Chem II Lab Final

Suppose that the absorbance of an aspirin tablet solution is 0.259. Also, the molar absorptivity of acetylsalicylic is 120. M-1 cm-1, and the path length of the light through the cuvette is 1.00 cm. Calculate the concentration of acetylsalicylic acid in the aspirin tablet solution using Beer's law: A = εbc.

0.0022

Enter the number of valence electrons in KrF2.

22.0

A buffer contains 0.500 M of formic acid (HCHO2) and 0.500 M of sodium formate (NaCHO2). Formic acid is a weak acid that dissociates in water as following: HCHO2(aq) + H2O(l) ⇌ H3O+(aq) + CHO2-(aq) The equilibrium constant expression for this equilibrium is K subscript straight a equals fraction numerator open square brackets straight H subscript 3 straight O to the power of plus close square brackets open square brackets CHO subscript 2 to the power of minus close square brackets over denominator open square brackets HCHO subscript 2 close square brackets end fraction equals 1.8 cross times 10 to the power of negative 4 end exponent Calculate the pH of the buffer solution. Recall that pH = -log[H3O+].

3.74

The gases NO2 and N2O4 are in equilibrium according to this equation: 2NO2(g) <=> N2O4(g) First, write down the equilibrium constant expression, Kc, for this reaction. Then, calculate the value of Kc if experiment shows the equilibrium concentrations to be 0.02 for NO2 and 0.016 for N2O4

40.0

Which of the following are true? Select all that apply.

A volumetric flask without a stopper is useless. A volumetric flask should be inverted and swirled before use.

The inside of the flask must be absolutely dry before it is fitted with the stopper assembly and weighed. The main reason is that

Any remaining water inside the flask would be hard to get to evaporate.

In this experiment, the hand-held device used to produce electricity is called a/an

DC generator

Which of the following count as one electron domain? (Select all that apply.)

Lone pairs of electrons The pair of electrons in a single bond. The four electrons in a double bond.

The pinch clamp on the flask assembly should be opened before heating the flask assembly in boiling water. If the clamp was left closed during heating, which of the following is the most likely problem that might result?

Pressure would buid-up in the flask, possibly blowing the stopper assembly into the ceiling.

How can you tell when vaporization of the unknown liquid is complete?

The ring of liquid around the boiling stone is nearly gone.

Which of the following is the least accurate method of determining the volume of liquid in a completely full Erlenmeyer flask?

Use the volume etched into the surface of the flask (e.g., 125 mL).

a cuvette is

a sample holder for measuring absorbance.

In today's lab bumping is prevented by

adding boiling stones to the flask.

A beaker contains the following equilibrium system: 2CrO42-(aq, yellow) + 2H+(aq) ⇌ Cr2O72-(aq, orange) + H2O(l) The H+ concentration is increased by adding concentrated HCl(aq) to the equilibrium mixture. The color of the mixture will

become more orange

Suppose a chemical reaction is initially at equilibrium, but then a change is made in the equilibrium system. To return to equilibrium, the equilibrium position will shift so as to

counteract the change

A blank is a solution that contains

everything that the sample contains except for the species to be determined.

The electronic geometry gives the "actual" shape of a molecule.

false

The volume occupied by air inside the Erlenmeyer flask is equal to the volume engraved on the flask.

false

As the "load" connected to the hand-held device increases, the effort required to operate the device increases. The effort needed to operate the device is best assessed by

how hard it is to turn the crank.

According to Beer's law, a plot of absorbance vs. concentration is

linear

According to Le Chatelier's principle, adding heat to a system is equivalent to

raising the temperature

A beaker contains the following equilibrium system (as before): 2CrO42-(aq, yellow) + 2H+(aq) ⇌ Cr2O72-(aq, orange) + H2O(l) If solid Na2CrO4 is added to the solution, the equilibrium will

shift to the right

A buffer solution resists changes in pH when

solution is diluted. a weak base is added. a strong acid is added. a weak acid is added. a strong base is added.

Electrical work is produced when the crank of the hand-held device is turned.

sometimes

The instrument used to measure the amount of voltage from the reaction between a metal and an ionic compound is a/an

voltmeter