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How many atoms are in a gram of carbon dioxide? Why do $1\ \text{g}$ of hydrogen and $16\ \text{g}$ of oxygen hold exactly the same number of atoms? How long would it take to count the molecules in one drop of water? In this workshop you learn two strategies, and then you solve problems that are a step harder than the chapter, with Python to check your work.

What's inside

1. Strategy: the mole map: Every mole problem uses the same three places: mass, moles and number of particles. Moles are always in the middle. Mass to moles is division by the molar mass, and moles to particles is multiplication by the Avogadro constant. The four steps are to write what you have and want, go to moles, move along the map, and count the atoms in a molecule last. The mass of one molecule is the molar mass divided by the Avogadro constant. Checking the size of the answer finds most mistakes.
2. Strategy: percent composition and mass ratios: The percent composition of a compound is the mass of each element divided by the molar mass, times 100. The ratio of the masses of the elements in a compound is the same in every sample.
3. Strategy: from percentages to a formula: A laboratory finds the percentage by mass of each element in a new compound. To turn the percentages into a formula, take 100 g, change each mass into moles, divide by the smallest number of moles, and make the ratio whole. This gives the simplest formula. The molar mass then tells how many times to multiply it to get the real formula.