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Science

Calculations and titrations

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It is important to know the correct quantities of reactants to use in a synthesis, and how much product has been produced. It is also important to know how pure the product is - this can be tested using the method of titration.

Calculating relative formula masses

Relative atomic mass

The relative atomic mass of an element shows its mass compared with the mass of atoms of other elements. The relative atomic mass of carbon is 12, while the relative atomic mass of magnesium is 24. This means that each magnesium atom is twice the mass of a carbon atom.

The relative atomic mass of each element can be found in the Periodic Table.

Relative formula mass

Relative atomic masses can be used to find the relative formula mass of a compound.

Here are two examples:

Question

Find the relative formula mass of sulfuric acid H2SO4.

Answer

Sulfuric acid has the formula H2SO4. The relative atomic masses of the elements in sulfuric acid are H = 1, O = 16 and S = 32.

Adding together the relative atomic masses of each atom in the formula of sulfuric acid gives the relative formula mass of sulfuric acid:

H2SO4 relative formula mass = (2 x 1) + 32 + (4 x 16) = 98

Question

Find the relative formula mass of sodium hydroxide NaOH.

Answer

Sodium hydroxide has the formula NaOH. The relative atomic masses of the elements in sodium hydroxide are H = 1, Na = 23 and O = 16.

Adding together the relative atomic masses of each atom in the formula of sodium hydroxide gives the relative formula mass:

NaOH relative formula mass = 23 + 16 + 1 = 40

Watch this illustrated podcast for a summary of chemical calculations involving mass.

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