Chemists need to write out formulas and equations to solve chemistry problems. It is important that chemists have a common set of rules for writing formulas and equations so they can communicate with other scientists. In this unit, we begin to name and write formulas for compounds, and learn how to write and balance chemical equations. Equations enable us to describe chemistry topics in mathematical terms and predict the outcomes of reactions. For example, what volume of steam is created if we turn one kilogram of ice into pure steam, at 200 degrees Celsius and sea-level air pressure? We can calculate the precise answer when we write the reaction out in the form of an equation!
Completing this unit should take you approximately 4 hours.
In this section, we investigate chemical formulas. There are two types of chemical formulas, empirical formulas and molecular formulas. Empirical formulas tell you the lowest whole number ratio of elements in a formula. The molecular formula is the actual formula of the compound. For example, glucose has a molecular formula of C6H12O6. This tells us that one molecule of glucose has 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. The empirical formula of glucose is CH2O. This tells us that for every one carbon atom, there are two hydrogen atoms and one oxygen atom.
Read this text, which reviews how to write chemical formulas, how to determine information about the compound such as molar mass, mole ratios, and percent composition.
Watch this video, which explains the difference between empirical and molecular formulas with multiple examples.
We can determine the empirical formula of an unknown compound from experimental data. Using instrumentation in the lab, we can determine the elemental mass composition of a compound. In other words, we can determine the percent of each element present in the compound. From that information, we can backtrack to determine the empirical formula of an unknown compound.
Watch these videos, which go through these types of problems step by step.
Watch this video for a worked example of how to calculate a mass percent.
If we know the molecular mass or molar mass of the unknown compound, we can determine the molecular formula of the unknown compound.
Watch this video, which shows an example of using percent composition data and a known molar mass to determine the molecular formula of an unknown compound.
Nomenclature is a fancy word for the process of naming compounds. In chemistry, we have a set of specific rules used to name compounds so scientists can communicate effectively. We will use this nomenclature for the remainder of the course, so you must be comfortable naming a compound correctly from its formula, and become comfortable determining the formula of a compound from its name. Here, primarily inorganic substance nomenclature is covered.
Read this text. The first section covers rules for naming inorganic molecular compounds. The second and third sections cover naming rules for ionic compounds. Be sure to focus on the systematic name for compounds that have a systematic name and a common name.
When we write a chemical equation, we are showing how elements or compounds react to form new compounds. When we write a chemical equation, we must make sure the number of each element on the reactant side equals the number of each element on the product side. This is called balancing a chemical equation, and it ensures that the law of conservation of mass is not violated.
When chemical reactions occur, the reactants react in whole-number mole ratios with each other. Products are formed in whole-number mole ratios. We read the equation for the hydrolysis of water (2H2O → 2H2 + O2) as "two moles of water react to yield two moles of hydrogen and two moles of oxygen". The coefficients before each formula are called stoichiometric coefficients. These coefficients are the mole ratios in the balanced equation. When we have a balanced equation, we can use the mole ratios of a balanced chemical equation to determine the amount of product that can be produced. These are called stoichiometry calculations.
Read this text to learn more about basic chemical arithmetic and stoichiometry.
Watch this video to see examples of how to systematically balance a chemical equation.
Watch these videos, which show all the types of stoichiometry problems we see in chemistry.
Pause each video after the problem is presented and try the problem yourself. Then, watch the rest of the video to check your work.
A balanced chemical equation shows us the numerical relationships between each of the species involved in the chemical change. Using these numerical relationships (called mole ratios), we can convert between amounts of reactants and products for a given chemical reaction.
Take this assessment to see how well you understood this unit.