Acids and Bases Laboratory Quiz

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Questions and Answers

What color does blue litmus paper turn in an acidic solution?

Red

What is the chemical formula for hydrochloric acid?

HCl

Name one common laboratory acid other than hydrochloric acid.

Sulfuric acid

What type of indicator changes its odor in acidic or basic solutions?

Olfactory indicator Signup and view all the answers

What color does red litmus paper turn in a basic solution?

Blue Signup and view all the answers

Give an example of a common base, other than sodium hydroxide.

Calcium hydroxide Signup and view all the answers

What is the chemical formula for acetic acid?

CH3COOH Signup and view all the answers

Is ammonium hydroxide ($NH_4OH$) an acid or a base?

Base Signup and view all the answers

What is the effect of an acid on methyl orange indicator?

Turns red Signup and view all the answers

What is the effect of a base on phenolphthalein indicator?

Turns pink Signup and view all the answers

How can you distinguish between three test tubes containing distilled water, an acidic solution, and a basic solution, respectively, using only red litmus paper?

Dip the red litmus paper in each solution. The solution that turns the red litmus paper blue is basic. Then, dip the blue litmus paper (formed in the basic solution) in the remaining two solutions. The solution that turns the blue litmus paper red is acidic. The solution that doesn't change the color of either litmus paper is distilled water. Signup and view all the answers

A student tests a solution with red litmus paper and observes no change. What can they conclude about the solution's nature?

The solution is either acidic or neutral. Signup and view all the answers

Explain how olfactory indicators can be used to differentiate between hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions.

Olfactory indicators have different odors in acidic and basic solutions. If an olfactory indicator, such as onion extract, smells different in one solution compared to another, one can be identified as acidic (HCl) and the other as basic (NaOH). Signup and view all the answers

If a solution changes methyl orange indicator from orange to red, what does this indicate about the solution?

The solution is acidic. Signup and view all the answers

A lab technician accidentally mixed hydrochloric acid ($HCl$) and sodium hydroxide ($NaOH$). How can they determine if the resulting solution is neutral, acidic, or basic using only phenolphthalein?

Add phenolphthalein to the solution. If the solution remains colorless, it is neutral or acidic. If the solution turns pink, it is basic. Signup and view all the answers

You have two unknown solutions, one acidic and one basic. Using only the chopped onion test, describe how you would identify each solution.

Prepare two samples of chopped onions. Add one solution to each sample. The solution in which the onion smell intensifies or remains the same is the acidic solution. The solution in which the onion smell diminishes or disappears is the basic solution. Signup and view all the answers

A student tests a solution with both red litmus paper and blue litmus paper. The red litmus turns blue, and the blue litmus remains blue. What can the student conclude about the solution?

The solution is basic. Signup and view all the answers

Explain why it is important to use a variety of indicators, rather than just one, when determining the acidity or basicity of a solution.

Different indicators change color at different pH levels. Using a variety of indicators provides a more precise estimate of the solution's pH and helps to confirm the results. Signup and view all the answers

If you add a drop of phenolphthalein to calcium hydroxide [$Ca(OH)_2$], what color change would you expect to see and why?

You would expect the solution to turn pink because calcium hydroxide is a base, and phenolphthalein turns pink in basic solutions. Signup and view all the answers

A solution is tested with methyl orange and the solution turns yellow. Describe the nature of the solution.

The solution is basic. Signup and view all the answers

Explain how the presence of a weak acid like acetic acid ($CH_3COOH$) affects the degree of dissociation of a strong acid like hydrochloric acid (HCl) in the same solution. What is this phenomenon called?

The presence of a weak acid like acetic acid suppresses the ionization of a strong acid like hydrochloric acid. This is known as the common ion effect. Signup and view all the answers

Imagine you have a mixture of three unknown solutions: hydrochloric acid (HCl), sodium hydroxide (NaOH), and sodium chloride (NaCl). You only have access to red litmus paper. Describe a step-by-step procedure to identify each solution.

