Learning Objective Calculate concentrations of solutions in molarity, molality, mole fraction and percent by mass and volume.
Key Points Stoichiometry deals with the relative quantities of reactants and products in chemical reactions. It can be used to find the quantities of the products from given reactants in a balanced chemical reaction, as well as percent yield.
To calculate the quantity of a product, calculate the number of moles for each reactant. Moles of a product are equal to the moles of a limiting reactant in one-to-one reaction stoichiometry. Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet.
There are 12 party invitations and 20 stamps. Each party invitation needs 2 stamps to be sent. How many party invitations can be sent? In this example are all the reactants stamps and invitations used up? No, and this is normally the case with chemical reactions. There is often excess of one of the reactants. The limiting reagent, the one that runs out first, prevents the reaction from continuing and determines the maximum amount of product that can be formed. What is the limiting reagent in this example?
Stamps, because there was only enough to send out invitations, whereas there were enough invitations for 12 complete party invitations. Aside from just looking at the problem, the problem can be solved using stoichiometric factors. When there is no limiting reagent because the ratio of all the reactants caused them to run out at the same time, it is known as stoichiometric proportions.
Before applying stoichiometric factors to chemical equations, you need to understand molar mass. Molar mass is a useful chemical ratio between mass and moles. The atomic mass of each individual element as listed in the periodic table established this relationship for atoms or ions. For compounds or molecules, you have to take the sum of the atomic mass times the number of each atom in order to determine the molar mass.
Using molar mass and coefficient factors, it is possible to convert mass of reactants to mass of products or vice versa. Almost every quantitative relationship can be converted into a ratio that can be useful in data analysis.
This ratio can be useful in determining the volume of a solution, given the mass or useful in finding the mass given the volume. In the latter case, the inverse relationship would be used. Percents establish a relationship as well. A percent mass states how many grams of a mixture are of a certain element or molecule. This is useful in determining mass of a desired substance in a molecule.
If the total mass of the substance is 10 grams, what is the mass of carbon in the sample? How many moles of carbon are there? Given volume and molarity, it is possible to calculate mole or use moles and molarity to calculate volume. This is useful in chemical equations and dilutions. These ratios of molarity, density, and mass percent are useful in complex examples ahead.
An empirical formula can be determined through chemical stoichiometry by determining which elements are present in the molecule and in what ratio. The ratio of elements is determined by comparing the number of moles of each element present. It yields 0. What is the empirical formula of the organic molecule? This is a combustion reaction. The problem requires that you know that organic molecules consist of some combination of carbon, hydrogen, and oxygen elements.
With that in mind, write the chemical equation out, replacing unknown numbers with variables. Do not worry about coefficients here. Since all the moles of C and H in CO 2 and H 2 O, respectively have to have came from the 1 gram sample of unknown, start by calculating how many moles of each element were present in the unknown sample.
Calculate the final moles of oxygen by taking the sum of the moles of oxygen in CO 2 and H 2 O. A mole is defined as the amount of a substance. More specifically, there are 6. Therefore, if you had 1 mole of feathers and 1 mole of bowling balls, you would have 6.
Now suppose you were asked the question, "Which weighs more, moles of feathers or moles of bowling balls? Although there is an equal number of both feathers and bowling balls, an individual bowling ball weighs much more than an individual feather, and so an equal number of bowling balls would weigh more than an equal number of feathers. Now, let's return to the number 6. This number is known as Avogadro's number and you should definitely commit it to memory.
Remember they are only guidelines to help if you run into trouble. You can see by simply adding a 2 in front of NO, we violate the first guideline even though it leads us to a balanced equation. Note: Typically a stoichiometric coefficient of "1" is not explicitly included when writing the chemical equation. Balanced, but without fractional coefficients. Balanced, without fractional coefficients.
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