Quantative Chemistry

Question 1

Relative atomic mass ( RAM, Ar )

Answer 1

The mass of the atom compared to the mass of carbon-12

Question 2

Relative formula mass (RFM, Mr)

Answer 2

The total value of all the Ar in a given substance

Question 3

The mole

Answer 3

Mass/Mr

Question 4

How to calculate the % of an element in a compound by mass?

Answer 4

% of element x = no. Of atoms of element x in compound x Ar / Mr of compound

Question 5

Interchanging Mr and Moles

Answer 5

The mass of a mole of a substance is equal to the substances Mr In grams
E.g Mr of H2O is 18, so one mol of water = 18g

Question 6

How to work out the Mr of a substance

Answer 6

Add up all the Ar of elements contained in the substance
E.g Mr of water = (H x2 =1x2) + (O = 16) = (2x1) + 16 = 18
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Mr = mass/mol
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The mass of moles a substance is equal to its Mr in grams, so if an unknown substance has 21g of mass and 0.25 moles, then one mole = 84g = Mr 84

Question 7

Heating to constant mass

Answer 7

Heating to constant mass means you heat the substance for a tracked amount of time, and record the mass at each interval e.g every two minutes. You continue to do this until the mass does not change; this is heating to constant mass.