AP Practice 3

A strong chemistry argument does more than announce an answer: it connects a claim to chemical evidence and explains why the evidence supports that claim. This original short free-response question (FRQ) practices that exam task type through Science Practice 6: Argumentation, while also requiring Science Practice 4: Model Analysis and Science Practice 5: Mathematical Routines.

Task type: Original, unofficial short free-response question
Suggested time: $10$–$12$ minutes
Primary science practice: 6. Argumentation
Supporting science practices: 4. Model Analysis and 5. Mathematical Routines

The investigation: Which antacid neutralizes more acid?

Two powdered antacids, $A$ and $B$, are tested by adding separate samples to excess hydrochloric acid. The remaining acid is then measured by titration with $0.100\ \mathrm{M}$ sodium hydroxide. Because any acid left over requires sodium hydroxide to neutralize, a smaller volume of sodium hydroxide indicates that the antacid consumed more hydrochloric acid.

The balanced neutralization reaction is

$$ \mathrm{HCl}(aq)+\mathrm{NaOH}(aq)\rightarrow \mathrm{NaCl}(aq)+\mathrm{H_2O}(l) $$

Each antacid sample has a mass of $0.500\ \mathrm{g}$. The initial hydrochloric acid contains $0.0500\ \mathrm{mol}$ of $\mathrm{HCl}$. The titration results are shown below.

Sample	Volume of $0.100\ \mathrm{M}$ $\mathrm{NaOH}$ used to titrate remaining $\mathrm{HCl}$
Antacid $A$	$18.0\ \mathrm{mL}$
Antacid $B$	$31.0\ \mathrm{mL}$

(a) Calculate the amount, in moles, of $\mathrm{HCl}$ remaining after reaction with antacid $A$.

(b) Determine which antacid consumed more $\mathrm{HCl}$ and support the conclusion with a quantitative argument using the data.

(c) A student claims that antacid $B$ must be more effective because it required a larger volume of $\mathrm{NaOH}$ during the titration. Evaluate the claim.

Worked reasoning

Part (a): Convert the titration measurement into chemical amount

The reaction ratio between $\mathrm{NaOH}$ and $\mathrm{HCl}$ is $1:1$. First convert the volume to liters:

$$ 18.0\ \mathrm{mL}=0.0180\ \mathrm{L} $$

Then use the molarity relationship:

$$ n=MV $$

$$ n_{\mathrm{NaOH}}=(0.100\ \mathrm{mol\ L^{-1}})(0.0180\ \mathrm{L}) =1.80\times10^{-3}\ \mathrm{mol} $$

Because the stoichiometric ratio is $1:1$,

$$ n_{\mathrm{HCl,\ remaining}}=1.80\times10^{-3}\ \mathrm{mol} $$

Point-worthy reasoning: The response must connect the measured sodium hydroxide volume to moles of sodium hydroxide, then use the balanced equation to identify the equal amount of remaining hydrochloric acid. A volume by itself is not yet a chemical amount.

Part (b): Build an evidence-based comparison

The initial amount of hydrochloric acid was

$$ n_{\mathrm{HCl,\ initial}}=0.0500\ \mathrm{mol} $$

For antacid $A$,

$$ n_{\mathrm{HCl,\ consumed}}

0.0500-0.00180

0.0482\ \mathrm{mol} $$

For antacid $B$, the amount of sodium hydroxide used is

$$ n_{\mathrm{NaOH}}

(0.100)(0.0310)

3.10\times10^{-3}\ \mathrm{mol} $$

Thus,

$$ n_{\mathrm{HCl,\ remaining}}=0.00310\ \mathrm{mol} $$

and

$$ n_{\mathrm{HCl,\ consumed}}

0.0500-0.00310

0.0469\ \mathrm{mol} $$

Antacid $A$ consumed more hydrochloric acid:

$$ 0.0482\ \mathrm{mol}>0.0469\ \mathrm{mol} $$

A complete argument is therefore: Antacid $A$ is more effective under these experimental conditions because less hydrochloric acid remained after treatment with $A$, so $A$ consumed $0.0482\ \mathrm{mol}$ of $\mathrm{HCl}$ compared with $0.0469\ \mathrm{mol}$ consumed by $B$.

Point-worthy reasoning: A high-scoring argument contains three linked parts: a clear claim, numerical evidence, and a chemical explanation connecting the evidence to the claim. Merely stating that $A$ is better does not establish the argument.

Part (c): Test the student’s interpretation

The student’s claim is incorrect. A larger volume of sodium hydroxide means that more hydrochloric acid remained to be titrated. Since antacid $B$ left $0.00310\ \mathrm{mol}$ of $\mathrm{HCl}$ while antacid $A$ left only $0.00180\ \mathrm{mol}$, $B$ neutralized less of the original acid.

Misconception check — “More titrant means more reacting power.”
In a back-titration, the titrant measures what is left over, not what the original substance consumed. A larger titrant volume can therefore indicate weaker performance by the tested material.

How the examiner sees the reasoning

Response feature	What it demonstrates
Correctly uses $n=MV$	5. Mathematical Routines
Uses the $1:1$ reaction ratio	Chemical interpretation of a representation
Subtracts remaining acid from initial acid	Quantitative model analysis
Identifies antacid $A$	Correct claim
Cites $0.0482\ \mathrm{mol}$ and $0.0469\ \mathrm{mol}$	Relevant evidence
Explains why lower leftover acid means greater neutralization	6. Argumentation

A frequent weak response compares $18.0\ \mathrm{mL}$ and $31.0\ \mathrm{mL}$ without explaining what those volumes represent. The stronger response analyzes the experimental model: titration measures residual $\mathrm{HCl}$, and residual acid is inversely related to acid consumed when the initial amount is fixed.

Error-review routine

After completing the question, classify any error as one of four types: measurement conversion—forgetting $\mathrm{mL}$ to $\mathrm{L}$; stoichiometric interpretation—missing the $1:1$ ratio; comparison logic—confusing leftover acid with consumed acid; or argumentation—giving a claim without numerical evidence and chemical reasoning. Rewrite only the sentence containing the error, then recompute the result without looking at the solution.

Retrieval check: If the initial acid amount were identical but Sample $C$ required $10.0\ \mathrm{mL}$ of the same sodium hydroxide solution, would $C$ have consumed more or less hydrochloric acid than $A$? Explain using the meaning of the titration volume.