<ol> <li>Test each solution with red litmus paper. 2. The solution that turns red litmus blue is NaOH. 3. Test the remaining two solutions by adding the NaOH to each; the one that generates heat (exothermic reaction) is HCl. 4. The remaining solution is NaCl.</li> </ol> Signup and view all the answers

If a solution of $NH_4Cl$ (ammonium chloride) is added to a solution of $NH_4OH$ (ammonium hydroxide), how does the pH of the $NH_4OH$ solution change, and why?

The pH decreases due to the common ion effect; the added $NH_4^+$ ions from $NH_4Cl$ suppress the dissociation of $NH_4OH$, reducing $OH^-$ concentration. Signup and view all the answers

Explain how an olfactory indicator like vanilla extract would behave differently in a strong acid versus a strong base. What chemical process causes this difference?

In a strong acid, vanilla extract might retain or enhance its original scent due to protonation of certain organic compounds. In a strong base, the scent could change or diminish due to deprotonation or hydrolysis of the scent compounds. Signup and view all the answers

You have two solutions: one is a strong acid and the other is a weak acid. Both have the same pH. Explain why the strong acid will cause a greater change in pH when diluted compared to the weak acid.

The strong acid is fully dissociated, so dilution directly lowers $[H^+]$. The weak acid's dissociation shifts to maintain equilibrium, partially offsetting the $[H^+]$ decrease, leading to a smaller pH change. Signup and view all the answers

How does the strength of an acid or base relate to its conductivity in solution, and why might this relationship not always hold true?

Stronger acids and bases generally have higher conductivity due to greater ion concentration. However, factors like ion size and mobility can affect conductivity independent of strength. Signup and view all the answers

Describe what would theoretically happen to the color of methyl orange indicator ($pK_a \approx 3.4$) in a solution buffered at pH 7.0.

Methyl orange would be yellow. Because the pH is well above its $pK_a$, the deprotonated form (yellow) would predominate. Signup and view all the answers

Explain why, at the same molar concentration, sulphuric acid ($H_2SO_4$) can neutralize twice the amount of sodium hydroxide (NaOH) compared to hydrochloric acid (HCl).

Sulfuric acid is a diprotic acid, meaning each molecule can donate two protons ($H^+$), whereas hydrochloric acid is monoprotic and can donate only one. Signup and view all the answers

You have a solution that turns blue litmus red but does not react with zinc metal. What does this suggest about the acidic properties of the solution?

The solution is likely a weak acid. It has sufficient $H^+$ ions to change litmus color but not enough to react visibly with zinc. Signup and view all the answers

Using Le Chatelier's principle, predict the effect of increasing the temperature on the ionization of water ($H_2O \rightleftharpoons H^+ + OH^-$). Given that the process is endothermic.

Increasing temperature will shift the equilibrium to the right, increasing ionization and both $[H^+]$ and $[OH^-]$, because the forward reaction (ionization) is endothermic. Signup and view all the answers

Consider a scenario where you have synthesized a novel amphoteric molecule with unknown acid dissociation constants ($pK_a$ values). Describe a multi-step experimental procedure, including specific techniques and calculations, to determine the distinct $pK_a$ values associated with this molecule in an aqueous solution at $25^{\circ}C$.

Perform a potentiometric titration using a calibrated pH meter and a strong acid/base titrant. Plot the titration curve (pH vs. volume of titrant). Identify the inflection points on the curve, which correspond to the $pK_a$ values. Use the Henderson-Hasselbalch equation to refine the results and determine buffer regions. Signup and view all the answers

You are tasked with designing a buffer system that maintains a pH of 7.4 under physiological conditions but must also function effectively at $4^{\circ}C$ (refrigeration temperature). Explain how temperature affects the ionization constant ($K_a$) of weak acids, and detail a method to adjust the buffer composition to compensate for these temperature-induced changes, ensuring optimal buffering capacity at both temperatures.

