 is the simples ratio of C2H4. The ratio of the subscripts 2:4 is simplified to 1:2. The empirical formula is sometimes but not always the same as the molecular formula. It is calculated using the percent composition of the compound.

It consists of the symbol for the elements combined in a compound with subscripts showing the smallest whole number mole raion of the different atoms in the compound, reduce using GCF

How to find Empirical Formula:

• Find grams for each element (number of grams is equal to percentage/100)

• Convert grams to moles

• Divide by the lowest number

• If the number is .5 than multiply by 2

• If the number is .33 or .67 multiply by 3

• If the number is .25 or .75 multiply by 4

Molecular Formula:

The actual formula for the molecule

How to find the molecular formula:

• Find the molar mass of the empirical formula

• Divide molecular (actual molar mass of the molecule)/empirical (the one you just found)

• Multiply the empirical formula by the number you just found

Stoichiometry:

Scientists perform chemical calculations or stoichiometry to measure the number of reactants and products in a chemical reaction.

How much product will be formed when specific amounts of the reactants combine?

How much of the reactants are required to create a specific amount of products?

It’s the calculation of reactants and products in a chemical reaction.

Use the ratio in the formula (mole ratio conversion)/use grams to moles conversion

Mole to Mole Stoichiometry:

In mole to mole stoichiometry, we start with moles and end with moles. The coefficients of each substance in a reaction are represented by moles.

For example, in N2 + 3H2 = 2NH3, there are 3 mol H2, 2 mol NH3 and1 mol N2.

These ratios can be used in calculations with dimensional analysis which is a problem solving method that uses the idea that any number, compound, or element can be multiplied by a ration from the chemical equation without changing its value; used to convert units of measurement.

Mass to Mass Stoichiometry:

In these calculations, reactants and products are given in mass.

You can take  N2 + 3H2 = 2NH3 one step further to calculate the grams of NH3 produced by using the grams in 1 mol NH3 (found in the periodic table).

To solve:

1. White a balanced equation for the chemical reaction

2. Start with the mass of the reactant given in the problem and convert the amount of reactant to number of moles

3. Use the mole ratio from the balanced equation to convert to the moles of product created

4. Convert the moles of product to grams of product

You don’t actually calculate each section of the problem individually, instead you calculate the entire problem as a whole.

Mass to Volume Stoichiometry:

Sometimes in gas reactants or products the measurements may be given in volume not mass.

To solve:

1. Write a balanced chemical equation for the reaction

2. Convert amount of reactant to number of moles

3. Use the mole ratio from the balanced equation to convert the amount (in moles) of product created

4. Convert the moles of product to liters of product using the STP conversion factor for gases

Volume to Volume Stoichiometry:

Equal volumes of all gases are the same at standard temperature and pressure (STP). You can get the volume ratios from the balanced equation.

Limiting/Excess Reactants:

When carrying out a reaction, you must use a number of reactants that you have available. Sometimes what you have does not equal the exact proportions in the equation. You may have less of a certain reactant. The reactant that is used up first in a chemical reaction is called a limiting reactant. In a chemical reaction, not having enough of a reactant limits the amount of product that can be made.

The excess reagent are reactants that are left over when the limiting reactant is gone. The limiting reagent is the reactant that is used up first in the equation.

For example suppose you have a recipe that makes 12 cupcakes but you need 36 cupcakes for a party. You will need to triple the ingredients for the recipe to get enough (or make the recipe three times). Suppose that you need two eggs for each set of 12 cupcakes and you only have five eggs. When you’ve used all the eggs assuming you don’t get more, your reaction is over no more cupcakes.

Use balanced equations and stoichiometry to identify the limiting reagent.

How:

1. Write the balanced equation

2. Using the amount of moles given for each reactant calculate how many moles of the substance will be produced

3. Identify the limiting reactant

Percent Yield:

Once you know the limiting reactant, you can calculate the yield of the products. The yield, or amount, of the product produced if all the limiting reactant was used is called a theoretical yield. In reactions occurring in an actual lab where there are experimental errors, it is normal for some of the reactants to remain unused. So we can calculate the actual yield.

Why is the actual yield unequal to the theoretical yield? Sometimes the reactants didn’t completely mix, the temperature wasn’t high enough for some of the reactants stuck to the side of the beaker. Some reactions are reversible so reactants reform over the course of the reaction. The actual yield is less than the theoretical yield.

Find out the efficiency of the reaction on a scale of 1 to 100 scientists determine the percent yield.

To calculate percent yield:

1. Right the balanced equation

2. Calculate the theoretical yield of the products from the amount of react and given

Summary:

The theoretical yield is the amount of product that would be produced if the limiting reactant completely reacted.

The actual yield is the amount of product actually produced in the reaction.

The percentage is the ratio of actual yield to theoretical yield.

If you were given the amount of each reactant, calculate the limiting reactant. Use that amount to calculate the theoretical yield of the product. Use the actual yield given a theoretical hear that your calculator to determine the percent yield.