Exam Review Guidance: Introduction to Chemistry (Modules 6–10)

Q1. You are given two liquids, one is an acid and the other is a base. State three tests that you could do to distinguish which is the acid and which is the base.

Background

Topic: Acids and Bases – Properties and Identification

This question tests your understanding of the physical and chemical properties that differentiate acids from bases, and how to use simple laboratory tests to identify them.

Key Terms and Concepts:

• pH: A measure of the hydrogen ion concentration; acids have pH < 7, bases have pH > 7.

• Litmus Paper: An indicator that turns red in acid and blue in base.

• Reaction with Carbonates: Acids react with carbonates to produce carbon dioxide gas; bases do not.

Step-by-Step Guidance

1. Test the pH of each liquid using pH paper or a pH meter. Record which sample has a lower pH (acid) and which has a higher pH (base).

2. Dip red and blue litmus paper into each liquid. Observe and record any color changes for each paper in each liquid.

3. Add a small amount of calcium carbonate (or another carbonate) to each liquid and observe if any gas bubbles (CO2) are produced.

Try solving on your own before revealing the answer!

Final Answer:

• Test their pH (acid will have pH < 7, base > 7).

• Test with red and blue litmus paper (acid turns blue litmus red, base turns red litmus blue).

• Add calcium carbonate (acid will produce CO2 gas, base will not).

These tests use observable properties to distinguish acids from bases.

Q2. Complete and balance the following acid-base neutralization reaction: Ca(OH)2 + HClO4 →

Background

Topic: Acid-Base Neutralization Reactions

This question tests your ability to write and balance chemical equations for neutralization reactions between a base and an acid.

Key Terms and Formulas:

• Neutralization Reaction: Acid + Base → Salt + Water

• Balancing Equations: Ensure the number of atoms of each element is the same on both sides.

Step-by-Step Guidance

1. Write the formulas for the reactants: Ca(OH)2 (calcium hydroxide, a base) and HClO4 (perchloric acid).

2. Predict the products: The cation from the base (Ca2+) combines with the anion from the acid (ClO4-) to form the salt, and water is formed from H+ and OH-.

3. Write the unbalanced equation: Ca(OH)2 + HClO4 → Ca(ClO4)2 + H2O

4. Balance the equation by adjusting coefficients so that the number of each type of atom is equal on both sides.

Try solving on your own before revealing the answer!

Final Answer:

Balanced equation: Ca(OH)2 + 2HClO4 → Ca(ClO4)2 + 2H2O

Two moles of acid are needed to neutralize one mole of base because Ca(OH)2 provides two OH- ions.

Q3. Write the net ionic equation for an acid-base neutralization reaction taking place during a typical titration.

Background

Topic: Net Ionic Equations for Acid-Base Reactions

This question tests your understanding of how to write net ionic equations, focusing on the actual chemical change during neutralization.

Key Terms and Formulas:

• Net Ionic Equation: Shows only the species that actually change during the reaction.

• Neutralization: H+ (from acid) reacts with OH- (from base) to form H2O.

Step-by-Step Guidance

1. Write the full ionic equation for a strong acid and strong base (e.g., HCl + NaOH).

2. Identify the spectator ions (ions that do not participate in the reaction).

3. Remove the spectator ions to write the net ionic equation, showing only the ions that form water.

Try solving on your own before revealing the answer!

Final Answer:

Net ionic equation: H+(aq) + OH-(aq) → H2O(l)

Alternatively, using hydronium: H3O+(aq) + OH-(aq) → 2H2O(l)

This shows the essential chemical change in a neutralization reaction.

Q4. What is the concentration of a solution of Ca(OH)2 if it takes 23.50 mL of Ca(OH)2 to neutralize 25.00 mL of 0.15 M H3PO4?

Background

Topic: Acid-Base Titration Calculations

This question tests your ability to use stoichiometry and titration data to calculate the unknown concentration of a solution.

Key Terms and Formulas:

• Molarity (M): $M = \frac{\text{mol}}{\text{L}}$

• Stoichiometry: Use the balanced equation to relate moles of acid and base.

• Balanced Equation: $3\text{Ca(OH)}_2(aq) + 2\text{H}_3\text{PO}_4(aq) \rightarrow \text{Ca}_3(\text{PO}_4)_2(aq) + 6\text{H}_2\text{O}(l)$

Step-by-Step Guidance

1. Calculate moles of H3PO4 using its volume and concentration: $\text{mol H}_3\text{PO}_4 = 0.15~\text{mol/L} \times 0.02500~\text{L}$

2. Use the stoichiometric ratio from the balanced equation to find moles of Ca(OH)2 needed to react with the calculated moles of H3PO4.

3. Calculate the concentration of Ca(OH)2 using the moles found and the volume of Ca(OH)2 used (in liters).

Try solving on your own before revealing the answer!

Final Answer:

Concentration of Ca(OH)2 is 0.24 M (0.24 mol/L).

We used the stoichiometry of the reaction and titration data to solve for the unknown concentration.

Q5. Write the balanced equation for the reaction between one molecule of the Bronsted-Lowry acid H3PO4 and one molecule of the Bronsted-Lowry base NH3.

Background

Topic: Bronsted-Lowry Acid-Base Reactions

This question tests your understanding of proton transfer reactions and how to write equations showing conjugate acid-base pairs.

Key Terms and Concepts:

• Bronsted-Lowry Acid: Proton (H+) donor.

• Bronsted-Lowry Base: Proton (H+) acceptor.

• Conjugate Acid-Base Pairs: The acid forms its conjugate base after donating a proton; the base forms its conjugate acid after accepting a proton.

Step-by-Step Guidance

1. Identify H3PO4 as the acid (proton donor) and NH3 as the base (proton acceptor).

2. Write the equation showing H3PO4 donating a proton to NH3.

3. Determine the products: H2PO4- (conjugate base) and NH4+ (conjugate acid).

Try solving on your own before revealing the answer!

Final Answer:

H3PO4 + NH3 → H2PO4- + NH4+

This shows the acid donating a proton to the base, forming their conjugate pairs.