SCSA Chemistry Science Inquiry Skills
15 sample questions with marking guides and sample answers · Avg. score: 68.4%
A chemist performed a series of titrations and published the results in a scientific journal. From the point of view of the chemist, the titration data is
primary.
secondary.
personal.
investigative.
Reveal Answer
primary.
Primary data is data collected directly by the researcher for their own specific purpose. Since the chemist performed the titrations themselves, the data is primary.
secondary.
Secondary data is data collected by someone else. If another scientist used this published data, it would be secondary data to them, but not to the original chemist.
personal.
Personal data refers to information relating to an identified or identifiable individual, such as names or addresses, not experimental scientific results.
investigative.
While the chemist is conducting an investigation, "investigative" is not a standard statistical classification for the source of data.
An electrolysis cell consumed a charge of 4.00 C in 5.00 minutes.
This represents a consumption of
mol of electrons.
mol of electrons.
mol of electrons.
mol of electrons.
Reveal Answer
mol of electrons.
The number of moles of electrons is calculated by dividing the total charge by Faraday's constant (). Thus, .
mol of electrons.
This result comes from incorrectly multiplying the charge by the time in minutes before dividing by Faraday's constant. The given value of 4.00 C is already the total charge.
mol of electrons.
This value is obtained by dividing Faraday's constant by the time in minutes, which does not yield the moles of electrons.
mol of electrons.
This is the result of dividing Faraday's constant by the charge (), rather than using the correct formula of charge divided by Faraday's constant ().
Which statement best outlines the difference between systematic and random errors?
Systematic errors
will consistently produce measured values either above or below the actual value, while random errors will produce measured values above and below the actual value.
can be reduced through multiple trials and averaging of results, while random errors cannot be reduced.
are generally accurate but not precise, while random errors are generally precise but may not be accurate.
are difficult to avoid, while random errors can be readily improved.
Reveal Answer
will consistently produce measured values either above or below the actual value, while random errors will produce measured values above and below the actual value.
Systematic errors shift measurements in one consistent direction (either always too high or always too low), whereas random errors cause unpredictable fluctuations both above and below the true value.
can be reduced through multiple trials and averaging of results, while random errors cannot be reduced.
This statement has it backwards. Random errors can be reduced by taking multiple trials and averaging the results, but systematic errors will persist regardless of how many trials are averaged.
are generally accurate but not precise, while random errors are generally precise but may not be accurate.
This is the opposite of the truth. Systematic errors reduce accuracy by shifting the mean away from the true value, while random errors reduce precision by scattering the data points.
are difficult to avoid, while random errors can be readily improved.
Systematic errors can often be identified and eliminated through proper equipment calibration, whereas random errors are inherent to the measurement process and cannot be completely avoided.
Scientific posters communicate the findings of scientific investigations.
Which section of a scientific poster should explain the reason for undertaking an investigation?
discussion
conclusion
introduction
methodology
Reveal Answer
discussion
The discussion section is used to interpret the results, explain their significance, and compare them to existing literature, not to state the initial reason for the study.
conclusion
The conclusion summarizes the main findings of the investigation and their broader implications, rather than explaining why the study was started.
introduction
The introduction provides the background information, context, and the specific rationale or reason for undertaking the investigation.
methodology
The methodology section details the procedures, materials, and techniques used to conduct the investigation, not the reason for doing it.
Consider an acid-base titration between hydrochloric acid solution and ammonia solution. Which of the following actions is least likely to cause an error when calculating the concentration of hydrochloric acid?
cleaning the pipette with distilled water before each titration
rinsing the sides of the conical flask with distilled water during the titration
measuring the ammonia solution in a 20 mL measuring cylinder
leaving the funnel in the burette for each titration
Reveal Answer
cleaning the pipette with distilled water before each titration
Rinsing the pipette with distilled water dilutes the solution being transferred, which alters the number of moles delivered and causes a calculation error.
rinsing the sides of the conical flask with distilled water during the titration
Adding distilled water to the conical flask does not change the total number of moles of acid or base present, so it will not affect the titration results or the calculated concentration.
measuring the ammonia solution in a 20 mL measuring cylinder
A measuring cylinder is not precise enough for analytical titrations; a volumetric pipette must be used to avoid significant volume measurement errors.
leaving the funnel in the burette for each titration
Leaving the funnel in the burette can allow additional drops of solution to fall into the burette during the titration, leading to inaccurate volume readings.
