Question 1

A 7.00 mL sample of saturated sodium borate solution was collected at 27°C. The sample's borate, B2O3(OH)4, was then completely titrated using 0.100 M HCl, and 6.40 mL HCl was required. Based on the reaction below, calculate the amount of borate in the original sample.

$$\mathrm{B_2O_3(OH)_4 (aq) + 2H^+ (aq) + 3H_2O (l) \rightarrow 4H_3BO_3 (aq)}$$

Accepted Answer

0.640 mmol

Question 2

A second saturated sodium borate solution was collected at 27°C. A 7.00 mL sample of that solution required 6.10 mL of 0.100 M HCl for complete titration. Calculate the borate concentration in the original solution in molarity (mol/L).

Accepted Answer

0.0871 mol/L

Question 3

The $$\Delta H_{sol} (kJ/$$mol) for several trials of the borax solubility experiment showed $$\Delta H^\circ = +11.5 kJ/$$mol and $$\Delta S^\circ = +34.2 J/($$mol·K). Calculate the $$\Delta G^\circ$$ for the process at 27°C.

Accepted Answer

$$\Delta G^\circ = +1.2 kJ/$$mol

Question 4

For the dissolution of borax, the solubility increases as temperature increases. Which of the following best explains this observation?

Accepted Answer

The process is endothermic and entropy increases

Question 5

Consider the electrolysis apparatus used in Lab. As shown in the diagram, a solution of 1.00 M NaCl is electrolyzed. If the cell voltage is 4.50 V, calculate the theoretical minimum voltage required for the electrolysis of NaCl(aq) to produce H$$_2(g) $$and Cl$$_2(g) at $$the electrodes.

$$\mathrm{2Cl^- (aq) \rightarrow Cl_2$ (g) + 2e^-}$$
$$\mathrm{2H_2O (l) + 2e^- \rightarrow H_2$ (g) + 2OH^- (aq)}$$

Accepted Answer

2.19 V

Question 6

For the electrolysis experiment, which change would increase the cell potential for a 1.00 M NaCl solution?

Accepted Answer

Increase NaCl(aq) to more than 1.00 M

Question 7

When the electrolysis cell operates systematically, which electrode will increase in mass?

Accepted Answer

The anode where oxidation occurs

Question 8

A particular coordination compound strongly absorbs light with wavelength 517 nm. Calculate the crystal field splitting energy ($$\Delta) in kJ/$$mol.

Accepted Answer

231 kJ/mol

Question 9

The coordination compound K$$_3$$[Cr(C$$_2O$_4$$)$$_3$$] was synthesized during Lab. Choose the correct coordination number for the chromium ion in this compound.

Accepted Answer

6

Question 10

Consider the following balanced reaction equation for preparing a coordination compound:

$$\mathrm{Ni(NH_3$)_6Cl_2 (s) + 5 NH_3$ (aq) + 6 NaOH (aq) \rightarrow Na_2$[Ni(NH_3$)_6] (aq) + 2 OH^- (aq)}

If $$you add 1.00 g of Ni(NH$$_3$$)$$_6Cl$_2 to 1.00 g of $$solid NaOH, calculate the mass of Na$$_2$$[Ni(NH$$_3$$)$$_6$$] produced, assuming 100% yield. The molar mass of Ni(NH$$_3$$)$$_6Cl$_2 is 231 g/$$mol, and Na$$_2$$[Ni(NH$$_3$$)$$_6$$] is 220 g/mol.

Accepted Answer

0.952 g

Question 11

In two different steps of the Qual procedure, you added potassium hexacyanoferrate(II) to confirm a cation. Which cation is confirmed by this coordination compound?

Accepted Answer

Fe$$^{3+}$$

Question 12

During the Qual cation procedure, how was the sulfide ion separated and how was its result confirmed?

Accepted Answer

By adding Na$$_2S $$and observing a black precipitate

Question 13

Some cations listed below can be present in the Qual cation analysis. If your unknown sample contains only Ag$$^+$$ and Pb$$^{2+}$$, which reagent would you add to precipitate only Ag$$^+$$ and not Pb$$^{2+}$$?

Accepted Answer

NH$$_3$$

Question 14

Flame tests are often used to detect preliminary indications that certain cations are present in Qual samples. Which cation is indicated by a green flame?

Accepted Answer

Cu$$^{2+}$$

Question 15

When HCl is added during Step 2 of the Qual cation procedure, the goal is to make PbCl$$_2$$ precipitate. The $$K_{sp} of $$PbCl$$_2 is$$ $$1.7 	imes 10$$^{-5}$$ at 25°C. If the initial Pb$$^{2+}$$ concentration is 0.10 M, calculate the minimum concentration of Cl$$^-$$ required to make PbCl_2$ begin precipitating.

Accepted Answer

1.3 	imes $$10^{-2}$ M

Question 16

If too much HCl is added when trying to precipitate PbCl$$_2 in $$the Qual procedure, PbCl$$_2$$ may form instead of AgCl. What is a possible risk in this scenario for trying to confirm the presence of Ag$$^+$$?

Accepted Answer

Ag$$^+$$ will not precipitate and will be lost in the filtrate

Question 17

In Step 4 of the Qual cation procedure, a buffer solution is made by adding NH$$_3$$ and NH$$_4Cl to $$the unknown sample. What is the main purpose of this buffer?

Accepted Answer

To maintain a constant pH and allow selective precipitation of cations

Question 18

The half-cell reaction for the reduction of nitrate to ammonia is:

$$\mathrm{NO_3$^- + 3H^+ + 3e^- \rightarrow \frac{1}{2} N_2$ + 1.5 H_2O}$$

Given the standard reduction potentials $$E^\circ(NO$_3^-) = +0.96 V $$and $$E^\circ(Fe$^{3+}/Fe$^{2+}) = +0.77 V$$, what is the cell potential for the reaction:

$$\mathrm{NO_3$^- + 3Fe$$^{2+}$$ + 4H^+ \rightarrow 3Fe$$^{3+}$$ + 2H_2O + NO}$$

Accepted Answer

0.19 V

Question 19

A student observes a yellow precipitate after adding Na$$_2S $$and HCl and heating it. Which cation is most likely present?

Accepted Answer

Cd$$^{2+}$$

General Chemistry II Lab Exam Study Guide: Solutions, Equilibria, Electrochemistry, and Qualitative Analysis

스터디 가이드 - 연습 문제

학습 가이드 - 플래시카드