analytical chemistry lab practical

average formula

x̄ = (sum of values) / n

standard deviation formula

density formula

density = mass / volume

volume formula

volume = mass / density

which is more accurate and precise, volumetric flask or beaker?

volumetric flask

if you see: find volume of water from mass, do this . . .

divide by density (1.00 g/mL)

if you see: a precision question, then . . .

its likely asking for standard deviation

if you see: an accuracy question, then . . .

its likely wanting you to compare something to the true value

dilution equation

C₁V₁ = C₂V₂

calibration curve equation

y = mx + b (absorbance vs concentration)

solve for concentration from absorbance

x = (y − b) / m

percent recovery

% recovery = (measured / expected) × 100

if you are being asked to find concentration from absorbance, then . . .

rearrange line equation to solve for x

if you see: spike recovery, then . . .

(spiked − original) / added × 100

should your concentration ever be negative?

no, there must be a calibration error

what is le chatliers principle?

a system shifts to counteract stress

if you add reactant, the equilibrium will shift . . .

right

if you add product, the equilibrium will shift . . .

left

if the temperature of an endothermic reaction is increased, the equilibrium will shift . . .

right

if the temperature of an exothermic reaction is increased, the equilibrium will shift . . .

left

if you see heat being added, heat is treated like . . .

a reactant

beer’s law

A = εbc

if you’re solving for concentration from beers law, then what’s the new formula?

c = A / (εb)

how do you get moles from mass?

mol = mass / molar mass

what is λmax?

wavelength of max absorbance

if you see: find molar absorptivity, then . . .

ε = A / (bc)

in indicator equilibrium, what do the colors yellow and blue generally translate to?

yellow is for acids and blue is for bases

how do you find molarity?

M = mol / L

how do you find moles from molarity?

mol = M × V

what does equivalence point mean?

mol acid = mol base

what makes polyprotic acids different?

they have multiple equivalence points

if you see: find unknown concentration, then . . .

M₁V₁ = M₂V₂

if you see: find the equivalence point from graph, then . . .

find the largest peak in the derivative graph

if you see: find two endpoints, then . . .

it is a polyprotic acid

what is the cell potential equation?

E°cell = E°cathode − E°anode

what is the nernst equation?

E = E° − (0.0592/n) log Q

what happens at the anode?

oxidation

what happens at the cathode?

reduction

if you see: concentratoon changes, then . . .

use nernst

if you are taking a stock solution and diluting it, what formula are you using?

C₁V₁ = C₂V₂

if heat is a reactant, then . . .

it is endothermic

if heat is a product, then . . .

it is exothermic

if you primarily see more yellow, what is the dominant species?

the acid

if you primarily see blue, what is the dominant species?

the base

explain the variables in beers law? (A=εbc)

a is absorbance, epsilon is molar absorptivity, b is path length (usually 1), and c is concentration

how do you make a calibration curve in excel?

highlight x (concentration) and y (absorbance), then insert scatter plot. be sure to add trendline and R² equation

how do you calculate average in excel?

=Average(range)

how do you calculate standard deviation in excel?

=STDEV.S(range)

how do you approximate derivative in Excel? how do you calculate it?

Δy/Δx = (y₂ − y₁) / (x₂ − x₁) or (pH₂ − pH₁) / (V₂ − V₁)

how do you graph titration data?

x is volume added, y is pH