Chapter 3 - Mass Relationships in Chemical Reactions

Question 1

What is the definition of an atomic mass unit?

Answer 1

Exactly 1/12 the mass of a Carbon-12 atom.

Question 2

Is the atomic mass of an element in the periodic table absolute or an average? Explain.

Answer 2

The atomic mass in the periodic table is an average of the atomic masses of all the isotopes of an element with respect to the proportions in which they occur in nature.

Question 3

What is the international definition of a mole (mol)?

Answer 3

A mole is equal to the number of atoms there are in 12g of a Carbon-12 atom.

Question 4

How many units are in a mole ( Avagadro’s Number )?

Answer 4

6.022 x 10²³

Question 5

Explain molar mass and its relationship to the atomic mass of an element.

Answer 5

Molar mass is the mass (g or kg) of 1 mole of a substance. By convention, 1 mole of Carbon-12 is exactly 12 g and since the atomic mass of an element is relative to the atomic mass of a Carbon-12 atom (12 amu), the molar mass (g or kg) of an element is equal to its atomic mass (amu).

Question 6

Explain how we can translate back and forth between the mass of an element, the number of moles of an element, and the number of atoms of an element.

Answer 6

We employ 2 conversion factors that can be used to translate between three, in the following fashion.

Mass of element ⇔ Numer of moles of element ⇔ Number of Atoms of element

Between “Mass” and “Number of Moles”, we have
1 mole of X/1 molar mass of X
1 molar mass of X/1 mole of X

Between “Number of Moles” and “Number of atoms”, we have

1 mole of X/6.022 x 10²³ atoms
6.022 x 10²³ atoms/1 mole of X

We use Dimensional Analysis to solve for whatever we’d like

Question 7

Explain molecular mass and how it relates to atomic mass. Are the combined masses of ionic compounds treated any differently?

Answer 7

Molecular mass is the sum of the atomic masses of the elements that comprise a molecule. Knowing the molecular mass of a substance will allow you to know the mass of the molecule.

It should be noted that ionic compounds follow the same paradigm as molecular masses, except that the term used is formula mass.

Question 8

What is the most accurate and direct method for determining atomic and molecular masses?

Answer 8

Mass spectrometry

Question 9

How do you calculate the percentage composition by mass for a compound?

Answer 9

% comp by mass = (num_mols * molar mass)/molecular mass