AP Chemistry

How many grams of Cr can be obtained by the electrolysis of a Cr(NO3)3 if 10 amps are passed through the cell for 6 hours?

38.8 g

19.4 g

103 g

12.5 g

56.3 g

Explanation

$Cr^{3+} + 3 e- \rightarrow Cr(s)$

$mass = \frac{10 A , x, 6 hr , x , \frac{3600 s}{1 hr}}{ \frac{96485 C}{mol e-} } , x , \frac{1 mol , Cr}{3 mol e-} , x , \frac{51.996 g , Cr}{1 mol , Cr}= 38.8 g$

Which of the following would most likely form a homogeneous solution?

Water and benzene (C6H6)

NaNO3 and octanol (C8H18O)

NH4Cl and water

A and B

All would form homogeneous solutions

Explanation

Like dissolves like. The NH4Cl and water mixture involves an ionic solid and a polar solvent. Examples a and b involve mixing polar/non-polar solvents and ionic and non-polar solvents.

How many lone pairs of electrons exist on the following molecule?

BF3

Explanation

.. ..

:F — B — F: Boron can have a violated octet (6 e-) and each F has 3 lone pairs

.. | .. for a total of 9 pairs of unpaired electrons

:F:

Where does the flattest region of a titration curve of the titration of a weak acid with a strong base occur?

At the pKa of the acid

At the pKb of the base

At a pH greater than 7

At a pH of 7

Explanation

In this question, titration curve would graph the pH of acid solution versus the amount of base added. Since the base is strong and the acid is weak, we can conclude that the pH will be slightly greater than 7 at the equivalence point. The equivalence point is found in the steepest region of the curve.

The half-equivalence point is the flattest region of the titration curve and is most resistant to changes in pH. This corresponds to the pKa of the acid. Within this region, adding base (changing the x-value) results in very little deviation in the pH (the y-value). This region is also the buffer region for the given acid.

Which of the following statements best describes ionic compounds?

3-D arrays of charged particles

Formed when molecules share electrons

Neutral particles that donate electrons

Malleable compounds that lack structural stability

Explanation

The definition of ionic compounds are three-dimensional arrays of atoms held together by strong ionic bonds. Ions are charged particles that have either gained or lost a certain number of electrons. They have great crystalline strength because of the strong electrostatic forces between the ions.

How many electrons are involved in the following reaction?

$MnO_4^-+ I^-\rightarrow I_2+Mn^{2+}$

1 e-

2 e-

4 e-

5 e-

10 e-

Explanation

The common factor between 2 e- and 5 e- is 10. Therefore the number of electrons involved is 10 e-.

Which of the following is what determines the strength of an acid?

The Ka

The Kb

Its physical state

How many bonds the central atom makes

Electronegativity values

Explanation

The Ka is the acid dissociation constant, and thus it is what determines how strong the acid is. Stronger acids dissociate to a greater extent and produce lower pH values.

A buffer using acetic acid (pKa=4.76) is titrated with NaOH. What is the pH at half the equivalence point?

2.38

4.76

7.00

9.52

12.36

Explanation

The pH at half the equivalence point is equal to the pKa of the acid.

A chemistry student is trying to calculate how long it will take a power source of to heat a sample of ice from $-35^{o}C$ to $75^{o}C$ . Given that the specific heat capacity of ice is $2.06\frac{J}{g^{o}C}$ , the specific heat capacity of liquid water is $4.184\frac{J}{g^{o}C}$ , and the heat of fusion for water is $334.16\frac{J}{g}$ , how long will this process take?

There is not enough information to determine the amount of time needed for the process described

Explanation

In order to solve this problem, we'll need to break it up into steps.

Step 1: Calculate the amount of energy necessary to raise the sample of ice from $-35^{o}C$ to $0^{o}C$ . To do this, we'll need to use the following equation:

$q_{1}=mc_{H_{2}O(s)}\Delta T$

$q_{1}=( 20\cdot 10^{3}g)\left ( 2.06\frac{J}{g^{0}C} \right )\left ( 35^{o}C \right )=1.44\cdot 10^{6}J$

Step 2: Calculate the amount of energy necessary to convert the sample at $0^{o}C$ from ice to water. We'll need to make use of the following equation:

$q_{2}=m\Delta H_{fus(H_{2}O)}$

$q_{2}=\left ( 20\cdot 10^{3}g \right )\left ( 334.16\frac{J}{g} \right )=6.68\cdot 10^{6}J$

Step 3: Calculate the amount of energy necessary to convert the sample of water from $0^{o}C$ to $75^{o}C$ .

$q_{3}=mc_{H_{2}O(l)}\Delta T$

$q_{3}=\left ( 20\cdot 10^{3}g \right )\left ( 4.184\frac{J}{g^{o}C} \right )\left ( 75^{o}C \right )=6.28\cdot 10^{6}J$

Step 4: Sum the amount of energy from the previous 3 steps. This value is the total amount of energy for the entire process.

$q_{Total}=q_{1}+q_{2}+q_{3}=1.44\cdot 10^{7}J$

Step 5: Now that we know the total amount of energy needed for the process, we need to calculate the time based on the amount of power provided.

$P=\frac{q_{Total}}{t}$

$t=\frac{q_{Total}}{P}=\frac{\left ( 1.44\cdot 10^{7}J \right )}{(100\frac{J}{s}) }=1.44\cdot 10^{5}s$

$t=(1.44\cdot10^{5}s)\left ( \frac{1hr}{3600s} \right )=40hrs$

Where does the flattest region of a titration curve of the titration of a weak acid with a strong base occur?

At the pKa of the acid

At the pKb of the base

At a pH greater than 7

At a pH of 7

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