Relative formula mass

Question 1

How do you calculate Mr (relative formula mass) of a substance?

Answer 1

add together the Ar (relative atomic mass) of all the atoms in its formula

Question 2

Why do Mr values not have units?

Answer 2

they are just numbers

an Mr value is the mass of a molecule compared with 1/12th the mass of a 12C atom
(Ar is the same but for an atom)

M in Mr stands for molecular

relative molecular mass only applies to covalent substances

Question 3

Calculate the Mr of aluminium oxide, Al2O3.

Ar: Al = 27, O = 16

Answer 3

atoms in Al2O3:
(2 x Al) + (3 x O)
Mr = (2 x 27) + (3 x 16)
= 54 + 48
= 102

Question 4

Calculate the Mr of calcium nitrate, Ca(NO3)2

Ar: Ca = 40, N = 14, O = 16

Answer 4

(1 x 40) + (2 x 14) + ( 6 x 16)

= 164

Question 5

Calculate the relative formula mass, Mr, of the following substances:

a) H2O
b) CO2
c) NaOH
d) CCl4
e) CuCl2
f) Na2SO4
g) Al(OH)3
h) Al2(CO3)3

(Ar: H = 1, C = 12, O = 16, Na = 23, Al = 27, S = 32, Cl = 35.5, Cu = 63.5)

Answer 5

a) 18
b) 44
c) 40
d) 154
e) 134.5
f) 142
g) 78
h) 234

Question 6

mass number

Answer 6

total number of protons and neutrons in the nucleus of an atom

Question 7

atomic number

Answer 7

number of protons (and electrons) in an atom

Question 8

What do Mr and Ar stand for?

Answer 8

relative formula mass

relative atomic mass