Topic 3 Mathematics of formulas and equations:



Topic 3 Mathematics of formulas and equations:

• As we enter the topic of math and chemistry – round all calculations to one decimal place before multiplying and adding unless otherwise instructed.

• WE know that one of the measurements used to describe substances is atomic mass.

o Atoms of different elements have different masses

o Atomic mass unit (amu) is used to describe the mass of an atom

o a system devised to express masses on a relative scale

▪ based on an experimental standard

▪ based on the mass of carbon-12

▪ atomic mass is based upon the mass of an atom expressed relative to the mass of carbon -12

How do you find the MASS of an entire molecule of a compound?

➢ H2O

➢ 2 hydrogens 1.0 amu 2(1.0) = 2.0 amu

➢ 1 oxygen 16.0 amu 1(16.0) = 16.0 amu

➢ 2.0 amu + 16.0 amu = 18.0 amu = formula mass for H2O

The sum of the atomic masses of all the atoms in a compound = FORMULA MASS.

Example : acetic acid = vinegar

HC2H3O2

What is the formula mass for vinegar?

H 4 atoms x 1.0 amu = 4.0 amu

C 2 atoms x 12.0 amu = 24.0 amu

O 2 atoms x 16.0 amu = 32.0 amu

60.0 amu

Formula mass of a sample should be greater than the atomic masses of any of the individual elements.

Example:

H2CO3 carbonic acid – carbonated drinks

H 2 atoms x 1.0 amu = 2.0 amu

C 1 atom x 12.0 amu = 12.0 amu

O 3 atoms x 16.0 amu = 48.0 amu

62.0 amu

What is a mole?

• We know that the gram is the desired unit of mass in the lab

• A way to relate the atomic mass of one atom of an element in amu to an amount of that element in grams is seen in the following:

o Need to find the number of atoms in a given mass of an element – we use carbon as a standard.

o it was determined experimentally that the number of atoms of an element whose mass in grams is numerically equal to 6.02 x 10 23 = called a mole ( mol)

A mole of any element is defined as the number of atoms of that element equal to the number of atoms in exactly 12.0g of carbon-12.

6.02 x1023 H atoms in 1 g of hydrogen

6.02 x1023 O atoms in 16g of oxygen

6.02 x1023 Pb atoms in 207 g of lead

Mole establishes the relationship between AMU and the gram.[pic]

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