CHEM 1364 Exam 3 – Step-by-Step Study Guidance

Q1. 45.4 g of NaCl is dissolved in 350 mL of water. What is the molarity of the solution?

Background

Topic: Solution Concentration (Molarity)

This question tests your ability to calculate the molarity of a solution given the mass of solute and the volume of solvent.

Key Terms and Formulas

• Molarity (M) = moles of solute / liters of solution

• Moles = mass (g) / molar mass (g/mol)

Step-by-Step Guidance

1. Calculate the molar mass of NaCl: $M_{NaCl} = M_{Na} + M_{Cl}$

2. Convert 45.4 g NaCl to moles: $n = \frac{45.4\ g}{M_{NaCl}}$

3. Convert 350 mL to liters: $V = 350\ mL \times \frac{1\ L}{1000\ mL}$

4. Set up the molarity formula: $M = \frac{n}{V}$

Try solving on your own before revealing the answer!

Final Answer: 2.22 M

Using the calculated moles and volume in liters, $M = \frac{0.776}{0.350} = 2.22$ M.

This means the solution is quite concentrated.

Q2. Which one of the following compounds is insoluble in water?

Background

Topic: Solubility Rules

This question tests your knowledge of common solubility rules for ionic compounds in water.

Key Terms and Concepts

• Soluble: Dissolves in water to form an aqueous solution.

• Insoluble: Does not dissolve significantly in water; forms a precipitate.

• Key solubility rules: Most nitrates, alkali metal salts, and ammonium salts are soluble. Most silver halides (like AgBr) are insoluble.

Step-by-Step Guidance

1. Recall the solubility rules for each compound:

2. Check if KOH (potassium hydroxide) is soluble (alkali metal hydroxide).

3. Check if Ca(NO3)2 (calcium nitrate) is soluble (all nitrates are soluble).

4. Check if AgBr (silver bromide) is soluble (most silver halides are insoluble).

5. Check if Na2S (sodium sulfide) is soluble (alkali metal salts are soluble).

Try solving on your own before revealing the answer!

Final Answer: AgBr

Silver bromide is insoluble in water due to the low solubility of most silver halides.

Q3. What is the oxidation state of N in NCl3?

Background

Topic: Oxidation States

This question tests your ability to assign oxidation numbers to elements in a compound.

Key Terms and Formulas

• Oxidation state: The hypothetical charge an atom would have if all bonds were ionic.

• Sum of oxidation states in a neutral molecule is zero.

• Cl usually has an oxidation state of -1.

Step-by-Step Guidance

1. Assign the oxidation state of Cl as -1.

2. Let the oxidation state of N be x.

3. Set up the equation: $x + 3(-1) = 0$

4. Solve for x.

Try solving on your own before revealing the answer!

Final Answer: +3

Nitrogen must be +3 to balance the three -1 charges from chlorine in a neutral molecule.

Q4. Which element undergoes oxidation in the reaction NCl3(l) + 3H2O(l) → NH3(g) + 3HOCl(aq)?

Background

Topic: Redox Reactions

This question tests your ability to identify which element is oxidized (loses electrons) in a chemical reaction.

Key Terms and Concepts

• Oxidation: Increase in oxidation state (loss of electrons).

• Reduction: Decrease in oxidation state (gain of electrons).

Step-by-Step Guidance

1. Assign oxidation states to N, Cl, H, and O in both reactants and products.

2. Compare the oxidation state of each element before and after the reaction.

3. Identify which element's oxidation state increases (that element is oxidized).

Try solving on your own before revealing the answer!

Final Answer: Cl

Chlorine's oxidation state increases from -1 in NCl3 to +1 in HOCl, indicating oxidation.

Q5. Which of the following are physical changes?

Background

Topic: Physical vs. Chemical Changes

This question tests your ability to distinguish between physical and chemical changes.

Key Terms and Concepts

• Physical change: Change in state or appearance without changing chemical composition (e.g., melting, evaporation).

• Chemical change: Produces new substances (e.g., combustion, charring).