Temperature affects $K_a$ (ionization constant) due to changes in equilibrium. To compensate, determine the enthalpy of ionization, $H$, for the buffering acid. Use the van't Hoff equation to calculate $K_a$ at $4^{\circ}C$. Adjust the ratio of acid to conjugate base in the buffer system to maintain pH 7.4 at the specified temperature. Verify buffer capacity through titration. Signup and view all the answers

Aqueous solutions of $FeCl_3$ are acidic. Explain the chemical processes involved at a molecular level, using appropriate chemical equations, which results in the solution having a pH less than 7. Furthermore, explain how increasing the concentration of $FeCl_3$ would affect the pH.

$Fe^{3+}$ undergoes hydrolysis. $Fe^{3+}(aq) + H_2O(l) ightleftharpoons [Fe(OH)]^{2+}(aq) + H^+(aq)$. This releases $H^+$ ions, decreasing the pH. Increasing the concentration of $FeCl_3$ shifts the equilibrium to the right, further increasing the $H^+$ concentration and lowering the pH. Signup and view all the answers

Consider a scenario: A research lab has discovered a new organic compound. Initial tests indicate it behaves as a weak base. Describe a comprehensive experimental procedure, including specific analytical techniques, to determine the base dissociation constant ($K_b$) of this compound. Detail the calculations necessary to derive $K_b$ from the experimental data.

Titrate the weak base with a standardized strong acid (e.g., HCl) using a pH meter. Record the pH changes during titration. Determine the equivalence point using the first or second derivative method. Calculate the $K_b$ using the pH at the half-equivalence point where [base] = [conjugate acid]. Signup and view all the answers

Explain, using principles of chemical thermodynamics, why strong acids completely dissociate in aqueous solution while weak acids only partially dissociate. Include in your explanation the role of entropy and enthalpy in determining the extent of dissociation, and how the hydration of ions influences the process.

Strong acids have a large, negative Gibbs free energy change (G) for dissociation due to a combination of favorable enthalpy (H) from strong hydration of ions and increased entropy (S) from increased particle number. Weak acids have a less negative G due to weaker ion hydration and smaller entropic gains, leading to incomplete dissociation. Signup and view all the answers

A solution is prepared by mixing 50.0 mL of 0.20 M $NH_3$ with 50.0 mL of 0.10 M HCl. Calculate the pH of the resulting solution, given that the $K_b$ for $NH_3$ is $1.8 imes 10^{-5}$.

First, HCl reacts with $NH_3$: $NH_3 + H^+ ightarrow NH_4^+$. Moles of $NH_3 = 0.05L \cdot 0.2M = 0.01 mol$. Moles of $H^+ = 0.05L \cdot 0.1M = 0.005 mol$. After reaction, 0.005 mol $NH_3$ and 0.005 mol $NH_4^+$ remain. This forms a buffer. Use the Henderson-Hasselbalch equation: $pH = pK_a + log([NH_3]/[NH_4^+])$. $pK_a=14 - pK_b = 14 - (-log(1.8 imes 10^{-5}))=9.26$. Since $[NH_3]=[NH_4^+]$, $pH=9.26$. Signup and view all the answers

Describe an experimental setup to measure the enthalpy change of neutralization when a strong acid reacts with a strong base in a calorimeter. Detail the steps required for accurate data collection and explain how heat loss to the surroundings can be minimized. Further, explain how you would calculate the enthalpy change of neutralization from the experimental data.

