Chemistry Counting Particles too small to see

John Dalton ( ) Proposed Atomic Theory Each element is made up of tiny, indivisible atoms. Different elements have different atoms. Compounded substances (compounds) are composed of fixed groupings of atoms called molecules (“little lumps of matter”)

Joseph Louis Gay-Lussac ( ) Discovered that gases at constant pressure and temperature always combine in definite volume ratios. 1 Liter of hydrogen gas + 1 Liter of chlorine gas always produced 2 Liters of hydrogen (mono)chloride. 2 Liters of hydrogen gas + 1 Liter of oxygen gas always produced 2 Liters of water vapor

Joseph Louis Gay-Lussac ( ) Concluded equal volumes of gas must have the same number of particles. 1 Liter of hydrogen gas has the same number of hydrogen particles as 1 Liter of oxygen gas. 2 Liters of water vapor has 2X the number of particles as 1 Liter of oxygen gas.

Consequences of Gay- Lussac’s Findings How is it possible to get two volumes (liters) of product when you only started with one volume (liter) of reactants? But the results continued to yield two volumes of gas C Instead of one Should Equal 5 atoms of gas A 5 atoms of gas B 5 molecules of gas C

Amedeo Avogadro ( ) Explained Lussac’s findings by: Realizing that some gas particles are actually diatomic molecules instead of atoms. Diatomic means they are made up of two of the same atoms bonded together. Equal 5 molecules of gas A 5 molecules of gas B 10 molecules of gas C

Empirical Formula Based on the ratio of volumes of reactants and products, a chemical formula for the product can be deduced 2 volumes of hydrogen combine with 1 volume of oxygen to produce 2 volumes of water Thus, the formula for water must be H 2 O.

Relative Mass of Atoms Scientists also noticed that gases combined in specific mass proportions. Two volumes of hydrogen gas (each with a mass of 0.5g) always combined with one volume of oxygen gas (with a mass of 8g) Thus, oxygen’s mass relative to hydrogen is

This means: 1 volume of oxygen has the same number of particles as 1 volume of hydrogen, but the mass of the volume oxygen is 16 times greater than the mass of the volume of hydrogen. If the relative mass of hydrogen is 1, then the relative mass of oxygen would be 16. Relative Mass of Atoms

The Mole Weighable amount of an element or compound. If the relative mass of hydrogen is 1 and the relative mass of oxygen is 16, then H 2 O molecule would have a relative mass of 18. Relative mass of 1 Relative mass of 1 Relative mass of 16

Molecular Mass The mass of the smallest unit of an element or compound. Determined by the mass of the elements in the formula and the number of atoms. Measured in Atomic Mass Units (amu) because it is more convenient than grams The molecular mass of H 2 O is 18 amu

Molar Mass The number of grams equivalent to the molecular mass of an element or compound. 1amu is equivalent to 1gram If the molecular mass of H 2 O is 18amu, then the molar mass of H 2 O is 18 grams. 1 Molar mass of a substance = 1 mole of substance.

Number of Particles in a Mole Thus, one molar mass (mole) of any substance (element or compound) has the same number of particles. 1 gram of substance = x amu. This means there are x particles of a substance in one mole of that substance.

1 mol of a substance = 1 molar mass 1molar mass = x particles The molar mass of the substance is equal to the molecular mass of the substance determined by the chemical formula for the substance. How Much?