Quantitative chemistry (Calculations)

1. Calculate Mr (relative formula mass)                (click for more examples)

Q1. Calculate the relative formula mass (Mr) of Fe2O3.

Relative atomic masses (Ar): O = 16   Fe = 56

Mr = (56 x 2) + (16 x 3)

Mr = 160

Q2. Calculate the relative formula mass (Mr) of zinc nitrate Zn (NO3)2.

Relative atomic masses (Ar): Zn = 65  N = 14    O = 16 

Mr = 65 + (14 x 2) + (16 x 6)

Mr = 189



2. Calculate percentage mass                              (click for more examples)

Q3. Calculate the percentage by mass of iron in iron oxide (Fe2O3.) Relative atomic masses (Ar): O = 16   Fe = 56

Q4. Calculate the percentage by mass of zinc in zinc nitrate Zn (NO3)2.

Relative atomic masses (Ar): Zn = 65  N = 14    O = 16 



3. Ar of Isotopes           (click for more examples)

Q5. The table below shows the mass numbers and the percentage abundance of two isotopes of potassium. 

Calculate the relative atomic mass (Ar) of potassium.

Give your answer to 1 decimal place. (3)



4. Concentration  (click for more examples)

Q6. A solution of hydrochloric acid contains 3.2 g of hydrogen chloride in 50 cm3

Calculate the concentration of hydrogen chloride in g per dm3. (3)



5. Moles and Avogadro's constant (click for more examples)

number of atoms / ions / molecules = number of moles x Avogadro's constant

Avogadro's constant = 6.02 × 1023  

Q7. 0.00625 moles of bromine react with sodium.

Calculate the number of bromine molecules in 0.00625 moles of bromine. (3)

1 mole of bromine contains 6.02 × 1023 bromine molecules.

Number of molecules = 0.00625 x 6.02 × 1023 

Number of molecules = 0.037625 x 1023

Number of molecules = 3.76 x 1021 molecules



6. Mole = Ar or Mr in grams (click for more examples)

For an atom, a mole is the relative atomic mass (Ar) expressed in grams.

Ar of H = 1, so the mass of 1 mole of hydrogen is 1 g.

Ar of C = 12, so the mass of 1 mole of carbon is 12 g.



For a compound, a mole is the relative formula mass (Mr) expressed in grams.

Mr of NaCl = 23 + 35.5 = 58.5, so the mass of 1 mole of NaCl = 58.5 g

Mr of Al2O3 = (27 x 2) + (16 x 3) = 102, so the mass of 1 mole of Al2O3 = 102 g

Q8. Calculate the mass of 1 mole of CaCO3 (2).

Mr of  CaCO3 = 40 + 12 + (16 x 3) = 100 

So the mass of 1 mole of CaCO3 = 100 g



7. Mass = Mr x Moles