chem43a owl

What has higher melting points?

Inorganic bc large structures with strong ionic/covalent bonding

In gravity filtration, solution is filtered by pouring it into folder filter paper in filter funnel, why?

to remove solid impurities from solution or liquid product.

Which of the following can be used to heat volatile chemicals in a microscale experiment? More than one answer may be correct.

aluminum heat block, water bath, sand bath

What is the result of boiling the impure product with too much solvent and then cooling on ice?

no crystals produced

What happens to the crystal size if the impure crystals are dissolved in sufficient boiling solvent and then cooled on ice rather than in air?

smaller crystals

An inexperienced chemist adds too little boiling solvent to an impure sample of product and then leaves this mixture to cool. The crystals:

form with impurities coated onto the pure crystals.

Which of the following statements is true when considering temperature during a recrystallization?

Ideally the hot solvent used will keep the impurities dissolved OR not dissolve them at all.

It is important that the sample is dissolved in just enough hot solvent.

In a recrystallization, after the crystals have formed (with cooling on standing), the solution can be cooled with ice. Why is this done?

Less product is needed to saturate cold solvent so the yield is higher.

At the end of a reaction it is important to remove the solvent from a solid product (more than one answer may be correct)

so that the melting point can be determined.

so that the yield can be determined.

so that clean NMR spectra can be obtained that do not contain solvent peaks.

what is a flash point

The lowest temperature at which the liquid has sufficient vapor pressure to ignite when in the presence of an ignition source

In silica gel chromatography, which compounds elute first?

non-polar compounds

What is a good R _f value in order to ensure that a compound will elute over a reasonable timeframe?

around 0.3

What can you do to elute a compound that is not coming off the column?

change to more polar solvent so they have something to compete with

How to calculate Rf

baseline to middle of point/baseline to solvent front

A liquid is said to be boiling when

the vapor pressure of the liquid is equal to the atmospheric pressure.

Which is the best description of a fractional distillation?

A technique to separate liquid mixtures with boiling points close to each other.

In a vacuum distillation, what is the effect of using reduced pressure? (lower altitude)

The boiling point of the fractions are decreased

SAFETY: How much should you fill your distillation pot?

Less than half full

Which of the following is an appropriate use for distillation apparatus? More than one answer may be correct.

measure bp of liquid

remove liquid product from reaction mixture

What rate should the liquid come across in a fractional distillation?

few drops per min

When performing a fractional distillation, how can you tell when your liquid is beginning to distill over? More than one answer may be correct.

Liquid droplets start to condense on the thermometer bulb.

The temperature remains constant after initially rising.

Which of the following factors affect boiling point? More than one answer may be correct.

polarity and pressure

What features make a sample suitable for analysis by gas chromatography? (More than one answer may be correct.)

thermal stability, high vapor pressure, appreciable volatility

Which of the following statements concerning the principle of separation in gas chromatography is true when using a liquid stationary phase? (More than one answer may be correct.)

The lower the vapor pressure of a compound, the lower the amount that will be in the mobile phase at equilibrium, hence the slower the compound will travel through the column.

What is the consequence of the gas flow rate being too high?

The sample won’t have a chance to equilibrate with the stationary phase, leading to poor separation.

What characteristic odor would you expect to observe if an alcohol reacted with acidified ethanoic acid?

fruity

altitude increases, air pressure decreases

vapor pressure of water does not need as much pressure to boil = lower bp