Groundwater, in addition to dam water and desalinated seawater, is part of the water supply to Perth homes. Groundwater contains a wide variety of chemicals that can affect the quality of drinking water. One of the contaminants is iron, often found in the form of iron(II) hydrogencarbonate.
The iron can be removed by the addition of chlorine gas. Enough calcium hydrogencarbonate is added to maintain a slightly basic pH. The reaction can be represented by the following equation:
7.00 g of chlorine gas is bubbled through 30 000 L of groundwater containing 39 010 mg of iron(II) hydrogencarbonate to which 16.22 g of calcium hydrogencarbonate has been added. Calculate the mass of iron(III) hydroxide that will be precipitated.
Reveal Answer
Moles calculation
| Descriptor | Marks |
|---|---|
| 1 | |
| 1 | |
| 1 |
Limiting reagent determination
Marking Bands| Descriptor | Marks |
|---|---|
Correctly determines is limiting reagent with full justification (e.g., comparing required vs available moles) | 3 |
Determines is limiting reagent with partial justification | 2 |
Attempts to determine limiting reagent | 1 |
None of the above | 0 |
Final calculation
| Descriptor | Marks |
|---|---|
| 1 | |
| 1 |
Calculate the concentration of calcium chloride in the final solution.
Reveal Answer
| Descriptor | Marks |
|---|---|
| 1 | |
| 1 |
Use the following information to answer the question.
A group of students conducted a series of titrations to determine the concentration of acetic acid in vinegar using the following steps:
i. A sample of vinegar was pipetted into a volumetric flask that had been rinsed with the vinegar and then deionised water added up to the mark.
ii. The volumetric flask was stoppered, and the diluted solution mixed thoroughly.
iii. Aliquots of the diluted vinegar solution were pipetted into conical flasks that had been rinsed with deionised water and a few drops of indicator added to each flask.
iv. A standardised sodium hydroxide solution was added to a burette that had been rinsed with deionised water.
v. Two samples of diluted vinegar were titrated against the sodium hydroxide solution and both values were used to calculate the concentration of the vinegar.
Which of the following does not explain why Step v contributed to the errors in the titration?
There were insufficient titrations
hence the sample size was too small.
to determine if either was an outlier.
to reduce random errors by averaging.
to identify the colour change.
Reveal Answer
hence the sample size was too small.
This is a valid explanation for the error. Two titrations represent a very small sample size, which decreases the reliability of the calculated concentration.
to determine if either was an outlier.
This is a valid explanation for the error. With only two titration values, it is impossible to identify if one of the results is an anomaly or outlier; typically, at least three concordant results are required.
to reduce random errors by averaging.
This is a valid explanation for the error. Performing multiple titrations and averaging the concordant results helps to minimize the impact of random errors, which cannot be effectively achieved with just two trials.
to identify the colour change.
This does not explain the error. The ability to identify the colour change at the endpoint depends on the choice of indicator and the observer's perception, not on the number of titrations performed.
The electrolysis of water is used to produce oxygen, , gas.
The gas produced is piped into a 200.0 L fixed-volume gas storage container at 22.0 °C.
When more is added, the pressure in the container increases.
What mass of needs to be added to increase the pressure by 250.0 kPa?
Reveal Answer
Incorrect. This value is the reciprocal of the correct mass (). The mass should be found by calculating the moles of gas and multiplying by the molar mass.
