DOH124 Lecture Notes - Lecture 3: Molecular Mass, Chemical Formula, Atomic Number

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30 Jun 2018
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Percent composition
- Percent composition is the percentage of an element that is a particular mass.
- This is used to work out the atomic mass of an element.
- The atomic mass of an element is:
Atomic mass = (mass of isotope1)(fraction of isotope1) + (mass of
isotope2)(fraction of isotope2) + ….
Law of multiple proportions
- If two elements A and B combine to form more than one compound, the masses
of B that can combine with a given mass of A are in the ratio of small whole
numbers.
- Water- 8.0 g of oxygen combines with 1.0 g of hydrogen
- Hydrogen peroxide: 16.0 g of oxygen combines with 1.0g of hydrogen.
- Ratio of mass of oxygen per gram of hydrogen in the two compounds is 2:1.
- Hydrogen peroxide contains twice as many atoms of oxygen per hydrogen atom
as water.
How do we figure out how much of a substance is a mole?
- If two elements A and B combine to form more than one compound, the masses
of B that can combine with a given mass of A are in the ratio of small whole
numbers.
- Water- 8.0 g of oxygen combines with 1.0 g of hydrogen
- Hydrogen peroxide: 16.0 g of oxygen combines with 1.0g of hydrogen.
- Ratio of mass of oxygen per gram of hydrogen in the two compounds is 2:1.
- Hydrogen peroxide contains twice as many atoms of oxygen per hydrogen atom
as water.
Periodic table
- Elements arranged in increasing atomic number
- Arranged in groups (with similar chemical properties) and periods (horizontally)
- Contains masses of information
Molecules and Molecular Compounds.
- Only the noble gases are normally found as isolated atoms.
- Most matter consists of molecules or ions.
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Document Summary

Percent composition is the percentage of an element that is a particular mass. This is used to work out the atomic mass of an element. The atomic mass of an element is: Atomic mass = (mass of isotope1)(fraction of isotope1) + (mass of isotope2)(fraction of isotope2) + . If two elements a and b combine to form more than one compound, the masses of b that can combine with a given mass of a are in the ratio of small whole numbers. Water- 8. 0 g of oxygen combines with 1. 0 g of hydrogen. Hydrogen peroxide: 16. 0 g of oxygen combines with 1. 0g of hydrogen. Ratio of mass of oxygen per gram of hydrogen in the two compounds is 2:1. Hydrogen peroxide contains twice as many atoms of oxygen per hydrogen atom as water. Arranged in groups (with similar chemical properties) and periods (horizontally) Only the noble gases are normally found as isolated atoms. Most matter consists of molecules or ions.

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