photo.jpg

## Chemical Reactions, Dalton's Atomic Theory and Atomic Mass ### 1L of Nitrogen Gas Combines with 3L of Hydrogen Gas - In the reaction: *Nitrogen (g) + Hydrogen (g) → Ammonia (g)* - 1L of Nitrogen (1 Vol) combines with 3L of Hydrogen (3 Vol) to produce 2L of Ammonia (2 Vol). - This is a ratio...

## Chemical Reactions, Dalton's Atomic Theory and Atomic Mass ### 1L of Nitrogen Gas Combines with 3L of Hydrogen Gas - In the reaction: *Nitrogen (g) + Hydrogen (g) → Ammonia (g)* - 1L of Nitrogen (1 Vol) combines with 3L of Hydrogen (3 Vol) to produce 2L of Ammonia (2 Vol). - This is a ratio of 1:3:2 ### Gay-Lussac’s Law - The Law of Definite Proportions applies to the gaseous volumes in this reaction. - In other words, the volumes of gases reacting and produced show a definite ratio, even when in the gaseous state. **Can you tell?** - If 10 volumes of dihydrogen gas react with 5 volumes of dioxygen gas, how many volumes of water vapor would be produced? - 10 volumes of Hydrogen + 5 volumes of Oxygen -> 10 volumes of water ### Avogadro's Law - Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. - The reaction of hydrogen and oxygen to produce water vapor: - 2 volumes of Hydrogen gas + 1 volume of Oxygen gas ->2 volumes of water vapor gas. ### Dalton's Atomic Theory (Proposed in 1808) - Matter consists of tiny, indivisible particles called atoms. - All the atoms of a given element have identical properties including mass. - Compounds are formed when atoms of different elements combine in a fixed ratio. - Chemical reactions involve only the reorganization of atoms. Atoms are neither created nor destroyed in a chemical reaction. **Can you recall?** - **What is an atom and a molecule?** - **What is the order of magnitude of the mass of one atom?** - **What are isotopes?** ### Atomic Mass - The term “atomic mass” refers to the mass of an atom. It is measured in atomic mass units (amu). - 1 amu is defined as one-twelfth of the mass of a carbon-12 atom. - The mass of a carbon-12 atom is 1.66056 x 10^-24 g. ### Unified Atomic mass Unit (Da) - Da represents the unified mass unit, which is equivalent to amu. **Problem 1.1:** The mass of an oxygen atom is 26.56896 x 10^-24 g. What is the mass of oxygen in u? - Solution: 26.56896 x 10^-24 g / 1.66056 x 10^-24 g/u = ~16u - The mass of an oxygen atom is 16 u ### Other Information - The molecular mass of water is 18 u (2 H + 1O). - The molecular mass of oxygen is 32 u (2 O). - The molecular mass of Hydrogen is 2 u (2 H).