# Chemistry unit 2 topic 3

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Mass & atomic no.
Mass number – top number - total number of protons and neutrons in an atom. Atomic number - bottom number - number of protons
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Isotopes
Atoms of the same element with different numbers of neutrons are called isotopes.
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Ars
relative atomic masses, the Ar of an element in grams is called one mole of atoms of the element. E.g – Ar of Ca is 40
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Mrs
relative formula mass of a substance found by adding up the relative atomic masses in its formula e.g – Mr of CaCl2 = 40+ (35.5X2)= 111
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What is the mass of one mole of Soduim hydroxide? Ars: Na=23, O=16, H=1?
40g
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Percentages
What is the percentage of carbon in CO2? – Ars: C=12, O=16. Mr of CO2 = 12 + (16X2) =44. (12/24)X100 = 27.3%
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Empirical formula
simplest ratio of the atoms or ions in a compound. In covalent compounds it is not always the same as the molecular formula (ethane C2H6, but empirical formula CH3) n=m/Mr – number of moles =mass / relative atomic mass
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What is the empirical formula of the hydrocarbon that contains 80% carbon?
-
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% Yield – actual mass / theoretical mass X 100
reasons for not getting 100% yield – reaction incomplete / reversible, reactant impure, product lost on equipment or to surroundings, side reactions.
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## Other cards in this set

### Card 2

#### Front

Atoms of the same element with different numbers of neutrons are called isotopes.

Isotopes

### Card 3

#### Front

relative atomic masses, the Ar of an element in grams is called one mole of atoms of the element. E.g – Ar of Ca is 40

### Card 4

#### Front

relative formula mass of a substance found by adding up the relative atomic masses in its formula e.g – Mr of CaCl2 = 40+ (35.5X2)= 111

40g