Synthesis reaction - two substances (generally elements) combine and form a compound.
Ionic compounds as products are always solids.
Decomposition reaction - a compound breaks up into the elements or in a few to simpler compounds.
Combustion reaction - occur when a hydrocarbon reacts with oxygen gas (also known as burning).
Fire triangle: a fuel (hydrocarbon), oxygen to burn it with, spark to ignite it.
Products in combustion are always carbon dioxide and water (although incomplete burning does cause some by-products like carbon monoxide).
Don't forget to re-check the equation at the end, as amounts may have shifted.
Formula weight (FW) - sum of the atomic weights for the atoms in a chemical formula.
Molecular weight (MW) - the sum of the atomic weights of the atoms in a molecule.
Ionic compounds form three-dimensional crystal structures, so they only have empirical formulas and formula weights, not molecular formulas or molecular weights.
Molar mass - mass of 1 mol of a substance.
This is the mass listed on the periodic table (e.g. carbon has a mass of 12.01 g/mol, also 12.01 amu).
Avogadro's Number is ~6.022 * 10^23
It's the number of particles in 1 mol.
A mol is a unit like a dozen, it's a measure of items of a type.
1 mol H2O contains 2 mol hydrogen and 1 mol oxygen
Moles are used to convert between grams and number of atoms or molecules of a substance — they're the connector unit.
Learn by example:
A compound of B and H is 81.10% B. What is its empirical formula?
When given percentages, use 100g to calculate
81.10g B * (1 mol / 10.81g) = 7.502 mol B
100 - 81.10g H * (1 mol / 1.01g) = 18.713 mol H
Divide each calculated result by lower number of moles:
7.502 / 7.502 = 1
18.713 / 7.502 = about 2.5 (you'll have to round, there's always experimental error)
Then, multiply to get a whole-number empirical formula from that ratio
For this you need the molecular mass, which is found through experimentation.
Then, take the mass of the empirical formula (e.g. C2H3 = 27 g/mol) and compare with molecular mass (e.g. 81 g/mol), to find how much you need to multiply the empirical formula by to get the molecular formula (e.g. 81/27 = 3, so molecular formula is C6H9).
A hydrate is a salt that when crystallized forms an aqueous solution that incorporates a fixed amount of water in its crystalline matrix, even while appearing dry.
The number of moles of water present per one more of anhydrous salt is usually a simple whole number.
in these problems, you are given the mass of the hydrocarbon, and resulting masses of CO2 and H2O.
C is determined by calculating moles of CO2 produced.
H is determined by calculating moles of H2O produced, then multiplying by two (because it's H2, there's 2 mol H per 1 mol H2O).
If the hydrocarbon is of the form CH_O or CH_N, you can calculate the mass of the O or N by subtracting the masses of C and H from the mass of the hydrocarbon.
Then, find the moles of O or N, and find the empirical formula by dividing moles by the lowest number of moles (like usual) and finding the ratio.
The limiting reactant is the one that will be used up first in the reaction, and restricts the amount of product that can be created.