# Stoichiometry Flashcards

## From the basic definition of atomic and molecular weights, through percent composition, to balancing complex chemical equations, use these cards to master the topic of stoichiometry as tested in most introductory undergrad chemistry courses and on the AP Chemistry exam.

Define and give the value for:

**one mole**

A **mole** is the number of particles of a substance that must be present in a sample such that the sample’s mass in grams is equal to the substance’s atomic weight in AMU.

One mole is equal to 6.02 x 10^{23} particles.

What is the mass of one mole of helium atoms?

(one helium atom has a mass of 4 AMU)

4 g

One mole of atoms or molecules is the exact number such that the entire sample will have a mass (in g) of the individual atom of molecule’s mass in AMU.

Define:

**atomic weight**

**Atomic weight** is the mass, in grams, of one mole of a naturally occurring element.

To calculate atomic weight, one must take into account both the weight of all the naturally-occurring isotopes of that element, and their proportional abundance.

Ex: natural Cl appears as two significant isotopes, ^{35}Cl (75% abundance) and ^{37}Cl (25% abundance). The atomic weight of Cl is therefore:

(0.75 * 35) + (0.25 * 37) = 35.5

Define

**atomic mass**

**Atomic mass** is the mass, in Atomic Mass Units (AMU), of one atom of a particular isotope of an element.

It also represent the mass in grams of one mole of the isotopes, and can be found by adding together the number of protons and neutrons in that particular isotope.

Ex: the atomic mass of ^{235}U, with 92 protons and 143 neutrons, is 235 AMU.

Define:

**molecular weight**

**Molecular weight** is the weight of one mole of molecules of a substance.

It can also be calculated by adding together the atomic weights of all the atoms in the molecule.

What is the molecular weight of glucose, C_{6}H_{12}O_{6}?

180.20 AMU

[6 * C] + [12 * H] + [6 * 16] =

[6 * 12.01] + [12 * 1.01] + [6 * 16.00]

These 3 masses are worth memorizing. Remember, approximation is almost always good enough, so let C = 12, H = 1, and O = 16, for a total of 180 AMU.

Define:

**molecular formula**

A **molecular formula** shows the total number and type of atoms in each molecule. This is the full, unsimplified formula.

Ex: The molecular formula of glucose, with 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, is: C_{6}H_{12}O_{6}

Define:

**empirical formula**

An **empirical formula** is a simplified ratio of whole numbers for the different elements in a compound.

Ex: The empirical formula of glucose, with 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, is: CH_{2}O

What is the molecular formula of this molecule?

C_{4}H_{8}

The molecular formula is the total of all the atoms present in a single molecule of the substance.

What is the empirical formula of this molecule?

CH_{2}

The empirical formula is the ratio of the number of atoms in a substance, expressed as the lowest common denominator.

In this case, take the molecular formula, C_{4}H_{8}, and divide both subscripts by 4 to get to the final answer.

How do you calculate the mass of one mole of a substance?

- Find the atomic weight of each element in the molecular formula.
- Multiply that weight by the subscript (number of that atom present).
- Add these numbers together to get the total sum.

Ex: 1 mole of H_{2}O has a mass of 18 g.

(1 * 2) + (16 * 1) = 18

What is the weight of:

1 mole of carbon dioxide, CO_{2}?

44 g

To calculate the weight of 1 mole of a substance, add the atomic weights of every atom in the molecular formula. Remember approximation is almost always good enough on most chemistry tests, especially on the AP Chemistry Exam.

Weight (CO_{2}) = 12 + 2(16) = 44 g

How many moles are in:

32 g of SO_{2}?

1/2 a mole

1 mole of SO_{2} has a mass of: 32 + 2(16) = 64 g

Thus, 32 g is one half a mole.

Give the formula for:

**percent composition**

**Percent composition** is a calculation of the proportion of a substance that a particular element makes up, by weight.

What is the percent composition of oxygen in glucose, C_{6}H_{12}O_{6}?

53.30%

This reaction is a **disproportionation reaction**. What does that mean?

Hg_{2}Cl_{2} → HgCl_{2} + Hg

**Disproportionation reactions** are a subset of redox reactions, in which one species (in this case, the Hg atom) acts as both the oxidizing agent and the reducing agent.