Relative formula mass
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How do you calculate Mr (relative formula mass) of a substance?
add together the Ar (relative atomic mass) of all the atoms in its formula
Why do Mr values not have units?
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
Calculate the Mr of aluminium oxide, Al2O3. Ar: Al = 27, O = 16
atoms in Al2O3: (2 x Al) + (3 x O) Mr = (2 x 27) + (3 x 16) = 54 + 48 = 102
Calculate the Mr of calcium nitrate, Ca(NO3)2 Ar: Ca = 40, N = 14, O = 16
(1 x 40) + (2 x 14) + ( 6 x 16) = 164
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)
a) 18 b) 44 c) 40 d) 154 e) 134.5 f) 142 g) 78 h) 234
mass number
total number of protons and neutrons in the nucleus of an atom
atomic number
number of protons (and electrons) in an atom
What do Mr and Ar stand for?
relative formula mass relative atomic mass