Calculations with chemical formulas and equations Flashcards
Molecular Mass
- Sum of the atomic masses of all the atoms in a molecule of the substance.
- Molecular mass of ethylene, C2H4
- Carbon: 2 • 12.01 amu = 24.02 amu
- Hydrogen: 4 • 1.008 amu = 4.032 amu
- Total: 28.05 amu
- Mass spectrometer is used to determine molecular mass.
Formula mass
- The sum of all the atomic masses of all atoms or ions present in a formula unit of any compound, whether molecular or not.
Formula mass of Na2SO4
- Sodium: 2 • 22.99 amu = 45.98 amu
- Sulfur: 1 • 32.07 amu = 32.07 amu
- Oxygen: 4 • 16.00 amu = 64.00 amu
- Total: 142.1 amu
Mole
the quantity of a given substance that contains as many molecules or formula units as the number of atoms in exactly 12g of carbon-12
Avogadro’s number
- The number of atoms in a 12 g sample of carbon-12
- Named after the Itailian scientist, Amadeo Avagadro (1776-1856)
- 6.0221367 x 1023 equals to 1 mole.
Mole Calculations
- enables to inter-convert from moles to grams, grams to moles and find the number of molecules in a given mass.
- molar mass = mass/moles
Percent Composition
- Mass percentage of each element in the compound.
- Proportion of the constituent elements expressed as the number of grams od each element per 100 grams of the compound.
- Knowing the total mass, we can determine the contribution of each element to the total
Mass Percentage from the Formula
parts of A per hundred parts of the total, by mass
- Steps
- Find Molar mass
- Divide total mass of each element in the formula by the molar mass and multiply by 100.
Emperical Formula
- Also known as simplest formula
- smallest whole number ration of atoms present in a compound.
- Sometimes the empirical formula and the molecular formula is the same, but most of the time, the molecular formula is a whole number multiple of the empirical formula.
Empirical Formula from the composition
- Either from the masses of elements or from the percent composition.
- steps
- assume 100.0 g of the substance
- convert the gram of each element into moles using each element’s molar mass
- divide each value by the smallest value
- if whole numbers, use them as subscripts
- if not whole numbers, multiply by the smallest number that would convert them to whole numbers.
What is the simple rhyme to remember to determine empirical formula.
- Percent to mass
- mass to mole
- divide by small
- multiply until whole.
Molecular formulas from empirical formula
- Molecular mass = n x (empirical formula mass)
Stoichiometry
- Calculation of the quantities of reactants and products involved in a chemical reaction.
- Quantitative relationship among reactants and products
Molar interpretation of chemical equation
- A chemical equation can be interpreted in terms of molecules, ions, formula units and moles.
- 2H2 + O2 → 2H2O can be interpreted as
- 2 moles of H2 + 1 mole of O2 → 2 moles of H2O
- 2 grams of H2 + 1 gram of O2 → 2 grams of H2O
Mole ratio
the ratio between the number of moles of any two species in a chemical reaction.
Limiting Reactant
- Also known as Limiting reagent
- Limits the amount of product that can be formed
- It is completly used up when a reaction is complete
- The moles of products are always determined by the starting moles of the limiting reagent.
- To determine the limiting reagent, use stoichiometry to find the product for each of the element. The smallest amount is the limiting reagent.
Theoretical Yield
Maximum amount of product that can be obtained from given amounts of reactants according to the chemical equation.
Actual Yield
Measured mass of the product
Note: actual yield may be less than the theoretical yield because of:
- Many reactions do not go to completion
- Two or more reactions may occur simutaneously producing desired and undesired products
- Accurate amount of product may be difficult to measure
- starting materials may not be pure.
Percentage Yield
Ratio of actual yield to theoretical yield and multiplied by 100
Both the actual and theoretical yield must have the same units to find percentage
