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Updated Mar 26, 2026

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2 pages

Understanding Basic Stoichiometry

Mira

@miraa._noui

Chemistry comes alive when we understand how atoms interact and... Show more

# Stoichiometry

* C/12-carbon: STANDARD 12 amu, all other atomic masses are RELATIVE

→ mass spectrometer:

* mass of atoms/molecules, pa

Atomic Mass and The Mole

Atoms are incredibly tiny, but we can measure them using relative mass. Carbon-12 is our standard at exactly 12 atomic mass units (amu), with all other elements measured in comparison. Scientists use mass spectrometers to determine these masses by ionizing atoms, accelerating them through an electric field, and measuring their deflection.

When you look at the periodic table, the atomic masses shown are actually weighted averages of all naturally occurring isotopes. This average is calculated using the percent abundance of each isotope. The mole connects these microscopic measurements to the macroscopic world we can observe.

One mole contains exactly 6.022 × 10²³ particles (Avogadro's number) and has a mass equal to the atomic or molecular weight in grams. For example, one mole of carbon weighs 12.01 grams, while one mole of water (H₂O) weighs 18.02 grams. Remember that some elements naturally exist as diatomic molecules (Br₂, I₂, N₂, Cl₂, H₂, O₂, F₂).

Quick Tip: When determining empirical formulas from combustion analysis, follow a systematic approach: calculate the mass of each element, convert to moles, find the simplest whole-number ratio, and then scale up to the molecular formula if needed.

Chemical Reactions and Stoichiometry

Chemical reactions occur when atoms rearrange to form new substances. This process involves breaking bonds (which requires energy) and forming new ones (which releases energy). When balancing chemical equations, remember that the number of atoms of each element must be the same on both sides.

Certain reaction types follow predictable patterns. In combustion reactions, hydrocarbons combine with oxygen to produce carbon dioxide and water. Metal carbonates decompose to form metal oxides and carbon dioxide. For organic compounds, knowing the general formulas helps: alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), and alkynes (CₙH₂ₙ₋₂).

The limiting reactant in a chemical reaction determines how much product can actually form. To identify it, calculate the theoretical yield of product from each reactant - the one producing less product is limiting. The difference between theoretical yield (calculated from stoichiometry) and actual yield (measured in lab) gives us percent yield: $actual/theoretical$ × 100.

Remember This: In mass spectrometry, all particles enter the deflection chamber with the same kinetic energy, but their different mass-to-charge ratios result in different deflection paths. This principle allows us to identify elements and determine isotope abundances.

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AP Chemistry

•

Updated Mar 26, 2026

•

2 pages

Understanding Basic Stoichiometry

Mira

@miraa._noui

Chemistry comes alive when we understand how atoms interact and combine. This summary explores atomic mass, moles, chemical formulas, and reaction calculations - essential concepts that help explain everything from why compounds form to how we measure chemical reactions in... Show more

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Atomic Mass and The Mole

Quick Tip: When determining empirical formulas from combustion analysis, follow a systematic approach: calculate the mass of each element, convert to moles, find the simplest whole-number ratio, and then scale up to the molecular formula if needed.

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Chemical Reactions and Stoichiometry

Remember This: In mass spectrometry, all particles enter the deflection chamber with the same kinetic energy, but their different mass-to-charge ratios result in different deflection paths. This principle allows us to identify elements and determine isotope abundances.