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CHEM - 2.1.3 Amount of Substance > Determination of formulae > Flashcards

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Empirical formula from mass

Step 1: Convert mass into moles of atoms
Step 2: To find the smallest whole-number ratio, divide by the smallest whole number.
* If percentage is given, use it instead of mass in the same way


Determination of a molecular formula

Step 1: Convert % by mass into moles of atoms
Step 2: Find the smallest whole-number ratio and empirical formula
Step 3: Find the relative mass of empirical formula
Step 4: Divide the given relative molecular mass of compound by the empirical mass
Step 5: Multiply the empirical formula by that factor


What are anhydrous and hydrated salts?

- All solid salts have a lattice structure made of ions. In some salts, water is incorporated in the lattice.
- When water is in a lattice, it is called water of crystallisation.
- This is shown in formula with a large dot between the compound formula and water units.
- A solid salt containing water of crystallisation is hydrated.
- A salt is anhydrous if it doesn't contain water of crystallisation.


Finding the formulas of hydrated salts

Many hydrated salts lose their water of crystallisation when heated, to become anhydrous.
Step 1: Find the mass of water lost by subtracting the mass of anhydrous from hydrated.
Step 2: Find the moles of water lost.
Step 3: Find the moles of anhydrous formed.
Step 4: Find the smallest whole number ratio of anhydrous salt and water.


PAG 1: Formula of hydrated salt

1. Weight empty crucible.
2. Add hydrated salt and record the weight of crucible + hydrated salt.
3. Use pipe-clay triangle to support the crucible containing the hydrated salt on a tripod. Heat the crucible and contents gently for about 1 minute. Then heat it strongly for further 3 mins.
4. Leave crucible to cool. Then weigh the crucible and anhydrous salt.


Assumptions made when calculating hydrated salt formula

Assumption 1: All of the water has been lost.
Assumption 2: No further decomposition occurred after heated.