Use a bomb calorimeter to minimize heat loss. Add known volumes of equimolar strong acid and base. Record the temperature change ($T$). Calculate heat absorbed by the calorimeter: $q = CT$ (C is the calorimeter's heat capacity). Enthalpy change of neutralization: $H = -q/n$, where n is moles of water formed. Correct for heat loss (e.g., using a cooling curve). Signup and view all the answers

How do the Hammett acidity function ($H_0$) and Z acidity function provide a more accurate measure of acidity in highly concentrated acid solutions compared to pH? Explain the theoretical basis for these acidity functions, and describe an experimental method to determine the $H_0$ value of a given superacid system.

pH is inadequate at high concentrations due to activity coefficient deviations. $H_0$ and Z correct for this by measuring the protonation of neutral indicators. $H_0 = pK_{BH^+} - log([BH^+]/[B])$. Use UV-Vis spectroscopy to measure the ratio $[BH^+]/[B]$ for a series of indicators with known $pK_{BH^+}$ values in the acid. Extrapolate to find $H_0$. Signup and view all the answers

You have a mixture containing a strong acid, a weak acid, and a neutral salt. Design a sequential titration procedure using both acid-base titrants and selective precipitation reactions to quantify each component independently. Explain the chemical principles behind each step, including appropriate indicator choices and pH ranges for accurate endpoint determination.

First, titrate the strong acid with a strong base using an indicator like methyl orange (pH 3.1-4.4). Then, add a precipitating agent to selectively precipitate the neutral salt (e.g., $AgNO_3$ for $Cl^-$ salts). Finally, titrate the weak acid remaining in solution with a strong base using phenolphthalein (pH 8.3-10.0), after removing the precipitate. Calculate each concentration from titration data. Signup and view all the answers

Explain the concept of superacidity. Provide four examples - with chemical formulas - of superacids, and delineate the criteria used to classify these substances as such. Compare and contrast the properties and applications of two specific superacids, highlighting their strengths and weaknesses.

Superacidity is acidity greater than 100% sulfuric acid and $H_0 < -12$. Examples: $HF/SbF_5$, $HSO_3F/SbF_5$, $CF_3SO_3H$, magic acid ($HSO_3F/SbF_5$). $HF/SbF_5$ is a strong protonator, useful in organic synthesis, but highly corrosive. $CF_3SO_3H$ is less corrosive and easier to handle, but less effective for certain reactions. Signup and view all the answers

Using only red litmus paper, how can you distinguish between distilled water, an acidic solution, and a basic solution?

Add red litmus paper to each solution. If it turns blue, it's a base. If it remains red, test with the other solutions. The one that doesn't change the color of the changed litmus paper is distilled water, and the remaining one is acidic. Signup and view all the answers

What is the purpose of using indicators like litmus paper, phenolphthalein and methyl orange?

To determine whether a substance is acidic or basic (or neutral) by observing a change in color. Signup and view all the answers

Explain what olfactory indicators are and provide an example from the text.

Olfactory indicators are substances whose odor changes depending on whether they are placed into an acidic or basic medium. Finely chopped onions are an example. Signup and view all the answers

Write the chemical formula for sulfuric acid.

$H_2SO_4$ Signup and view all the answers

List three common laboratory acids mentioned in the text.

Hydrochloric acid (HCl), sulfuric acid ($H_2SO_4$), and nitric acid ($HNO_3$). Signup and view all the answers

What would you expect to observe if you tested a solution of calcium hydroxide [$Ca(OH)_2$] with red litmus paper?

The red litmus paper would turn blue. Signup and view all the answers

Imagine you have a solution that remains colorless when tested with both red litmus and blue litmus paper. What can you conclude about the solution?

The solution is likely neutral. Signup and view all the answers

Describe a procedure to identify if an unknown solution is either $H_2SO_4$ or $CH_3COOH$ using only the materials listed in Activity 2.1. What observations would lead you to differentiate them?

Test both solutions with methyl orange, and measure the intensity of the resulting red color. Sulfuric acid will produce a more intense red color than acetic acid, because it is a stronger acid. Signup and view all the answers

Propose a modification to Activity 2.2 that could allow for a more quantitative measurement of acidity or basicity using the onion olfactory indicator. What additional materials would you need?

