• ### Unit 4: Chemical Formulas and Equations

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.

• ### 4.1: Chemical Formulas

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.

• ### 4.2: Stoichiometry and Limiting Reagents

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.