Regents Examination in Physical Setting / Chemistry: How to Answer Part C (Extended-Response) Questions

An extended-response question is an examination question that requires the test taker to do more than choose among several responses or fill in a blank. You may need to perform numerical calculations, draw and interpret graphs, and provide extended written responses to a question or a problem.

Part C of the New York State Regents Examination in Chemistry— Physical Setting contains extended-response questions. This section is designed to provide you with a number of general guidelines for answering them.

Solving Problems Involving Numerical Calculations
To receive full credit you must:
Provide the appropriate equation(s).
Substitute values and units into the equation(s).
Display the answer, with appropriate units and to the correct number of significant figures.
If the answer is a vector quantity, include its direction.
You should write as legibly as possible. Teachers are human, and nothing irks them more than trying to decipher a careless, messy scrawl. It is also a good idea to identify your answer clearly, either by placing it in a box or by writing the word “answer” next to it.
A final word: If you provide the correct answer but do not show any work, you will not receive any credit for the problem!
The following is a sample problem and its model solution.

Problem
A 5.00-gram object has a density of 4.00 grams per cubic centimeter. Calculate the volume of this object.

Solution:

Rearranging the equation gives



Graphing Experimental Data
To receive full credit you must:
Label both axes with the appropriate variables and units.
Divide the axes so that the data ranges fill the graph as nearly as possible.
Plot all data points accurately.
Draw a best-fit line carefully with a straightedge. The line should pass through the origin only if the data warrant it.
If a part of the question requires that the slope be calculated, calculate the slope from the line, not from individual data points.
A graph should have a title, and the independent variable is usually drawn along the x-axis.
The following is a sample problem and its model solution.

Problem
A student attempts to estimate absolute zero in the following way: He subjects a sample of gas (at constant pressure and mass) to varying temperatures and measures the gas volume at each of the temperatures.

The accompanying table contains his experimental data.
 

Using axes that are appropriately labeled and scaled, draw a graph that accurately displays the student’s data.
Estimate the student’s value for absolute zero by extending the graph to the Celsius temperature at which the volume of the gas is 0 milliliter.

Solution
The first step is to construct an appropriate set of axes if one is not provided. We will assume that you must start from scratch. Since temperature is the independent variable, you need to place it along the x-axis. Also, since absolute zero is –273.15°C, you must scale the axes properly.

Here is one appropriate set of axes:

Second, you must plot the data points carefully on the axes as shown below:

Your third task is to draw the graph. You might be tempted to “connect the dots,” but then you would miss the significant relationship between volume and temperature. If you examine all of the plotted points, you will note that they fall approximately on a straight line.
Therefore, the next step is to draw a best-fit straight-line graph. This is a graph in which the data points are most closely distributed on both sides of the line.

The accompanying graph shows the best-fit straight line.

Note that the graph extending beyond the data points (above and below) is a dashed line. Such extensions are known as extrapolated data, and they are based on the assumption that the gas will continue to behave as it did within the experimental range for which the student collected data.

(You may ask: Why this particular line? It seems as though many lines could have been drawn using the plotted data. Actually, there is only one best-fit straight line, and it is calculated by using an advanced statistical technique known as linear regression. This technique was used to draw the line shown above. At this point, it is sufficient for you to provide an “eyeball” estimate of the best-fit straight line.)
Your final task is to inspect the graph closely and to estimate absolute zero. The calculated value is –263°C. (This corresponds to an experimental error of 3.7%.)

Drawing Diagrams
To receive full credit you must:
Draw your diagrams neatly and label them clearly.
Bring a straightedge and a protractor with you so that you can draw neat, accurate diagrams.
The accompanying diagram represents a zinc–copper electrochemical cell containing an agar–KCl salt bridge.

This is the type of diagram you may be asked to draw as part of an examination question.


Writing a Free-Response Answer
To receive full credit you must:
Use complete, clear sentences that make sense to the reader.
Use correct chemistry in your explanations.
A sample question and a model answer follow.

Question
Describe in detail the technique used to determine the concentration of a dilute hydrochloric acid solution using a dilute sodium hydroxide solution of known concentration.
Name any equipment or other chemicals that you would use. You need not describe any mathematical calculations.

Model Answer
Since this question requires an extended answer containing a series of steps, we decide to use an outline form.
The technique is called titration, and it is described in the steps given below.
Place a known volume of the hydrochloric acid solution in a beaker of suitable size.
Add a drop or two of an acid–base indicator such as phenolphthalein.
Pour the sodium hydroxide solution carefully into a burette, using a small funnel.
Open the stopcock to allow the trapped air to escape. Then close the stopcock and wipe the tip of the burette with a tissue to remove any clinging liquid.
Record the initial volume in the burette.
Add the base slowly to the acid solution, with continuous stirring, until the phenolphthalein just changes from colorless to faint pink.
Record the final volume in the burette.
Repeat the experiment at least once.
Rinse the apparatus with water to remove all traces of acid and base.
Calculate the concentration of the hydrochloric acid solution.

Writing and Balancing Equations in a Chemistry Examination
For all of the suggestions that follow, the reaction that occurs between aqueous solutions of sodium sulfate and barium nitrate is used as an example and is referred to as the given reaction.

If you are asked to write a word equation, be certain to include the correct names of the reactants and products and their phases in the reaction.

For the given reaction, the word equation is

 

If you are asked to write a balanced equation, you are usually expected to balance using smallest whole-number coefficients.

For the given reaction, the balanced equation is



If you are asked to write an .
 

For the given reaction, the ionic equation is

 

If you are asked to write a net ionic equation, you should omit all spectator ions, that is, all ions appearing unchanged on both sides of the equation.
 

For the given reaction, the net ionic equation is