CHEM 119 Lab Exam

CHEM 119 Lab assesment

• Conceptual understanding and application of pertinent calculations

Precision v Accuracy

• Accurate = one that is close to the correct or accepted value (TIP: think A+)

• Precise = measurements that have a high degree of reproducibility (recurring answers)

Weighing on a balance

• make sure to be at room temp, if not it can generate air currents

Measuring volumes

• least accurate = marked graduations on a beaker or erlenmeyer flask

  • measured within great distances (25,50 mL)

• Instead, use a graduated cylinder

• meniscus = forms when water is in a narrow glass container, curving in a concave fashion, at air-water interface

  • read at eye level, bottom of the meniscus

Density

• “how much stuff is squeezed into this much space?” or packed into it, kind of like states of matter

• mass/volume

• varies with temperature, but for solids and liquids it is small

deviation

• difference between the measurement, x, and the mean (average) value

• standard deviation: statistical measure of the amount of spread in a data series

Conductivity

• affect the electrical conductivity of the solution

• electrolytes dissociate in water to form ions

• EXTENT of dissociation or ionization of a compound can be determined by measuring conductivity

  • e.g. the greater the amount of ions, the greater the conductivity

molar concentration (M)

• M = mol/L

• moles of solute/volume of the solution in liters

variables

• independent: can be modified and adjusted during an experiment, typically horizontal axis

• dependent: responds to change and adjustments, typically vertical axis

• controlled: do NOT change

Pressure (P)

• the force that the gas exerts per unit area of the surface

• unit of kilopascals (kPa)

  • 1 atm = 101.325 kPa

  • 1 atm = 760 torr

  • 1 torr = 0.133322 kPa

Temperature (T)

• measure of the thermal energy of a substance

• kelvin or celsius

• TK = TC + 273.15

empirical model

• model that mathematically describes trends in experimental data

• trend between two variables

• coefficient of determination (R²): How well does this model fit the observed data?

  • higher value of R² indicates model fits the data better

  • not how good your data is, but how good your model fits your data

direct proportional relationship vs indirect

• y = kx

• 1/y = kx

Aqueous solutions

• one or more type of solutes dissolved in water as solvent

• if solute particles can dissociate, they can conduct electricity

• the more ions, the greater the solution’s conductivity

• conductivity is a temperature-dependent (conductivity typically increases w increasing temps)

nonelectrolytes vs weak electrolytes

• dissolve in water, do NOT dissociate into ions = non

• small portion forms ions = weak

strong electrolytes

• break apart completely into ions

concentration/molarity

• measure of the amount of solute per unit volume of solution

• brackets are used during molarity talk

• M1V1 = M2V2 ONLY works in direct dilution (pouring in the same beaker) then serial dilution (pouring in diff beakers bc you scoop and changing the amt.)

aliquot

volume sample

calibration curve

• standard curve

• shows how a dependent variable varies across a range of standard solutions

• allows us to find missing statement with as much context as possible

absorbance

• a sample blocks the passage of light due to interactions of molecules in sample with light

• correlates with solute concentration

  • A (absorbance) = K(x) + b

  • k and b are empirically determined constants

nanometers phases (in order)

• 10^-9

• reduction: (NaBH₄) dissociated aqueous metal ions react w reducing agent to form elemental metal atoms

• nucleation phase: particle grows from a few bonded atoms to where it begins to resemble the end

• growth phase: particles grow to final size

• etching agent: (also a oxidizing agent) (H₂O₂) to remove atoms to get to that optimal shape

• reaching equilibrium

• capping reagent: (NaBr and sodium citrate) covers surface to prevent addition of more and to prevent bulking solid

titration

• laboratory method, adding a known amt. of either acid or base to know the other amt.

• titration curve = pH on the y-axis vs. volume of titrant added on x-axis

Bronsted- Lowry Acid

a susbtance that can donate a proton, H+ ion

e.g. hydrogen-ion donors or proton donors

• would have

Bronsted-Lowry Base

• any substance that can accept a proton, H+ ion

• hydrogen-ion acceptor or proton acceptor

• to distinguish, look at where the Hydrogen atom went

Lewis acid

electron pair acceptor

Lewis Base

electron pair donor

to find pH formula

pH = - log [H+]

equivalence point

• chemically equivalent amount of acid and base are reacted

math shown

• net ionic equation, balancing equation

• identify bronsted-lorey acid and base

• spectator ions

• theoretical maximum

• re-visit carrying out chemical reactions!! this is where most of the math is!!

Law of Conservation of Matter

mass is neither created nor destroyed

percent yield

100% (actual yield)/(theoretical yield)

percent atom economy

( mass of atoms in desired product per mole of reactions / mass of atoms in all reactants per mole of reactions) x 100%

• the proportion of reactant atoms that end up in the desired product

electronegativities (ascending order)

• Nonpolar Covalent: < 0.4

• Polar Covalent: 0.5 to 1.6

• Ionic: > 2.0

Modeling Empirical Relationships

• Charle’s Law (Pressure - Temperature)

  • directly proportional relationships

• Boyle’s Law (Pressure - Volume)

  • inversely proportional

• Leak Test

  • leak = # of moles (n) no longer constant, invalidate relationship trying to measure (PV = nRT = Ideal Gas Law) (R is a constant and so is n in this experiment hence leak test to make sure it is constant)

    • if on the same side of = sign, then inverse to make the total. If on opp. sides of = sign, then direct to be balance/equal.

• Thermal Equilibration

  • wait after adding ice before reading? to let the gas inside the flask reach same temp as water bath

• Submerging Flask

  • submerged all the way to the bottom of the stopper to make sure to min. air that can remain at room temp.

Preparing Solutions & Calibration Curves

• Beer-Lambert Law: Absorbance is directly proportional to concentration.

• conductivity is proportional to total concentration of dissolved ions

• ex: a sample of juice allows 25% of light to pass through. convert to decimal and A = -log (x)

Preparation and Optical Properties of Silver Nanoparticles

• the color of the solution is determined by the size and shape of nanoparticles

• wavelength max is where solution absorbs light most strongly, shift in this is change in physical size of nanoparticles

Performing Titrations: Acid-Base Titration

• Equivalence Point vs Endpoint

  • equivalence point = theoretical point where moles of titrant are equal to moles of substance being titrated. determined by stoichiometry

  • endpoint = physical point where it closely shows as close to the equivalence point as possible

• Primary Standard vs Secondary Standard

  • Primary = KHP, used to determine the exact concentration of another solution

  • Secondary = NaOH, concentration is determined by titrating it against primary standard

• Q’s

  • If a student over-titrates and the solution turns a very dark pink, how will this affect the calculated molarity of the unknown acid? will make an overestimation of acid concentration

Carrying Out Chemical Reactions