3.2

Question 1

Molecular formula is…

Answer 1

Number of atoms of each element in a molecule

Question 2

Elements that exist as molecules

Answer 2

H2
O2
N2
F2
(At2)
Br2
I2
Cl2
P4
S8

Question 3

Empirical formula

Answer 3

Simplest whole number ratio of atoms of each element in a compound

Question 4

Which substances is empirical formula important for? Why?

Answer 4

Substances that don’t exist as molecules, incl metals, some non metals (eg C and Si) and ionic compounds
These substances form crystalline structures of atoms/ions.

Would be impossible to base formula on actual number atoms/ions - numbers would go into billions of billions, and depend on size of crystal

Question 5

Difference between relative formula mass and relative molecular mass

Answer 5

Relative formula mass for giant crystalline structures, while relative molecular used for molecules

Question 6

Relative molecular mass definition

Answer 6

Weighted mean mass of molecule of a compound compared with 1/12 the mass of a carbon-12 atom

Question 7

How to calculate relative molecular mass

Answer 7

Add relative atomic masses of elements making up molecule

Question 8

Relative formula mass definition

Answer 8

Weighted mean mass of the formula unit of a compound compared with 1/12 the mass of a carbon-12 atom

Question 9

How to calculate relative formula mass

Answer 9

Add together relative atomic masses of elements in empirical formula

Question 10

What happens when coloured hydrated salts (eg hydrated copper(II) sulfate) heated?

Answer 10

Bonds holding water within crystal broken
Water driven off
Leaves behind anhydrous salt (eg white copper(II) sulfate)
without water, crystalline structure lost, white powder left

Question 11

How ro work out formula of hydrated salt

Answer 11

Calculate amount in mol of water and of anhydrous salt formed
Find the smallest whole number ratio

Question 12

2 assumptions made when working out formula of hydrated salt

Answer 12

Assumption 1: All water has been lost
If anhydrous and hydrated different colours, can be fairly sure when all water gone. However, only see surface if crystals - some water may still be inside. If anhydrous and hydrated similar colour, quite difficult to see. So, solution is to HEAT TO CONSTANT MASS, until mass of residue no longer change, suggests all water gone

Assumption 2: no further decomposition
Many salts decompose if heated - eg CuSO4 decomposes to form black CuO. Difficult to jusge if no colour change