Use a series of solutions with known pH values and immerse the cloth strips in each. Then, create a scale of odor intensity corresponding to each pH value. This would require a set of standard solutions or a calibrated pH meter. Signup and view all the answers

You have solutions of $NaOH$ and $NH_4OH$ of equal concentration. Without using any additional indicators or laboratory equipment, describe a method to qualitatively determine which solution is more basic, relying only on your sense of smell after controlled exposure to air. Explain the chemical principle behind your observation.

Expose equal volumes of both solutions to the air and cautiously smell each from a distance. The $NH_4OH$ solution will emit a stronger ammonia odor due to the volatilization of $NH_3$ gas, indicating that the solution contains a higher concentration of dissolved ammonia, and is thus less basic than $NaOH$. Signup and view all the answers

How can red litmus paper be used to differentiate between distilled water, an acidic solution, and a basic solution?

Red litmus paper turns blue in a base. Therefore, the solution that turns red litmus blue is basic. The other two solutions will not change the red litmus paper's color. You can then use blue litmus paper (created in the first step) to test the remaining two solutions. The solution that turns blue litmus paper red is acidic, while the solution that does not change the color of either is distilled water. Signup and view all the answers

List three common acids that might be found in a school science laboratory.

Hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3) Signup and view all the answers

What is an olfactory indicator?

A substance whose odor changes in acidic or basic media. Signup and view all the answers

Describe a simple method to prepare olfactory indicator strips using common household items.

Chop onions finely, place them in a plastic bag with cloth strips, seal the bag tightly, refrigerate it overnight, and then use the cloth strips to test for acids and bases. Signup and view all the answers

If a solution turns blue litmus paper red, is it acidic or basic?

Acidic Signup and view all the answers

What color change would you expect to see when you add phenolphthalein to a basic solution?

Pink Signup and view all the answers

Explain why it is important to use a variety of indicators when testing unknown solutions.

Different indicators change color at different pH levels, providing a more comprehensive understanding of a solution's acidity or basicity. Using multiple indicators can help narrow down the pH range more accurately. Signup and view all the answers

A student tests a solution with both red litmus paper and blue litmus paper. The red litmus paper turns blue, and the blue litmus paper remains blue. What can the student conclude about the solution?

The solution is basic. Signup and view all the answers

Imagine you have a solution that is known to be either a very weak acid or a very weak base. Neither red nor blue litmus paper changes color noticeably when dipped into the solution. Suggest an alternative indicator that might provide a clearer result, and explain why it might be more suitable.

Methyl orange could be more suitable because it changes color within a pH range of 3.1 to 4.4, which is more sensitive to weakly acidic conditions. Litmus paper has a broader transition range and may not show a clear change with weak acids or bases. Another option is a universal indicator. Signup and view all the answers

You have three unknown solutions: one is $0.1 M$ HCl, another is $0.1 M$ NaOH, and the third is $0.1 M$ NaCl. You only have access to a pH meter that reads only integer values (no decimal places) and methyl orange indicator. Describe a procedure to identify each solution using only these tools, given the limitations of the pH meter.

Measure the pH of each solution. $0.1 M$ HCl will read pH 1, $0.1 M$ NaOH will read pH 13, and $0.1 M$ NaCl will read pH 7. Confirm these results using methyl orange. The HCl solution should turn red (pH < 3.1), the NaOH solution should turn yellow (pH > 4.4), and the NaCl solution should show an intermediate color (likely orange). Signup and view all the answers

Flashcards

Red Litmus Paper

A type of paper that turns blue in basic solutions and remains red in acidic solutions.

Acidic Solution

A solution with a pH less than 7 that donates protons (H+) in a reaction.

Basic Solution

A solution with a pH greater than 7 that accepts protons (H+) in a reaction.

Indicators

Substances that change color in response to acidity or basicity, helping to identify the nature of a solution.