Chemistry Lab Exam Flashcards
How do you make a measurement correctly?
Record all digits using the markings, plus one further estimated digit.
What is the difference between accuracy and precision?
Accuracy: how close experimental value is to the true value
Precision: how close the repeatedly measured values are to each other
What is the difference between random error and systematic error?
Random: experiment results give slightly different results as a result of limitations of instruments to produce exactly the same result each time. Low random error = high precision.
Systematic: experiment results are consistently too high or too low. Low systematic error = high accuracy.
How to measure precision?
Standard deviation: Measurement of precision or how close values are to each other.
s = sqrt[sigma (mean - x)^2/n-1]
How to measure accuracy?
percent error: measured value - true value
% error = [(mean - x ref)/x ref ] * 100%
What is the relationship between speed of light, wavelength and frequency.
speed of light = wavelength * frequency
What is planck’s equation?
Energy of wave = planck’s constant * frequency
How do we see colour?
We see the colours reflected from an object that absorbs its complementary colour.
How is light emitted?
Electrons in an atom exist at quantized energy levels. Upon the absorption of energy, an electron can be excited from ground state, and then return, emitting energy in the form of reflected light.
How to find the energy of a photon absorbed?
E photon = ∆E atom = Ef - Ei
How to find the energy of a photon emitted?
|E photon | = ∆E atom = Ef - Ei
How does a spectrophotometer work?
- White light shines through collimator lens and prism which scatters light into wavelengths.
- Wave length selector/slit allows a narrow range of selected wavelengths through to shine through the sample solution.
- The transmitted light hits a photocell detector which generates a current displayed on the output screen.
How do you calculate absorbance?
Absorbance = log (lnitial intensity / ∆Intensity)
OR in dilute samples use beer’s law
What is Beer’s Law?
Absorbance = molar absorption coefficient * path length * analyte concentration
How is absorbance influenced by fixed wavelength and pathlength?
Absorbance is proportional to concentration
In lab 2, what were we measuring with the spectophotometer? How does it form?
Measuring ascorbate in different samples through the formation of a red-coloured iron(ii)-bipyridine complex. The more ascorbate, the more Fe(iii) in FeCl3 is turned to Fe(ii) which binds to bipyridine to form the red complex.
What does an absorbance plot look like?
x axis: concentration of analyte
y axis: absorbance
slope of trendline: molar absorptivity* path length. Equals molar absorptivity when path length = 1.0
What is the difference between molarity and molality?
molarity: used for properties of solute; mol of solute/ L of solution
molality: used for properties of solvent; mol of solute / kg of solvent
What is a synthesis reaction?
Two or more elements and/or compounds react to form a single, more complex product
A + B = AB
What is a decomposition reaction
Single complex reactant breaks apart into multiple simple products
AB = A + B
What is the difference between single replacement and double replacement?
Single replacement: single elemental reactant replaces another element in a compound, forming a elemental product and compound product
Double replacement (metathesis): elements in two compounds switch places and result in a precipitate or molecular compound.
What is a combustion reaction?
When a hydrocarbon reacts with oxygen to produce CO2 gas and water vapour.
What is a pigment?
Usually inorganic compounds completely or nearly insoluble in water
What is verdigris? What is its formula?
Rich blue green coloured copper(II) acetate monohydrate. Its formumla is Cu(CH3COO)2*H2O
What is malachite? What is its formula?
Light green copper(ii) carbonate mineral. Its formula is CuCO3*Cu(OH)2
How was malachite formed in lab?
- Add Na2CO3 to CuSO4*H2O
- Add heat to give enough activation energy for SO4 to swap with CO3, creating aqueous Na2SO4 and insoluble CuCO3 with carbon dioxide gas and water as byproducts.
- Cool down and filter by vacuum filtration.
How is verdigris formed during lab?
- add NH3 to CuSO4*5H2O to form [Cu(NH3)4]SO4 + H2O and it will turn intense blue
- add NaOH to [Cu(NH3)4]SO4 to form solid Cu(OH)2 and aqueous Na2SO4 + NH3
- Filter out supernatant so only solid Cu(OH)2 remains
- Add solid Cu(OH)2 to pure CH3COOH to form Cu(CH3COO)2 and water then filter and dry.
How are pigments used to make paint?
Pigment is mixed with a binder to turn it into a liquid or paste like phase, allowing it to adhere to a surface.
What are hydrates and how do you name them?
They are ionic compounds that contain water molecules as an integral part to their crystal structure. To name, add name of salt, greek numerical prefix for molecules of water, and add hydrate at the end.
Why are different copper complexes different colours?
Colour of transition metals depends on the ligands bonded to the metal cations.
how do you find percent yield?
% yield = actual yield / theoretical yield * 100%
how to find limiting reactant?
- convert each reactant to moles
- Use stoich to find out how much product is made with each mole value
- Whichever produces less product is the limiting reactant
Why is cupric hydroxide washed before adding it to glacial acetic acid?
As hydroxide is basic, you need to wash it from the sample thoroughly as the next reaction is in acetic acid which we don’t want to neutralize.
Why is it more difficult to precipitate verdigris than malachite?
Verdigris is more soluble in water and therefore needs to be washed with glacial acetic acid. Malachite does not need glacial acetic acid.
Why are pigments made of transition metals coloured?
