Isotopes and Relative Atomic Mass

Isotopes: we can say Isotopes are the elements that have the same atomic number but a different mass number. The mass spectrometer shows the abundance of the isotopes.

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Relative Molar Mass

This is defined as the number of times one molecule of the element or compound is as heavy as 1/12th the mass of one atom of isotopic carbon – 12 having a mass of 12.00 unit.

Some given solution to some test questions

1. Calculate the percentage by mass of nitrogen in calcium trioxonitrate (v) [ Ca=40, O=16


Compound Ca(NO3)2

Relative molecular Mass of Ca(NO3)2

1 x 40 + 2 x 14 + 6 x 16 = 164g

percentage by mass of nitrogen= 28/164 x 100 = 17.03% approximately 17%

Isotopes and Relative Atomic Mass

2. Taking the symbol 16X to represent an atom of the element X, state 

(a) the atomic number of X (b) the number of the neutrons in an atom of X (c) the number of electrons in an atom (d) the mass of number of X. If another atom is represented as 18X what term would be used to state its relation to 16X and what is the different between them in terms of the number and situations of particles present?


  • the atomic number of X is 6
  • the number of the neutrons in an atom = Mass No. – Atomic No. = 16 – 6 = 10
  • the mass of number = 16
  • what is the different between them. The differences between them is that they have the same Neutron


3. What is the amount (in mole) of sodium trioxocarbonate (iv) in 5.3g of the compound? ( Na2Co3 = 106g)


Mole = Reacting Mass/molar mass

Mole = 5.3/106 = 0.05mole

Try this and solve

Calculate the percentage by mass of sulphur in tetraoxosulphate (vi) acid, H2SO4 ( H=1, S=32 O=16 ) what is your answer?

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