6. Quantifying Atoms and Compounds

Question 1

What is mass number?

Answer 1

Mass number is the total number of protons and neutrons in the nucleus of a particular isotope of an element

Question 2

What is the unified atomic mass units (u)

Answer 2

The standard unit of mass based on the mass of the C-12 isotope, where mass of an atom of C-12 equals 12 u; it is approximately equal to one nucleon (proton or neutron)

Question 3

What is relative isotopic mass?

Answer 3

Relative isotopic mass can be defined the mass of an individual isotope of an element on the relative atomic mass scale, on which the masses of particles are compared with the mass of the carbon-12 isotope,
Relative isotopic mass =
mass of an atom of the isotope
———————————————- x 12
mass of an atom of carbon-12

Question 4

What are the differences between mass number and relative isotopic mass?

Answer 4

• mass number is always a whole number as it is protons + neutrons and they are always whole
• relative isotopic mass is an accurate measurement made by comparing an atom’s mass to the mass of a carbon-12 atom. RIM are rarely whole numbers but they are always very close to an isotope’s mass number.

Question 5

What is Relative Atomic Mass?

Answer 5

Relative atomic mass is the weighted average of al the relative isotopic masses for the isotopes that make up the naturally occurring isotopic mixture of an element.

Question 6

What is the formula for Relative Isotopic Mass?

Answer 6

Ar = RIM of first isotope x % Abundance) +
(RIM of second isotope x % Abundance)…
———————————————————————
100

Question 7

What is relative formula mass? (Mr)

Answer 7

Relative Formula mass is the sum of the relative atomic masses of the atoms in the formula of an ionic compound

Question 8

What is relative molecular mass? (Mr)

Answer 8

Relative molecular mass is the sum of the relative atomic masses of the atoms in a molecule of a covalent molecular compound.

Question 9

What are the Mr equal to?

Answer 9

Mr are equal to the sum of the relative atomic masses of the atoms in the formula of a compound, but RMM is used for covalent compounds and RFM is used for ionic compounds.

Question 10

What is Avogadro’s constant?

Answer 10

6.02 x 10^23, or the number of atoms in 12g of carbon-12

Question 11

How do you find the mass of one mole of an element?

Answer 11

To find the mass of one mole of an element, simply add ‘g’ to the relative atomic mass of that element.

Question 12

What is the molar mass of an element?

Answer 12

The molar mass is the mass of one mole of a compound expressed in grams per mol –>
g mol-1

Question 13

How do you calculate the molar mass of covalent compounds?

Answer 13

molar mass is numberically equal to the relative molecular mass, expressed in grams per mole
Eg. molar mass of water = mass of 1 mol of H2O molecules.
- mass of 1 mol of H2O molecules = 18.0g
- M(H2O)= 18.0gmol-1
18.0g of water contains 6.02 x 10^23 molecules of water.

Question 14

How do you calculate the molar mass of ionic compounds?

Answer 14

Mr of an ionic compound is found by adding the Ar of each atom in the formula of the compound.
Eg. Mr(CuSO4) = Ar(Cu) + Ar(Cu) + Ar(4xAr(O))

Question 15

Formula

Answer 15

n = number of moles
m = mass
M = molar mass
m m
n = ——- n M
M

Question 16

Formula for calculating percentage composition

Answer 16

% of x in compound =
mass of x in 1 mol of compound. 100
———————————————- x ———
molar mass of compound. 1

Question 16

What is percentage composition?

Answer 16

Percentage composition is the percentage contribution that each element makes to the mass of a compound.

Question 17

What is the empirical formula?

Answer 17

The empirical formula is the formula that shows the simplest numerical ratio in which atoms are combined.