Incorrect. This result comes from dividing the number of moles by the molar mass () instead of correctly multiplying them () to find the mass.
Correct. Using the ideal gas law (), the number of moles of added is calculated to be approximately . Multiplying this by the molar mass of () gives the correct mass.
Incorrect. This value is obtained by incorrectly using the temperature in degrees Celsius () instead of converting it to Kelvin () for the ideal gas law calculation.
A titration was performed to determine the concentration of an ethanoic acid, , solution using the following procedure:
- 25.00 mL of the solution was pipetted into a conical flask.
- A few drops of indicator were added to the flask.
- A burette was filled with standard sodium hydroxide, NaOH, solution.
- The solution was then titrated with the NaOH solution.
- Steps 1–4 were repeated until three concordant titres were obtained.
A systematic error could result if the
burette tap leaked during one of the titrations.
burette readings were recorded to the nearest 0.1 mL.
number of drops of indicator was not consistent for each titration.
actual concentration of the standard NaOH solution was lower than the stated concentration.
Reveal Answer
burette tap leaked during one of the titrations.
A leak during only one titration is a mistake or random error, as it does not consistently affect all trials in the same way.
burette readings were recorded to the nearest 0.1 mL.
Recording readings to the nearest 0.1 mL affects the precision of the measurements, which contributes to random error rather than a consistent systematic error.
number of drops of indicator was not consistent for each titration.
An inconsistent number of drops introduces random error, as the variation changes unpredictably between each individual trial.
actual concentration of the standard NaOH solution was lower than the stated concentration.
If the standard solution's concentration is lower than stated, a larger volume will be required for every titration, consistently skewing the calculated concentration of ethanoic acid higher. This consistent directional shift is the definition of a systematic error.
An example of a random error in a titration is
reading solution volumes to the bottom of the meniscus.
a gas bubble in the burette tap that comes out during a titration.
calculating the concentration of the primary standard incorrectly.
rinsing down the sides of the conical flasks during titrations.
Reveal Answer
reading solution volumes to the bottom of the meniscus.
Reading the volume to the bottom of the meniscus is the correct standard procedure for measuring liquids, not an error.
a gas bubble in the burette tap that comes out during a titration.
A gas bubble escaping during a titration causes an unpredictable change in the volume reading for that specific trial, which is the definition of a random error.
calculating the concentration of the primary standard incorrectly.
An incorrect calculation of the primary standard's concentration is a systematic error (or blunder) that will consistently skew all subsequent results in the same direction.
rinsing down the sides of the conical flasks during titrations.
Rinsing the sides of the conical flask with deionized water is a correct technique to ensure all reactants are in the main solution, not an error.
A water sample contains at least one of the following anions at concentrations of .
- bromide (Br)
- carbonate (CO)
Outline a sequence of tests that could be performed in a school laboratory to confirm the identity of the anion or anions present. Include expected observations and TWO balanced chemical equations in your answer.
Reveal Answer
-
Add aqueous nitric acid – bubbles indicate carbonate present:
Acid removes carbonate for further testing of sample
-
Add silver nitrate solution – creamy precipitate indicates bromide present
-
.
| Descriptor | Marks |
|---|---|
| 4 |
Demonstrates a sound understanding of anion testing with expected observation(s) and/or a correct chemical equation | 3 |
Demonstrates some understanding of anion testing | 2 |
Provides some relevant information | 1 |
None of the above | 0 |
Use the following information to answer the question.
A group of students conducted a series of titrations to determine the concentration of acetic acid in vinegar using the following steps:
i. A sample of vinegar was pipetted into a volumetric flask that had been rinsed with the vinegar and then deionised water added up to the mark.
ii. The volumetric flask was stoppered, and the diluted solution mixed thoroughly.
iii. Aliquots of the diluted vinegar solution were pipetted into conical flasks that had been rinsed with deionised water and a few drops of indicator added to each flask.
iv. A standardised sodium hydroxide solution was added to a burette that had been rinsed with deionised water.
v. Two samples of diluted vinegar were titrated against the sodium hydroxide solution and both values were used to calculate the concentration of the vinegar.