Step-by-Step Guidance

1. Identify which processes involve only a change in state (physical) and which involve a chemical reaction.

2. Butane combustion and steak charring are chemical changes.

3. Acetone evaporating and ice melting are physical changes.

4. Select the answer that includes only physical changes.

Try solving on your own before revealing the answer!

Final Answer: II and III

Evaporation and melting are physical changes; combustion and charring are chemical changes.

Q6. Which of the following metals will reduce Cr3+?

Background

Topic: Activity Series and Redox Reactions

This question tests your understanding of the metal activity series and which metals can reduce Cr3+ ions.

Key Terms and Concepts

• Activity series: A list of metals ranked by their tendency to lose electrons (be oxidized).

• A metal higher in the series can reduce the ions of metals below it.

Step-by-Step Guidance

1. Locate Cr on the activity series provided.

2. Identify which metals (Mg, Mn, Na, Fe) are above Cr in the series.

3. Metals above Cr can reduce Cr3+ to Cr(s).

4. Select the answer that includes all such metals.

Try solving on your own before revealing the answer!

Final Answer: I, II, and III

Mg, Mn, and Na are all above Cr in the activity series and can reduce Cr3+.

Q7. The solute of a solution may be a ___________.

Background

Topic: Solutions and Solubility

This question tests your understanding of the possible physical states of solutes in solutions.

Key Terms and Concepts

• Solute: The substance dissolved in a solvent to form a solution.

• Solutes can be solids, liquids, or gases.

Step-by-Step Guidance

1. Recall examples of solutions with solid, liquid, and gas solutes (e.g., salt in water, ethanol in water, CO2 in soda).

2. Determine which answer choice includes all possible states.

Try solving on your own before revealing the answer!

Final Answer: A, B, & C

Solutes can be solids, liquids, or gases in a solution.

Q8. What is the name of HCl?

Background

Topic: Nomenclature of Acids

This question tests your knowledge of naming binary acids.

Key Terms and Concepts

• Binary acid: An acid composed of hydrogen and one other element.

• HCl is a binary acid; its name follows the pattern "hydro- + root + -ic acid".

Step-by-Step Guidance

1. Recognize HCl as hydrochloric acid.

2. Recall the naming rule for binary acids.

3. Match the correct name to the formula.

Try solving on your own before revealing the answer!

Final Answer: hydrochloric acid

HCl is named hydrochloric acid according to acid nomenclature rules.

Q9. If you were to dilute 25.3 mL of concentrated sulfuric acid (18.4 M) to a total of 600 mL of water, what would be the molarity of the dilute acid?

Background

Topic: Solution Dilution

This question tests your ability to use the dilution equation to find the new concentration after dilution.

Key Terms and Formulas

• Dilution equation: $M_1V_1 = M_2V_2$

• $M_1$ = initial molarity, $V_1$ = initial volume

• $M_2$ = final molarity, $V_2$ = final volume

Step-by-Step Guidance

1. Identify $M_1 = 18.4$ M, $V_1 = 25.3$ mL, $V_2 = 600$ mL + $25.3$ mL (if total volume includes acid, otherwise just 600 mL).

2. Convert all volumes to liters or keep in mL as long as units are consistent.

3. Set up the equation: $M_1V_1 = M_2V_2$

4. Solve for $M_2$.

Try solving on your own before revealing the answer!

Final Answer: 0.776 M

After dilution, the new molarity is approximately 0.776 M.

Q10. Which of the following is a chemical property?

Background

Topic: Physical vs. Chemical Properties

This question tests your ability to distinguish between physical and chemical properties of substances.

Key Terms and Concepts

• Chemical property: Describes a substance's ability to undergo a specific chemical change (e.g., flammability).

• Physical property: Can be observed without changing the substance's chemical identity (e.g., color, melting point, texture).

Step-by-Step Guidance

1. Identify which property involves a chemical change (e.g., burning, reacting).

2. Eliminate physical properties from the choices.

3. Select the chemical property.

Try solving on your own before revealing the answer!

Final Answer: Flammability

Flammability is a chemical property because it describes the ability to burn (react with oxygen).