Atoms, Molecules And Stouchiometry

Question 1

Define the unified atomic mass unit

Answer 1

One twelve the mass of a C-12 isotope

Question 2

Define the relative atomic mass, Ar

Answer 2

The weighted average mass of atoms in a given sample of an element compared to the value of the unified atomic mass unit.

Question 3

What are isotopes?

Answer 3

Atoms of the same element with the same number of protons (atomic number) BUT different number of neutrons ( different mass/ nucleon number)

Question 4

Define the relative isotopic mass.

Answer 4

The weighted mass of one atom of the isotope compared to the value of the unified atomic mass unit.

Question 5

What is the relative abundance of a sample?

Answer 5

The relative abundance in a sample of an element is the percentage by mass of that isotope in that element.

Question 6

What is the formula for Ar?

Answer 6

Ar(element)= 
sum of (relative abundances x relative isotopic mass)/ sum of relative abundances

Question 7

Define the relative molecular mass

Answer 7

The weighted average mass of a molecule in a given sample of that molecule compared to the value of the unified atomic mass unit.

Only for covalent molecules

Question 8

Define the relative formula mass.

Answer 8

The weighted average mass of one formula unit of the compound compared to the value of the unified atomic mass unit.

Question 9

What is the mole?

Answer 9

The amount of substance that contains exactly 6.02 x 10 ^23 specified particles- AVOGADROS CONSTANT

Question 10

Review mole equations

Answer 10

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Question 11

What is the molecular formula?

Answer 11

A formula that shows the number of atoms of different elements which are present in one molecule of the compound.

Question 12

What is the empirical formula?

Answer 12

A formula that shows the whole number ratio of atoms present in a compound.

Question 13

Describe the steps taken to determine the molecular formula of a hydrocarbon.
Review general formula

Answer 13

1. A measured volume of GASEOUS hydrocarbon Is mixed with a measured volume of oxygen gas.
2. The GASEOUS mixture is ignited
   After complete combustion, the mixture is allowed to cool to rtp.
   (CO2 and O2 has left)
   NOTE THE VOLUME OF GAS COLLECTED.
3. The gaseous mixture is passed through aqueous sodium hydroxide.
   CO2 is absorbed as it is ACIDIC
   THE VOLUME OF GAS LEFT (oxygen) IS MEASURED.

Initial Volume of oxygen - final volume of oxygen = volume of oxygen burnt

Volume of gas left - volume of oxygen left= volume of carbon dioxide produced

Note: To balance firstly the carbon, then the hydrogen and finally the oxygen in combustion reactions of organic compounds

Question 14

Why do isotopes have similar chemical properties?

Answer 14

Chemical properties are determined by the number and arrangement of electrons in an atom. Isotopes have similar chemical properties as they have the same number of valence electrons.

Question 15

Why do isotopes have different physical properties?

Answer 15

• The only difference between isotopes is the number of neutrons
• Since these are neutral subatomic particles, they only add mass to the atom
• As a result of this, isotopes have different physical properties such as small differences in their mass and density