Stoichiometry Flashcards
Matter (+ divisions)
Matter is any substance that occupies space and has mass: can be a mixture (combo of two or more pure substances that retain their individual properties) or a pure substance (definite, constant composition).
Mixtures can be homogeneous (uniform composition, properties throughout [ex. salt water, metal alloys]) or heterogeneous (non-uniform composition, varying properties [ex. salad dressing—especially if you don’t shake it—, paint, garden soil]).
Pure substances can be elements (made up of atoms that each have the same atomic number [ex. Pb, Hg, Br]) or compounds (made up of a combo of atoms/ions in a fixed ratio, having different properties from the constituent elements [ex. water, carbon dioxide, sodium chloride]).
Air is homogeneous (here, elsewhere same). Vinegar too.
Temp
- Measure of avg KE (don’t all have same)
- SI unit is Kelvin
- Absolute zero = 0 K (all movt of particles stops)
- 273 K = 0ºC (formula for conversion: K = C + 273)
Kinetic theory of matter
Properties of three states (others [plasma, Bose-Einstein condensates, etc.]?):
Solid - fixed volume, shape; cannot be compressed; attractive forces between particles hold them in a close-packed arrangement; particles vibrate in fixed positions
Liquid - fixed volume; no fixed shape (takes shape of container it occupies); cannot be compressed; forces between particles weaker than in solids; particles vibrate, rotate, translate (move around)
Gas - no fixed vol, shape (expands to occupy space available); can be compressed; forces between particles taken as zero; particles vibrate, rotate, translate faster than in a liquid
Different materials have different phase diagrams, different phases exist at different temps, pressures.
Dry ice sublimation ex. (at room temp, straight to a gas). Iodine deposits and sublimates (purple gas and crystals).
Sublimation (solid to gas), deposition (gas to solid), melting (fusion [solid to liquid]), vaporization (liquid to gas), condensation (gas to liquid), freezing (liquid to solid).
In ex. heating curve of water (water to steam at 100, ice to water at 0 [above?]), when temp constant, energy going into breaking bonds.
Sponge not a proper solid (has air)?
Balancing chemical equations
Process of trial and error (once balanced, tells the ratios of now molecules combine in a reaction)
Arrow means “reacts to form…”
Balance last if alone
Changing subscripts would be changing the actual molecules
Chemical equations must be balanced so that they follows the Law of Conservation of mass (have to have same mass)
CHO
Mole and Avogadro’s #
Amt of a substance which contains the same # of chemical units (atoms, molecules, ions, electrons,…) as there are atoms in exactly 12 grams of pure carbon-12
6.023 x 10^23 (Avogadro’s #) chemical units
Unit is mol
One mole of atoms = 6.023 x 10^23 atoms = gram atomic mass of element (see periodic table [unit is gmol^-1])
One mole of molecules = 6.023 x 10^23 molecules = gram molecular mass (sum atomic masses of each atom to get the molecular mass)
One mol of Cl2 molecules weighs 2 x 35.45 = 70.9
One mole of H2SO4 contains two moles of H, one mole of S, and four moles of O
Solving problems using stoichiometry
Using mass and molar mass of A, calculate mols of A. Then, using mols of A and stoichiometric relationship, calculate mols of C. Using mols of C and molar mass of C, calculate mass of C.
Stoichiometry refers toc coefficients
Ratio: for that many, you react that many
Complexed with (for every ex. one mol, seven [seven molecules for every one])?
Copper sulfate blue b/c absorbs water out of the air (complexes w/ water [the dot symbol means “is complexed w/”])
Anhydrous means no water in it
% error, yield
% error tells you how far off you are from the known value (think 100% - whatever you got)
% yield = (actual/theoretical) x 100%
Something always happens (some falls out of container, doesn’t react fully, etc.)
