Stoichiometry - Formulae (3)
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1.
The empirical formula of a compound gives the simplest whole number ratio of atoms present in the compound. Explain what this statement means in terms of the composition of a chemical compound. Give an example to illustrate your answer.
The statement means that the empirical formula represents the fewest whole number of atoms of each element that are present in a compound. It's the simplest ratio. This is determined by dividing the number of atoms of each element in the compound by the greatest common divisor (GCD).
Example: Hydrogen peroxide (H2O2). The ratio of hydrogen to oxygen is 2:2, which simplifies to 1:1. Therefore, the empirical formula is HO. This indicates that for every one molecule of hydrogen peroxide, there is one hydrogen atom and one oxygen atom.
2.
The charges on the ions in an ionic compound are given as Na+ = +1 and Cl- = -1. The diagram below shows a model of the compound. Determine the chemical formula of the compound.
Diagram: (Imagine a diagram here showing 2+ cations and 2- anions. For example, two large yellow circles representing Na+ and two small blue circles representing Cl-.)
Answer: The chemical formula of the compound is NaCl. This is deduced from the diagram by observing the relative numbers of the ions and their charges. There is one sodium ion (Na+) for every one chloride ion (Cl-) to ensure the overall charge of the compound is neutral. (1 x +1 = 1 and 1 x -1 = -1, so 1 - 1 = 0. Therefore, the formula is NaCl.)
3.
State the formulae of the following compounds:
- Water
- Carbon Dioxide
- Ammonia
- Sodium Chloride
- Calcium Oxide
Water: H2O
Carbon Dioxide: CO2
Ammonia: NH3
Sodium Chloride: NaCl
Calcium Oxide: CaO