# Topic 2: Amount of Substance Hints

Hints for Topic 2, Amount of substance for AQA AS Chemistry.

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• Created by: M
• Created on: 15-10-11 22:58

## 2.1: Relative atomic and molecular masses, the Avo

1) The weighted average mass must be used to allow for the presence of isotopes, using their percentage abundance in calculations.

2) The word ‘entity’ is just a general word for a particle. It can refer to an atom, molecule, ion, or even an electron. It also refers to the simplest formula unit of a giant ionic structure.

3) We use the term ‘molar mass’ which is the mass per mole of substance. It has the units: kg mol^-3 or g mol^-3. The molar mass in g mol^-3 is the same numerically as Mr.

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## 2.2: The ideal gas equation

1) To convert degrees C to K add 273.

2) In the ideal gas equation, P must be in Pa, V must be in m^3 and T must be in K.

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## 2.3: Empirical and molecular formulae

1) If working in grams, the mass of 1 mole in grams is the same as the relative atomic mass of the element.

2) When calculating the empirical formula from percentages, check that the percentages add up the 100%.

3) Use relative atomic masses from the periodic table, not the atomic number.

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## 2.4: Moles in solution

1) To get a solution with a concentration of 1 mol dm^-3, must add the solvent to the solute until you have 1 dm^3 of solution.

2) 1 dm^3 = 1000 cm^3 = 1L

3) Small negative in “mol dm^-3” means ‘per’.

4) The error margin in a single measurement of an instrument is generally half the value of the smallest division.

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## 2.5: Balanced equations and related calculations

1) To convert m^3 to cm^3, multiply by 10^6.

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