Limiting reactants
In real-life industrial processes, reactions are run w/ an excess of all the reactants except one (called the limiting reactant, as it controls/limits how much product can be made)
Usually the most expensive or most difficult to procure
If given info abt mass/mols of more than one reactant, must first determine which one is limiting (see which can produce fewest molecules of product)
Once you find it, all calculations based on that one
Even if same # of mols, aren’t combined at same ratio
Empirical formula
Simplest ratio of atoms in a molecule (as opposed to actual ratio of atoms in a molecule [molecular formula])
Method for finding: make sure given percentages add up to 100% (figure out what’s missing if not) and assume that you have 100g off that simple (equal mass to percentage). Then, convert to mols (divide each element’s mass by its atomic mass) and divide by smallest # (if you can).
Doesn’t have to be perfect (they’ll be whole-# ratios).
Mr is relative molar mass. If given and asked for molecular formula, find molecular mass using empirical formula and see how many units would go into it. Then, multiple everything in the empirical formula by that # (other just the ratio [not how they are in real life]).
Mass spec does this too…
Concentration
= moles/vol (C = n/V)
Unit: mol dm^-3
Divide by 1000 to get cm^3
Use square brackets to denote concentration
Dilutions: C1V1 = C2V2 (mol = mol)
Adding water (no moles of the solute in water)
W/ reaction (if they don’t react 1:1, you’ll get it wrong)?
“just neutralize” = mols of one and other react stoichiometrically perfectly (neither in excess)
Ethanol could be anoth
Write balanced equation, will have a pair of vol and conc (find mols), use stoichiometry to fins mols of other
Titrations
Usually involve acids and bases (and redox?)
Has two parts: solution in the buret (titrant), which you will usually know the conc of, and a solution in the conical flask (analyte), which you will know the volume of (you’re usually trying to calculate the conc)
Endpoint = when they’re perfectly balanced
W/ an acid, indicator will be clear, but when add ex. NaOH and it becomes basic, turns pink (pH change to become basic [perfectly balanced]—SLIGHTLY in excess)
Overview: Record starting vol of titrant in burette, titrant added into conical flask (where reacts w/ analyte) until endpoint reached (mols of titrant exactly balances [stoichiometrically] w/ mols of analyte [just enough has been added to react all of the analyte], final vol recorded.
Calc overview: Write out (and balance) equation, calculate vol of titrant added, calculate mols of titrant added (using n = CV), use stoichiometry to calculate mols of analyte in flask, calculate conc of analyte using C = nV.
Until results are concurrent (within 0.1 of each other?). Discard first (rough). Air bubble? Greater vol for same uncert (minimize uncertainty)…
Over-shot endpoint (be specific [not just “human error”]). Drips on side = too=high measured value of NaOH. Use funnel, make sure valve closed
Base in bu
LabQuest strength (abt effectiveness, not ease)…
Molar vol of gas
One mol of any gas occupies 22.7 cm^3 at standard temp, pressure (100 kPa, 273 K = 0ºC)
Called the molar volume of gas (see table 2 of data book)
Formula n = VSTP/22.7 dm^3 mol^-1, where VSTP is the vol of gas in dm^3 at 273 K and 100kPa
B/c ONE mol
Ambient is SATP
In stoichiometry, first step is to always use info given to get mols (vol at STP is 22.7, can use use conc and vol, mass and molar mass [?])
Assumptions: gasses are not sticky (perfectly elastic collisions [when collide, bounce off each other]), vol of gas particles when condensed and vol of container negligible?
If all reactants and products are gases, vol of one species can be calculated from vol of another using stoichiometry of balanced equation (same way mols can be used)
ex. neither limiting, can react perfectly, so can only make so much?
Ideal gas equation
PV = nRT (V in dm^3, temp in K, pressure in kPa)
Used to relate mols of a gas to the pressure, vol, temp of a system
Only works for ideal gasses
Assumptions: no (or entirely negligible) IMFs between gas molecules, vol occupied by molecules themselves entirely negligible relative to vol of container
If not ideal, have to correct for it
For a solid, pure liquid, can find moles w/ molar mass
Mols constant b/c gas not escaping (can equate using gas constant?)
Piston means weight that can compress/expand vol
Boyle’s Law is a special case of the Idea Gas Law
Only applies to ideal gases held at a constant temp and constant # of mols of gas (only vol and pressure change)
Absolute pressure exerted by a given mass of an ideal gas inversely proportional to the volume it occupies (curve b/c denom [straight would be negative?])
