Physical: Formulae, equations & calculations

Question 1

define molecular formula

Answer 1

The molecular formula is the formula that shows the number and type of each atom in a molecule
E.g. the molecular formula of ethanoic acid is C2H4O2

Question 2

define empirical formula

Answer 2

The empirical formula is the simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound
E.g. the empirical formula of ethanoic acid is CH2O

Question 3

determine the empirical formula from mass
10 g of hydrogen and 80 g of oxygen.

Answer 3

H=1 O=16
10/1=10 mol 80/16 =5 mol- divide mass by atomic mass
10/5=2 5/5=1. 2:1
- divide by lowest to get ratio
H2O

Question 4

determine the empirical formula from percentage
85.7% carbon and 14.3% hydrogen.

Answer 4

c=12 h=1
85.7/12= 7.142 14.3/1=14.3
7.142/7.142= 1 14.3/7.142=2
1:2
CH2

Question 5

how to find molecular formula

Answer 5

relative formula mass of the molecular formula by the relative formula mass of the empirical formula, Multiply the number of each element present in the empirical formula by this number to find the molecular formula

Question 6

determine the molecular formula

The empirical formula of X is C4H10S and the relative molecular mass of X is 180
What is the molecular formula of X?

Answer 6

c=12 H=1 S=32
12 x 4=48 1x10=10 32x1=32
=90 relative empirical mass
180/90=2
multiply all elements by this
Molecular Formula of X is C8H20S2

Question 7

reminder

Answer 7

practice balancing equations and doing ionic equations

Question 8

what is the equation for percentage yeild

Answer 8

percentage yield= actual yield/ theoretical yield x100

will often have to use n=mas/mr to find predicted yeild mass

Question 9

what is the equation for atom economy

Answer 9

atom economy= mr of desried/ sum of mr of all reactants. x100
can also use mass instead of Mr

Question 10

Calculating water of crystallisation

10.0 g of hydrated copper sulfate are heated to a constant mass of 5.59 g.
Calculate the formula of the original hydrated copper sulfate.

(Mr data: CuSO4 = 159.6, H2O = 18.0)

Answer 10

CuSO4= 5.59g H2O=10-5.59=4.41g
5.59/159.6=0.035. 4.41/18=0.245
divide mass by mr to get moles
0.035/0.035=1 0.245/0.035=7
divide by smallest number for ratio
1:7
CuSO4.7H2O