Stoichiometry (Topic 3) Lecture Notes

Flashcards covering the fundamentals of stoichiometry, including atomic mass, the mole concept, molar mass conversions, molecular mass, percent composition, and empirical formulas.

What is the standard definition of atomic mass?

Atomic mass is the mass of an atom in atomic mass units ($amu$), defined by setting the mass of one atom of $^{12}C$ at exactly $12.00\,amu$.

How is the average atomic mass of an element calculated?

It is the weighted average of all the naturally occurring isotopes of the element, calculated by multiplying the mass of each isotope by its fractional abundance and summing the results.

What is the average atomic mass of Copper (Cu) given that $^{63}Cu$ (69.09%, $62.93\,amu$) and $^{65}Cu$ (30.91%, $64.9278\,amu$) are its stable isotopes?

$$(0.6909) \times (62.93\,amu) + (0.3091) \times (64.9278\,amu) = 63.55\,amu$$

Define the mole ($mol$).

The amount of a substance that contains as many elementary entities as there are atoms in exactly $12.00\,g$ of $^{12}C$.

What is the value of Avogadro's number ($N_A$)?

$1\,mol = 6.0221415 \times 10^{23}$ particles.

What is the relationship between atomic mass and molar mass for any element?

The atomic mass in $amu$ is numerically equal to the molar mass in grams ($g$); for example, 1 atom of $^{12}C$ weighs $12.00\,amu$ and 1 mole of $^{12}C$ weighs $12.00\,g$.

What is the conversion factor between grams and atomic mass units?

$1\,amu = 1.66 \times 10^{-24}\,g$ or $1\,g = 6.022 \times 10^{23}\,amu$

How do you calculate the number of atoms in a specific mass of an element?

First, divide the mass ($m$) by the molar mass ($M$) to find the number of moles ($n$), then multiply the number of moles by Avogadro's number ($N_A$).

Define molecular mass.

The sum of the atomic masses (in $amu$) in a molecule.

What is the molecular mass of sulfur dioxide ($SO_2$) if $S = 32.07\,amu$ and $O = 16.00\,amu$?

$$32.07\,amu + 2 \times (16.00\,amu) = 64.07\,amu$$

What is the chemical formula for urea?

$$(NH_2)_2CO$$

What is the formula mass of an ionic compound?

The sum of the atomic masses (in $amu$) in a formula unit of an ionic compound (e.g., $NaCl$).

State the formula for percent composition by mass.

$\text{Percent composition} = \frac{n \times \text{molar mass of element}}{\text{molar mass of compound}} \times 100\%$, where $n$ is the number of moles of the element in 1 mole of the compound.

What are the four main steps to determine an empirical formula from mass percent?

1. Convert mass percent to grams (assuming a 100 g sample). 2. Convert grams to moles using molar mass. 3. Divide by the smallest number of moles to find the mole ratio. 4. Convert to integer subscripts.

What is the empirical formula of ascorbic acid (vitamin C) if it is 40.92% C, 4.58% H, and 54.50% O?

$$C_3H_4O_3$$

What is the chemical formula for the mineral spodumene?

$$LiAlSi_2O_6$$