Relative Formula Mass

What is the relative formula mass of an element?

The average mass of all the isotopes of the element

What is the formula for relative formula mass?

Mr = The sum of Ar of elements x number of atoms in elements

What is the formula for percentage mass of an element?

(Ar x number of atoms of that element/ Mr of the compound) x 100

What is relative Atomic Mass(Ar)?

the average mass of an atom of an element relative to one-twelfth the mass of a carbon-12 atom

How to calculate relative atomic mass(Ar)?

Relative Formula masses that you should know:

Substance	Formula	Mr
Water	H₂O	18
Carbon dioxide	CO₂	44
Oxygen	O₂	32
Hydrogen	H₂	2
Nitrogen	N₂	28
Methane	CH₄	16
Ammonia	NH₃	17
Hydrochloric acid	HCl	36.5
Sulfuric acid	H₂SO₄	98
Sodium chloride	NaCl	58.5
Calcium carbonate	CaCO₃	100
Glucose	C₆H₁₂O₆	180

Relative Atomic masses you should know:

Element	Symbol	Ar
Hydrogen	H	1
Carbon	C	12
Oxygen	O	16
Nitrogen	N	14
Sodium	Na	23
Chlorine	Cl	35.5
Calcium	Ca	40
Sulfur	S	32

A carbon atom has a relative atomic mass of 12.

60% of the carbon atom has a mass number of 12.8, whilst the remaining 40% has a mass number

which is not known.

Calculate the mass number of the carbon isotope which is 40% of the Carbon 12 atom.

Give your answer in 2 decimal places if needed

((90 × 12) + 10z)/100

(1080 + 10z)/100

12×100 = 1210

1210 = 1080 + 10z

1210 - 1080 = 130

130 = 10z

130/10 = 13