Dr. King Chemistry II Lab Final

Initially, a beaker contains 50.0 mL of a buffer. Then, 5.0 mL of 0.10 M HCl solution was added to the buffer, and the pH of the solution was measured. Then, 10.0 mL more of the HCl solution was added. The total volume of HCl solution added was _______________. The final volume of solution in the beaker was ________________.

15.0 mL 65 mL

Enter the number of valence electrons in KrF2.

22

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 1.8 x 10^-4 Calculate the pH of the buffer solution. Recall that pH = -log[H3O+].

3.74

Beer's Law

A = εbc A: Absorbance ε: molar absorbitivity b: path length of light (usually 1 cm) c: concentration (mol/L)

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.

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

DC generator

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

Cooling curve and freezing point

Freezing begins when the liquid starts to super cool. The flat region of the curve.

The reaction, 2H2(g) + O2(g) ⇋ 2H2O(g), actually goes to completion, forming water, but if this is treated as an equilibrium, the equilibrium constant expression, Kc, is

Kc = [H2O]2 / [H2]2 + [O2]

Calculate Kc with equilibrium concentrations of compounds.

Kc= product of product concentrations / product reactant concentrations

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

Lone pairs of electrons

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.

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

Sometimes

When the dissociation constant of a weak acid, Ka, is calculates for this lab, which one of the following is used in the final step of the calculation?

The equilibrium concentration of the weak acid.

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.

A buffer solution resists changes in pH when (select all that are correct)

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

In today's lab bumping is prevented by

adding boiling stones to the flask.

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 water remaining inside the flask would vaporizes, filling the whole flask with water vapor, instead of air. Then, when cooled down, the water would condense back to a liquid, causing a large vacuum in 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.

Resonance structures

differ only in the arrangement of electrons

A blank is a solution that contains

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

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.

Polarity

identical groups: non-polar different groups: polar

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

making the system endothermic.

Suppose 1 mole of acetic acid (a weak acid) is placed in 1 L of water. In this solution dissociation of the acetic acid

occurs to only a small extent.

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.

Supercooling

temperature below freezing point shown by a dip in the curve.

Formal Charge

valence electrons - (# of bonds + non-bonded electrons)

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

voltmeter

Find moles of HA added to flask.

volume x molarity