Mathematics of Chemistry Laboratory

Significant Figures

The following digits are always significant:

all non-zero digits in the number
all zeros between non-zero digits in the number
all zeros right of the decimal point, and at the end of the number
all zeros to the left of the written decimal point in numbers ≥ 10

For addition and subtraction, the final result should have the same number of decimal places as the added/subtracted value with the smallest number of decimal places.

For multiplication and division, the final result should have the same number of significant figures as the multiplied/divided value with the smallest number of significant figures.

In the Laboratory

Mass determined by reading a balance should be recorded to as many decimal places as read off the balance, with absolute error as determined by the precision of the balance. Mass differences calculated using a tare should use that number of decimal places in the recorded result.

For graduated cylinders, a general rule is to record the amount to one more decimal place than is marked. For 10mL graduated cylinders, record to the nearest 0.01mL. For 25mL to 100mL graduated cylinders, record to the nearest 0.1mL.

Absolute Error

Use the precision determination given for the instruments used to determine the level of absolute error in recorded values.

For addition and subtraction, simply add absolute errors of each component in the calculation.

For multiplication and division, convert to relative error first. The relative error value of each component is determined by dividing the error value by the actual component value. The relative error values of all components are added, then multiplied by the final calculated result of the operations on the component values, to find the absolute error of the final result.

Basic Calculations

There are key calculations that are often used in the course of performing experiments in the chemistry laboratory.

Isolated mass of substance = [mass of container and substance] - [mass of container]
Percent yield = ([mass of product] / [mass of initial substance]) * 100

Key Scientific Terms

Solubility is the amount of a substance that dissolves in a given amount of solvent at a given temperature.

Stoichiometry

Stoichiometry deals with the quantitative relationships that exist between the reactants and products in chemical reactions. In stoichiometry, the Law of Conservation of Matter must be followed. The balanced chemical reaction equation gives the mole ratios of all reactants and products involved. The stoichiometric (equimolar) amount of reactants indicates the exact amount needed for all reactants to be used up simultaneously as the reaction proceeds to completion. If there are not stoichiometric amounts of all reactants, the reactant(s) that are used up at the completion of the reaction are the limiting reactant(s), and the remaining reactants are the reactants in excess.

Types of Chemical Reactions

Precipitation

In a precipitation reaction, an insoluble substance forms and separates from the solution. The solid is known as a precipitate, and the solution above the precipitate is called the supernatant. Generally, the experiment begins with two homogeneous solutions, and a precipitate forms when they are combined. After the maximum amount of substance that can dissolve in the solutions has dissolved, the remainder forms a precipitate.

Solutions with soluble components do not form a precipitate if they are not saturated. Ionic compounds generally dissociate in water.

Precipitation reactions are often used to isolate a particular ion from the solution. The process allows for selective removal of ions through properties of solubility.

The calculations for precipitation reactions are to be determined quantitatively; stoichiometry is heavily involved.

Acid-Base

In an acid-base reaction, molecules classified as acids and bases interact with each other in a particular way. There are multiple definitions of acid and base molecules. One of the most common of these is the Brønsted-Lowry definition, which indicates that an acid is a proton donor, while a base is a proton acceptor. Another common definition is the Lewis definition, which indicates that an acid is an electron acceptor, while a base is an electron donor.

Acids and Bases

Acids and bases are two classes of chemical molecules that interact in a particular way with each other when combined. There are multiple definitions of acid and base molecules. One of the most common of these is the Brønsted-Lowry definition, which indicates that an acid is a proton donor, while a base is a proton acceptor. Another common definition is the Lewis definition, which indicates that an acid is an electron acceptor, while a base is an electron donor.

Acids

Monoprotic acids involving hydrogen, per the Brønsted-Lowry definition, contain two bonded molecules - hydrogen and another - in a one-to-one ratio.

General Chemistry Laboratory/Printable version

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