mole concept and stoichiometry (part 1)

Question 1

The relative atomic mass (Ar) is…

Answer 1

The relative atomic mass is the ‘average’ mass of an atom of the element (as compared to the mass of 1/12 of an atom of 12C). Can be found in the periodic table.

Question 2

The relative molecular mass (Mr) is…

Answer 2

The relative molecular mass is the mass of one molecule of a covalent substance when compared to the mass of “. The relative molecular mass of any substance is the sum of the relative atomic masses of the constituent elements present in a molecule of that substance.

Question 3

The relative formula mass (Mr) is…

Answer 3

The relative formula mass is the mass of one formula unit of an ionic compound when compared to the mass of “. The relative formula mass of any substance is obtained by adding the relative atomic masses of the constituent elements present in the formula unit.

Question 4

Percentage composition by mass is…

Answer 4

Percentage composition by mass is the percentage by mass of each element in a compound. It is obtained by:
(Ar of element x number of atoms in formula / Mr of compound) x 100

Question 5

What is Avogadro’s number?

Answer 5

6.02 x 10^23 (NA). 1 mole of any substance contains 6.02 x 10^23 particles of that substance. (depending on context, particles can refer to atoms, molecules, ions, electrons etc.)

Question 6

What does the empirical formula of a compound tell you?

Answer 6

• The types of elements that are present in the compound

- The simplest whole-number ratio of the different types of atoms in it

Question 7

How to find mole ratio?

Answer 7

Number of moles / smallest mole

Question 8

What are the steps to find empirical formula?

Answer 8

1. Mass/g
2. Number of moles/mol (mass / relative atomic mass)
3. Mole ratio
4. Simplest whole number ratio

Question 9

Empirical formula vs molecular formula?

Answer 9

The molecular formula is the formula that shows the actual number of atoms of each element in one molecule of the compound. Where the empirical formula and molecular formula are different, the molecular formula is always a multiple of the empirical formula.

Question 10

How to find the molecular from the empirical formula?

Answer 10

If the empirical formula is AxBy, its molecular formula will be (AxBy)n, where n can be 1, 2, 3…
To find n, use the equation n = relative molecular mass / Mr from the empirical formula

Question 11

What is the molar volume of any gas at r.t.p (room temperature pressure)?

Answer 11

24dm3/mol (24dm3mol-1)

Question 12

How to find concentration (in g/dm3)?

Answer 12

mass of solute / volume of solution (in dm3)

Question 13

How to find concentration (in mol/dm3)?

Answer 13

No. of moles of solute (mol) / volume of solute (dm3)

Question 14

Relationship between g/dm3 and mol/dm3?

Answer 14

g/dm3 = molar mass (g/mol) x mol/dm3