# Chemistry

Topics: Chemical element, Neutron, Atomic number Pages: 3 (765 words) Published: April 9, 2013
Isotopes and average atomic mass
Isotope: element variations with different atomic mass but same atomic number Isotopic Abundance: the relative amount in which each isotope of an element Calculating: given 2 isotopes of an element [B: 10.01u] [B: 11.01u] 1) Given average atomic mass: 10.81u

If given percentages, you can find average atomic mass by adding each portion 2) Set variables for unknown values
Let x represent % abundance for 10/5 B
Let 1-x represent % abundance for 11/5 B
Calculate:
10.01x+(1-x)11.01=10.81
10.01x+11.01-11.01x=10.81
-1.00x=0.2
X=0.2
Mole: the SI unit that is used to measure how much of a substance. (balanced eqn coefficient are moles) Converting Moles -> number of particles
Number of Moles = number of particles / Avogadro’s constant Avogadro’s constant: 6.02214179 x 10^23
Converting Moles to Mass/molar mass
Molar Mass: how much grams per mol of an element = atomic mass Number of Moles = mass / Molar Mass
Percentage Composition
Laws of definite proportions: a law stating that a chemical compound always contains the same proportion of elements by mass Mass percentages (H in H2O) = molar mass of H (1.01) / molar mass of H2O (18.02) = 11.2% Empirical Formula

A formula that shows the smallest whole number ratio of elements within a compound Lactic Acid: a chemical name
Empirical Formula: CH2O
Molecular Formula: C3H6O3
Calculations: when given percentage composition of different elements 1) Find the number of moles for each element (assume % is per 100g for simplicity) 2) Divide all the moles by the smallest number mole to reduce the decimals 3) If not whole numbers yet, multiply by some special factors to reduce decimals 1.25 x ¼ = 1 Al2(SO4)3 + 3 Cu  :  2:3:1:3 ratio

To determine amounts, use ratio and plug in numbers correctly Limiting/excess reactants
Limiting:Reactants that are not completely used up in a reaction When this reactant is used up, the reaction stops
Excess Reactants:the...