The students found it difficult to obtain consistent results from their titrations. Which of the steps could have been responsible for the difficulties?
iv and v only
i, ii, iv and v only
i, iv and v only
iii, iv and v only
Reveal Answer
iv and v only
While steps iv and v do cause inconsistencies, this option misses step i. Rinsing the volumetric flask with vinegar adds unmeasured moles of acid, which is a significant source of error.
i, ii, iv and v only
Step ii is a standard and correct procedure for preparing a diluted solution. Mixing the solution thoroughly ensures homogeneity and would not cause inconsistent results.
i, iv and v only
Step i is incorrect because the volumetric flask should only be rinsed with deionised water. Step iv is incorrect because the burette must be rinsed with the titrant (NaOH) to prevent dilution. Step v is incorrect because titrations should be repeated until at least three concordant results are obtained.
iii, iv and v only
Step iii is a correct procedure. Conical flasks should be rinsed with deionised water, as rinsing them with the analyte would add extra, unmeasured moles of the substance being titrated.
A student hypothesised that polishing the zinc, Zn, electrode in an Fe–Zn galvanic cell would increase the current produced by the cell.
What would be the most valid method of testing this hypothesis?
researching the scientific literature to determine how polishing changes the structure of Zn
measuring the conductivity of a Zn electrode after polishing it
measuring the change in mass per unit time of the Fe electrode in the same Fe–Zn galvanic cell before and after the Zn electrode was polished
measuring the current produced by two different Fe–Zn galvanic cells, one using a polished Zn electrode and the other using an unpolished Zn electrode
Reveal Answer
researching the scientific literature to determine how polishing changes the structure of Zn
While researching literature can provide background information, it is not an experimental method to directly test the student's specific hypothesis about the cell's current.
measuring the conductivity of a Zn electrode after polishing it
Measuring the conductivity of the zinc electrode alone does not measure the overall current produced by the electrochemical reactions in the galvanic cell.
measuring the change in mass per unit time of the Fe electrode in the same Fe–Zn galvanic cell before and after the Zn electrode was polished
The change in mass per unit time of the electrode is directly proportional to the cell's current. Using the same cell before and after polishing controls for confounding variables, making it the most valid experimental design.
measuring the current produced by two different Fe–Zn galvanic cells, one using a polished Zn electrode and the other using an unpolished Zn electrode
Although it directly measures current, using two different cells introduces confounding variables, such as slight differences in electrolyte concentration or internal resistance, which could affect the results.
A student wanted to investigate how changing temperature would influence how rapidly oxalic acid solution would decolourise an acidified potassium permanganate solution.
The student was provided with the following chemicals and equipment:
- 0.1 mol L acidified potassium permanganate solution
- 0.1 mol L oxalic acid solution
- 250 mL conical flasks
- Bunsen burner
- tripod and gauze mat
- thermometer
- stop watches
- 5.00 mL, 10.00 mL, 20.00 mL and 25.00 mL pipettes
- distilled water
- 25.0 mL measuring cylinders.
State a hypothesis for this investigation.
Reveal Answer
Answer could include:
Increasing the temperature will decrease the time taken for the acidified potassium permanganate to decolourise (change from purple to pale pink/colourless as the rate of reaction increases with increasing temperature).
| Descriptor | Marks |
|---|---|
Writes a hypothesis that gives the relationship between the independent and dependent variables. | 2 |
Writes a hypothesis that includes the independent and dependent variables without giving their relationship. | 1 |
None of the above | 0 |
Identify the independent and dependent variables.
Reveal Answer
The independent variable is the temperature of the solution.
The dependent variable is the time taken for the potassium permanganate solution/mixture to decolourise.
| Descriptor | Marks |
|---|---|
Identifies the independent variable as temperature (of solution) | 1 |
Identifies the dependent variable as time taken (for potassium permanganate solution/mixture) to decolourise | 1 |
Identify two control variables.
