# Chemistry C2 3.2 Masses of atoms and moles.

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What do we use to compare the masses of atoms?
Relative atomic masses.
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What is the relative atomic mass of an element?
An average value for the isotopes of an element.
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How do e work out the relative formula mass of a compound?
By adding up the relative atomic masses of the elements in it.
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What is one mole of any substance?
Its relative formula mass in grams.
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Why do we use relative atomic masses (A_r) in calculations?
Because atoms are much too small to weigh.
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Where are relative atomic masses often shown?
In the periodic table.
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In the laboratory, what do we usually weigh substances in?
Grams (g).
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What is the relative atomic mass of an element in grams called?
One mole of atoms of the element.
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The relative atomic mass of an element (A_r) is an average value that depends on what?
The isotopes the element contains, however when rounded to a whole number it is often the same as the mass number of the main isotope of the element.
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What do we use an atom of 12 over 6 C for?
As a standard atom and compare the masses to of all other atoms with this.
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How is the relative formula mass (M_r) of a substance found?
By adding up the relative atomic masses of the atoms in its formula.
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What is a mole of a substance?
The relative formula mass of a substance in grams.
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Why is using moles of substances useful?
It is useful when we need to work out how much of a substance reacts or how much product we will get.
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What is the mass of one mole of sodium atoms?
23g.
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Why is the relative atomic mass of chlorine not a whole number?
It has (two main) isotopes and the relative atomic mass is an average value.
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Calculate the relative formula mass (M_r) of sodium sulfate, Na_2SO_4. (Relative atomic masses: Na=23, S=32, O=16).
(23x2)+32+(16x4)=142.
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What is the mass of one mole of magnesium carbonate, MgCO_3? (Relative atomic masses: Mg=24, C=12, O=16).
24+12+(16x3)=84g (must have g).
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**MAKE SURE THAT YOU CAN CALCULATE THE RELATIVE FORMULA MASS (M) OF A COMPOUND FROM ITS FORMULA**
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## Other cards in this set

### Card 2

#### Front

What is the relative atomic mass of an element?

#### Back

An average value for the isotopes of an element.

### Card 3

#### Front

How do e work out the relative formula mass of a compound?

#### Back ### Card 4

#### Front

What is one mole of any substance?

#### Back ### Card 5

#### Front

Why do we use relative atomic masses (A_r) in calculations?

#### Back 