3.2 Determination of formulae Flashcards

1
Q

What is the molecular formula

A

The number of atoms of each element in a molecule

Eg. H2, N2, O2, S8

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2
Q

What is the empirical formula

A

The simplest whole-number ratio of atoms of each element in a compound
Ratio is always the same
Important for substances that do not exist as molecules, eg. carbon, silicon, sodium chloride, which form giant crystalline structures of atoms or ions

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3
Q

what do carbon, silicon and sodium chloride form

A

Giant crystalline structures of atoms or ions

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4
Q

What is relative molecular mass

A

Mr, compares the mass of a molecule with the mass of an atom of carbon-12

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5
Q

How to calculate the relative molecular mass

A

Add together the relative atomic masses of the elements which make up a molecule

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6
Q

What is relative formula mass

A

Compares the mass of a formula unit with the mass of an atom of carbon-12
Calculated by adding the atomic masses of the elements in the empirical formula

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7
Q

What is the investigation of the chemical composition of a substance called

A

analysis

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8
Q

Wrongly rounding ratios

A

If you calculated a ratio as 1:1.67 don’t be tempted to round to 1:2

See if you can convert it into a whole number ratio by multiplying both by the same factor

In this case, by 3, so you get a ratio of 3:5

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9
Q

What is water of crystallisation

A

Water molecules that are a part of the crystalline structure of hydrated salts

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10
Q

What does anhydrous mean

A

Lacking water

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11
Q

Experiment to determine the water of crystallisation in hydrated crystals

A
  1. Weigh an empty crucible
  2. Add the hydrated salt into the weighed crucible
  3. Weigh the crucible and the hydrated salt
  4. Using a pipe-clay triangle, support the crucible on a tripod, heat it and its contents gently for one minute and then strongly for another three.
  5. Leave it to cool, then weigh
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12
Q

How accurate is an experimental formula

A

Some assumptions are made, all the water has been lost, and no further decomposition

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