Reveal Answer
Answers could include (any 2 of):
- concentration of acidified potassium permanganate solution
- concentration of oxalic acid solution
- volume of acidified potassium permanganate solution
- volume of oxalic acid solution
- stopwatch/timer
- person timing/observing.
| Descriptor | Marks |
|---|---|
1 mark for each correct point (any 2 of): concentration of acidified potassium permanganate solution, concentration of oxalic acid solution, volume of acidified potassium permanganate solution, volume of oxalic acid solution, stopwatch/timer, person timing/observing. | 2 |
Describe a procedure for this investigation.
Reveal Answer
A possible answer:
Fixed volumes of oxalic acid and acidified potassium permanganate are used. Temperature must be varied and measured, and time must be measured from mixing. Repeated trials should be conducted.
Additionally, an appropriate method for determining the end point of the reaction (decolourisation) is used, e.g. a white paper base.
| Descriptor | Marks |
|---|---|
Recognises that fixed volumes of both oxalic acid and acidified potassium permanganate are used (1 mark for recognising fixed volume of only one) | 2 |
Recognises that temperature must be varied and measured | 1 |
Recognises that time must be measured from mixing | 1 |
1 mark for each correct point (any 2 of): recognition of appropriate method for determining end point of the reaction (decolourisation) e.g. use a white paper base, recognition of the use of trials, recognition of the use of appropriate glassware, recognition that solutions are mixed in appropriate proportions e.g. 2:5 ratio of solutions | 2 |
Outline the difference between systematic and random errors. Use an example of each from this investigation to support your answer.
Reveal Answer
An answer could be:
Systematic errors produce consistently high or consistently low measurements compared to the true value. Random errors produce measurements that can fluctuate around the true value.
An example of a systematic error is only heating one solution. An example of a random error is not using the same measuring equipment during the reaction.
| Descriptor | Marks |
|---|---|
Recognises that systematic errors produce consistently high or consistently low measurements compared to the true value | 1 |
Recognises that random errors produce measurements that can be either high or low/fluctuate around the true value | 1 |
Provides an example of a systematic error (e.g. only heating one solution, using an inappropriate proportion of reactants, errors in calibration with equipment, inappropriate rinsing of glassware) | 1 |
Provides an example of a random error (e.g. parallax (reading of meniscus), judging the end point, use of stopwatch, reading thermometer, not using the same measuring equipment during the reaction, using measuring cylinder rather than pipette) | 1 |
Use the following information to answer the question.
A solution of citric acid, , was analysed by titration.
25.0 mL aliquots of the solution were titrated against a standardised solution of 0.0250 M sodium hydroxide, NaOH. Phenolphthalein indicator was used and the average titre was found to be 24.0 mL.
Which one of the following would have resulted in a concentration that is higher than the actual concentration?
The pipette was rinsed with NaOH solution.
The pipette was rinsed with solution.
The conical flask was rinsed with NaOH solution.
The conical flask was rinsed with solution.
Reveal Answer
The pipette was rinsed with NaOH solution.
Rinsing the pipette with NaOH would neutralize some of the citric acid before it is transferred to the flask. This decreases the required NaOH titre, resulting in a lower calculated concentration.
The pipette was rinsed with solution.
Rinsing the pipette with the solution it will transfer (citric acid) is the standard correct procedure and ensures the concentration remains accurate.
The conical flask was rinsed with NaOH solution.
Rinsing the conical flask with NaOH adds extra base to the flask, which neutralizes some of the citric acid before titration begins. This decreases the required NaOH titre, leading to a lower calculated concentration.
The conical flask was rinsed with solution.
Rinsing the conical flask with citric acid leaves residual acid in the flask, meaning there is more acid present than just the 25.0 mL aliquot. This requires a larger volume of NaOH to neutralize, leading to a higher calculated concentration.