Chemistry unit 5 test review
how to determine empirical formula?
Convert the mass of each element to moles using the molar mass from the periodic table. Divide each mole value by the smallest number of moles calculated. Round to the nearest whole number. Or if it is a .5 then multiply all whole numbers by 2. This is the mole ratio of the elements and is represented by subscripts in the empirical formula.
percent composition
Definition: the percent by mass of each element in a compound. Formula: % composition = total mass element/total mass compound(molar mass) x100
molecular formula (ALWAYS GIVEN)
a chemical formula of a molecular compound that shows the kinds and numbers of atoms present in a molecule of a compound. X= molecular formula mass(molar) / empirical formula mass
empirical formula
a formula with the lowest whole-number ratio of elements in a compound. (Simplest ratio)
Hydrates
compounds that have a specific number of water molecules attached to them. It is written by adding the formula for water to the end of the compound formula, separated by a dot. The water coefficient indicates the number of water molecules included per formula unit or the compound.
Naming a hydrate
ionic compound + prefix hydrate
molar mass of a compound
the sum of molar masses of the elements it contains. EXAMPLE: Sodium bicarbonate NaHCO3 Na 1x 23.0 H 1x 1.01 C 1x 12.0 O 3x 16.0 answer: 84.0 g/mol
Determining the formula of a hydrate
**salt is always the smallest number** Determine moles of salt and water. Example: a hydrate of aluminum bromide contains 71.16% AlBr3 and 28.84% H2O. What is the formula of the hydrate? AlBr3 • ___H2O 71.16g AlBr3 • 1mol/267g = 0.267 mol = 1 0.267 28.84g H2O • 1mol/18.0 g =1.60 mol = 6 0.267 AlBr3 • 6H2O
Finding the % of H2O in a hydrate
1. Find the mass of H2O 2. Mass of hydrate 3. Divide the mass of water by the hydrate. % H2O = mass H20/ mass Hydrate x100 Example: CuSO4 • 5H2O. 5(18.0g) = 90.0 g H2O Cu 1x 63.5 S 1x 32.1 = 160 salt + 90.0 = 250 hydrate O 4x 16.0 90.0 / 250g x100 = 36.0%