5 d-orbitals of transition metals do not have the same energy in a compound and get excited to different energy levels, emitting energy in a wavelength of visible light.
Why are solutions stirred while heated?
To distribute heat evenly throughout the solution.
What can a fume hood protect against?
Explosions, fumes, fires.
What is a calorie vs Calorie?
calorie: amount of heat needed to raise the temperature of one gram of water one degree celsius. 4.184 J
Calorie: aka kilocalorie or kcal; 1000 calories.
What is the difference between exothermic and endothermic reactions?
Endothermic: initial temp < final temp; heat is absorbed
Exothermic: initial temp > final temp; heat is released
How to find the heat change of the reaction?
From qcal:
qrxn = -qcal
From reaction:
qrxn = mass(substance) * C(sol’n)*∆T
how to find heat capacity of calorimeter
either qcal = Ccal *∆T
OR
Ccal = mass of sol’n * ∆T
how do you find heat of reaction?
given qcal and m sol’n:
qrxn = -qcal = -(m sol’nCs water∆T)
given substance properties:
qrxn = m_substnace *Cs substance * ∆T
How to find heat of neutralization?
qrxn/moles reacted
How to find molar enthalpy change / heat of neutralization?
enthalpy change (∆H) or q / number of moles
What is a colligative property?
Physical properties proportional to the NUMBER (not type) of solute particles contained in the solution. E.g. freezing point, boiling point, vapour pressure, and osmotic pressure.
What is molality?
m = mol solute/kg solvent
also can be found via
m = ∆Tf/Kf
How is freezing point of solvents affected through addition of solute?
Adding solute lowers the freezing point of the pure solvent.
What is freezing point depression?
∆Tf; difference between freezing point of pure solvent and freezing point of solution.
How to find molal freezing point depression constant? (Kf)
∆Tf/msolute
aka
Freezing point depression / molality of solute
How to find freezing point depression? (∆Tf)
Freezing point solvent - freezing point solution
OR
iKfm
where i = mols product formed per mol solute
Kf = freezing point depression constant
m = molality
How to find molar mass?
grams solute / mol solute
How to solve for the freezing point of a solvent in a cooling curve?
Put each linear equation on either side of the equals sign, solve for x, and plug in x into any equation to calculate the value of y.
What does courmarin look like?
Benzene + six membered ring with oxygen in place of C1 + oxygen sticking out of C2
What is an alkene?
Hydrocarbon with carbons connected via double bond
what is an ester?
Carbon double bonded to a single oxygen and single bonded to an oxygen attached to an R group
what is the equivalence point?
Moles of base = Moles of acid
Properties of a solid compound to be used in a standard solution?
- readily and cheaply available with 99% purity
- stable under normal conditions
- not hydroscopic or reactive with air
- reasonably soluble
- non toxic and environmentally friendly
Where to find pKa on graphs?
pH = pKa at midpoint between 0mL added and equivalence point 1
What is a zweitterion?
Structures with a positive and negative charge.
What is the difference between a monoprotic acid and a diprotic acid?
monoprotic: donates 1 mol H+ to neutralize 1 mol of base; one equivalence point
diprotic: donates 2 mol H+ to neutralize 2 mol base; two equivalence points
What is the acid ionization constant and the base ionization constant?
Ka = [H3O+][A-]/[HA]
Kb = [BH+][OH-]/[B]
What is the relationship between Ka and Kb?
Kw = Ka*Kb
What is a buffer made of?
Weak acid/base and conjugate base/acid.
What is the henderson-hasselbalch equation?
pH of acid buffer= pKa + log [CB]/[WA]
or
pOH of base buffer = pKb + log [CA/WB]
How to choose a good buffer?
The buffer region must be +/- 1 from the pKa of the weak acid/base.
How to solve for initial concentrations?
C1V1 = C2V2
C1: original concentration
V1: original volume
C2: new/required concentration
V2: new/required volume
how to determine purity of aspirin?
add FeCl3 which will bind to the phenol group in salycilic acid and turn purple.
what are the parts of thin layer chromatography?
- thin stationary phase
- chamber
- liquid mobile phase
What is the developing solvent?
mobile phase that moves up the plate via capillary action. substances soluble in solvent will travel faster and farther up
How to calcualte retention factor values?
Rf = distance solute travelled / distance solvent travelled
What are the parts of the national fire protection association diamond?
Red = Flammability
Yellow = Reactivity Hazard
White = Special Hazard
Blue = Health Hazard
How to clean a strong acid or base spill?
Neutralize acids with sodium bicarbonate, and neutralize bases with ascorbic acid. Test success of nutralization with paper, then use absorbent grains to clean up spill and sweep into labeled bag to put into solid waste.
What chemicals require special spill kits?
Hydrofluoric acid, perchloric acid, chromic acid and mercury
Steps of the copper cycle
- Dissolve copper wire with HNO3
- Add NaOH dropwise until solution forms dark blue CuOH precipitate
- Heat mixture until it turns black/brown and leave to settle
- Add HCl to dissolve CuOH and creates aqueous blue CuCl complex
- Add NH3 to form aqueous blue ammonium complex
- Add H2SO4 to create aqueous bright blue CuSO4
- Add Zn to create red-brown Cu(s) and aqueous ZnSO4
- Add HCl so Zn is washed out from solid Cu
