How Much? C2.3

What is an Isotope?


An Isotope is a variant of an element with a different no. of neutrons. This means the atomic mass changes.

What Changes?

The number of Neutrons

What Stays the Same?

The number of protons

What would happen if the no. of protons were to change?

The atom would change to another element

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Relative Atomic Mass & Relative Formula Mass

RAM = the mass of an atom compared to 1 atom of carbon

The Relative Atomic Mass of Carbon is 12. The relative Atomic Mass of Hydrogen is 1.

RFM= The sum of all the the relative atomic masses added together

For Example:


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1 Mole = RAM

1 Mole of Carbon = 12g

1 Mole of Zinc= 65g

Moles within a Mass

No. of Moles= Mass of Element/Compound divided by RFM of Element/Compound

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Percentage Mass

Percentage Mass is used to show the percntage of an element within a Compound.

This is worked out by:

(RAM x No. of atoms of element) divided by (RFM OF Whole Compound)


Ex. Oxygen in CO2

(16x2)/44= 0.727...

x100 = 72.7%

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Moles Within A Mass

No. of Moles= Mass of element/compound Divided by Mr of element or compound

No. Of moles in 42g of carbon = 42/12= 3.5 moles

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Empirical Formula

You can use information about reacting masses to calculate the formula of a compound.


Suppose 3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?

Use the fact that the Ar of sulfur is 32 and the Ar of oxygen is 16.

stepactionSO 1 find masses 3.2 3.2 2 look up given Ar values 32 16 3 divide masses by Ar 0.1 0.2 4 find the ratio 1 2

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Percentage Yield

In real experiments, 100% of all the chenicals do not react; some alway get left behind. Percentage Yield is the percentage of a product that is produced as a result of a certain reaction.

Percentage Yield = (actual yield / predicted yield) x100

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Balancing Equations

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