Unit 5: Stoichiometry (Into to the mole) Flashcards
A mole is
AKA Avogadro’s Constant
is a chemist’s unit of counting.
- 1 mole = 6.02x10^23 items
Molar Mass is
the mass of ONE MOLE or the mass of 6.02x10^23 items of that substance
- Symbol= M
- Unit= gram/mole
- the molar mass of an atomic element is its mass that’s found on the periodic table
When calculating problems involving mass, make sure that….
the mass is in grams
1 tonne x 1000 = kg
1kg x 1000 = g
1g x 1000 = mg
Law of Constant Composition
A compound contains elements in CONSTANT PERCENTAGES, regardless of how the compound is prepared or where it is found in nature.
- Equation: % composition= (mass of element [atomic mass x number of atoms] / total mass of compound) x 100%
Molecular Formula
a formula that indicates the actual numbers of atoms in one molecules of a compound.
- Ex. C8H18
Empirical Formula
the simplest ratios of atoms, or the simplified formula.
- This might not be the actual formula of a compound
Molecular & Empirical formulas in Ionic & molecular compounds
In ionic compounds, the empirical & molecular formula is the same (rmr criss cross)
In molecular compounds, the empirical and molecular compound can be different. (cuz u don’t criss cross and simplify)
In chemistry, do the mass ratios matter or do the mole ratios matter?
mole ratios
A hydrate is
an ionic compound that contains water molecules in its structure
–> when many salts crystallize out of aqueous solution, they incorporate water molecules in a fixed ratio into their ionic crystal lattice
- the water in the crystal doesn’t usually present a problem as most salts are destined for aqueous solutions anyway.
- Water is an integral part of hydrates and must be accounted for in both the name and the formula (SEE PAGE 22 TO SEE HOW)
An anhydrate is
the substance that remains after the water is removed from a hydrate
–> When a hydrate is heated, the water molecule are driven off as steam, leaving behind the water-free anhydrate
- some anhydrous salts are hygroscopic, meaning that they can absorb water from the air to form hydrates
Desiccants are
hygroscopic salts that are being used to keep the air dry in a container
- Ex. Silicate salts pouches are sometimes are used as desiccants in boxes or cases with binoculars, guitar, shoe, etc. –> think about the pouches found in seaweed that is used to stop the seaweed from becoming soggy
Combustion Analysis
Used to determine the empirical & molecular formulas for compounds that only carbon & hydrogen (CxHy) & other elements
PROCESS
1- Weigh a sample of the compound to be analyzed & place it in the apparatus
2- The sample is burned with a plentiful oxygen supply to ensure complete combustion
3- The H2O & CO2 are drawn through 2 tubes. One tube contains a substance that absorbs the water, the other contains a substance that absorbs CO2
–> Weigh each of these tubes before & after the combustion. The increase in mass in the first tube= mass of water formed in combustion, & the increase in mass in the second tube= mass of CO2 formed in combustion
To determine the empirical formula of the unknown in combustion analysis, we assume that
- all the carbon in the compound has been converted into CO2, & trapped in tube 2
- all the hydrogen in the compound has been converted into H2O, & trapped in tube 1
- If the compound contains another element, we can calculate the mass of the element by subtracting the mass of carbon & hydrogen from the total mass of the original sample
- RMR THAT WE ARE COMPARING MOLE RATIOS NOT MASS ONES OT SOLVE PROBLEMS
List all equations
