Ch 2 Quantitative Analysis

False

A pipet continuously delivers higher volume than it is calibrated for. This is an example of gross error

False

Method errors are random errors that can be avoided by analyzing standard samples of known concentration

False

To calculate the standard deviation, the true value must be known

True

To calculate the percentage error, the true value must be known

Sig Figs

Minimum number of digits needed to write a given value in scientific notation without loss of PRECISION

• communicate the true precision and reliability of measured data

• ensure that results reflect the limitations of the measurement process

Gravimetric Analysis

• measuring the MASS of a substance

• The mass of a product is used to calculate the quantity of the original analyte

• precipitation

  • ex) Chloride (Cl-) can be measured by precipitating the anion with Ag+ and find the mass of AgCl

    • Ag+ + Cl- →AgCl(s)

precipitation

ideally should be:

• pure

• insoluble

• easily filterable

• possess a known composition

Larger precipitated particles are desired since:

• have less surface area for impurities to attach

• will not clog or pass through a filter

• THERMODYNAMICALLY FAVORABLE

  • since energy of atoms on the surface of a crystal is higher than the enrgy of atoms inside the crystal

Dialysis

the size of colloidial particles can be demostrated by

colloidal precipitation

• particles with diameters of 1-500 nm

• pass through most filters

• larger than most moleucles but too small to precipitate (suspension)

Crystal growth

for a short time after a precipitant is added to analyte, the solution contains more dissolved solute than should be present at equilibrium → SUPERSATURATED

Crystallization occurs in two stages

nucleation and particle growth

Nucleation

solutes form clusters of sufficient size then reorganize into an ORDERED STRUCTURE capable of growing into larger particles

• ex) scratches on a glass surface

particle growth

molecules, ions, or other nuclei condense onto the nucleus to form a larger cyrstal

Amorphous particles have

no crystalline order

the amorphous particle

crystalline aragonite grows out of the surface and consumes

highly

in precipitation, nucleation proceeds faster than particle growth in a ___ supersaturated solution

fast nucleation produces

tiny particles or worse, a colloid

to slow nucleation,

a less supersaturated solution is preferred

Techniques that promote particle growth include

• raising the temp to increase solubility

• adding precipitant slowly with vigorous mixing to prevent local supersaturation

• using a larger volume of solution to lower concentrations of analyte and precipitant

Coagulation

• ionic compounds are usually precipitated in the presence of electrolyte (compound that dissociates into ions when it dissolves

• tiny coooidal particles coagulate (come together) to form larger crystals

• heating can increase kinetic energy, allowing particles to come close enough to coalesce

digestion

a period of standing in the presence of the hot mother liquor

• this promotes slow recyrstallization of the precipitate

• particles size increases and impurities tend to be expelled from the crysal

Washing

washing a precipitate on a filter can remove droplets of liquid containing excess solute

but many require electrolyte to maintain coherence

some precipitates

peptization

result in loss of product through the filter

What to use for wasing AgCl (s)

AgCl will peptize if washed with water, so washed with dilute HNO3 instead

ignition

invovles strong heating

used to change the chemical form of some precipiates

• more suitable for weighing